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LT1578/LT1578-2.5 1.5A, 200kHz Step-Down Switching Regulator
FEATURES
s s s s s s s s s s s s
DESCRIPTIO
1.5A Switch Current High Efficiency--Low Loss 0.2 Switch Constant 200kHz Switching Frequency 4V to 15V Input VoltageRange Minimum Output: 1.21V Low Supply Current: 1.9mA Low Shutdown Current: 20A Easily Synchronizable Up to 400kHz Cycle-by-Cycle Current Limit Reduced EMI Generation Low Thermal Resistance SO-8 Package Uses Small Low Value Inductors
The LT (R)1578 is a 200kHz monolithic buck mode switching regulator. A 1.5A switch is included on the die along with all the necessary oscillator, control and logic circuitry. The topology is current mode for fast transient response and good loop stability. The LT1578 is a modified version of the LT1507 that has been optimized for noise sensitive applications. It will operate over a 4V to 15V input range. In addition, the reference voltage has been lowered to allow the device to produce output voltages down to 1.2V. Quiescent current has been reduced by a factor of two. Switch on resistance has been reduced by 30%. Switch transition times have been slowed to reduce EMI generation. The oscillator frequency has been reduced to 200kHz to maintain high efficiency over a wide output current range. The pinout has been changed to improve PC layout by allowing the high current, high frequency switching circuitry to be easily isolated from low current, noise sensitive control circuitry. The new SO-8 package includes a fused ground lead that significantly reduces the thermal resistance of the device to extend the ambient operating temperature range. Standard surface mount external parts can be used including the inductor and capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
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Portable Computers Battery-Powered Systems Battery Chargers Distributed Power Systems
TYPICAL APPLICATION
Efficiency vs Load Current 3.3V Buck Converter
INPUT 5V TO 15V C3* 10F TO 50F
90 85
+
VIN BOOST
C2 0.33F VSW
EFFICIENCY (%)
L1** 15H
D2 1N914 OUTPUT** 3.3V, 1.25A
80 75 70 65 60 55 50 0 0.25 0.50 0.75 1.00 LOAD CURRENT (A) 1.25 1.50 VOUT = 3.3V VIN = 5V L = 25H
LT1578 OPEN = ON SHDN GND FB VC CC 100pF D1 1N5818 R1 8.66k R2 4.99k
* RIPPLE CURRENT RATING IOUT/2 ** INCREASE L1 TO 30H FOR LOAD CURRENTS ABOVE 0.6A AND TO 60H ABOVE 1A SEE APPLICATIONS INFORMATION
+
C1 100F, 10V SOLID TANTALUM
1578 TA01
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1578 TA02
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LT1578/LT1578-2.5
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW VSW 1 VIN 2 BOOST 3 GND 4 8 SYNC 7 SHDN 6 FB/SENSE 5 VC
Input Voltage .......................................................... 16V BOOST Pin Above Input Voltage ............................. 10V SHDN Pin Voltage ..................................................... 7V SENSE Pin Voltage .................................................... 4V FB Pin Voltage (Adjustable Part) ............................ 3.5V FB Pin Current (Adjustable Part) ............................ 1mA SYNC Pin Voltage ..................................................... 7V Operating Junction Temperature Range LT1578C ............................................... 0C to 125 C LT1578I ........................................... - 40C to 125C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LT1578CS8 LT1578IS8 LT1578CS8-2.5 LT1578IS8-2.5 S8 PART MARKING 1578 1578I 157825 578I25
S8 PACKAGE 8-LEAD PLASTIC SO
JA = 80C/ W WITH FUSED GROUND PIN CONNECTED TO GROUND PLANE OR LARGE LANDS
Consult factory for Military grade parts.
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TJ = 25C. VIN = 5V, VC = 1.5V, Boost = VIN + 5V, switch open, unless otherwise noted.
PARAMETER Feedback Voltage Sense Voltage (Fixed 2.5) All Conditions Sense Pin Resistance Reference Voltage Line Regulation Feedback Input Bias Current Error Amplifier Voltage Gain (Notes 2, 10) Error Amplifier Transconductance (Note 10) VC Pin to Switch Current Transconductance Error Amplifier Source Current Error Amplifier Sink Current VC Pin Switching Threshold VC Pin High Clamp Switch Current Limit Slope Compensation (Note 8) Switch On Resistance (Note 7) Maximum Switch Duty Cycle Minimum Switch Duty Cycle (Note 9) Switch Frequency Switch Frequency Line Regulation Frequency Shifting Threshold on FB Pin Minimum Input Voltage (Note 3) Minimum Boost Voltage (Note 4) 4.3V VIN 15V
q q q
ELECTRICAL CHARACTERISTICS
CONDITIONS All Conditions
q
MIN 1.195 1.18 2.46 2.44 5.7
TYP 1.21 2.5 9.5 0.01 0.5 400 1050 1.5 110 130 0.8 2.1 2 0.3 0.2 94 94 8 200 0 0.74 4.0 2.3
MAX 1.225 1.24 2.54 2.56 13.7 0.03 2 1300 1700 190 200
UNITS V V V V k %/V A Mho Mho A/ V A A V V A A % % % kHz kHz %/ V V V V
I (VC) = 10A
q
200 800 400 40 50
VFB = 1.1V VFB = 1.4V Duty Cycle = 0 VC Open, VFB = 1.1V, DC 50% DC = 80% ISW = 1.5A VFB = 1.1V
q q
q
1.5
3.5 0.35 0.45
q q
90 86 180 160 0.4
VC Set to Give 50% Duty Cycle
q
4.3V VIN 15V f = 10kHz ISW 1.5A
q q q q
220 240 0.15 1.0 4.3 3.0
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LT1578/LT1578-2.5
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TJ = 25C. VIN = 5V, VC = 1.5V, Boost = VIN + 5V, switch open, unless otherwise noted.
PARAMETER Boost Current (Note 5) VIN Supply Current (Note 6) Shutdown Supply Current Lockout Threshold Shutdown Thresholds Synchronization Threshold Synchronizing Range SYNC Pin Input Resistance Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Gain is measured with a VC swing equal to 200mV above the switching threshold level to 200mV below the upper clamp level. Note 3: Minimum input voltage is not measured directly, but is guaranteed by other tests. It is defined as the voltage where internal bias lines are still regulated so that the reference voltage and oscillator frequency remain constant. Actual minimum input voltage to maintain a regulated output will depend on output voltage and load current. See Applications Information. Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the internal power switch. Note 5: Boost current is the current flowing into the boost pin with the pin held 5V above input voltage. It flows only during switch on time. Note 6: Input supply current is the bias current drawn by the input pin with switching disabled. CONDITIONS ISW = 0.5A ISW = 1.5A VSHDN = 0V, VIN 15V, VSW = 0V, VC Open
q
ELECTRICAL CHARACTERISTICS
MIN
q q q
TYP 9 27 1.9 20 2.42 0.37 0.45 1.5 40
VC Open VC Open Device Shutting Down Device Starting Up
q q q
2.34 0.13 0.25 250
MAX 18 50 2.7 50 75 2.50 0.60 0.7 2.2 400
UNITS mA mA mA A A V V V V kHz k
Note 7: Switch on resistance is calculated by dividing VIN to VSW voltage by the forced current (1.5A). See Typical Performance Characteristics for the graph of switch voltage at other currents. Note 8: Slope compensation is the current subtracted from the switch current limit at 80% duty cycle. See Maximum Output Load Current in the Applications Information section for further details. Note 9: Minimum on-time is 400ns typical. For a 200kHz operating frequency this means the minimum duty cycle is 8%. In frequency foldback mode, the effective duty cycle will be less than 8%. Note 10: Transconductance and voltage gain refer to the internal amplifier exclusive of the voltage divider. To calculate gain and transconductance referred to the sense pin on the fixed voltage parts, divide values shown by the ratio 2.5/1.21.
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Voltage Drop
0.5 125C SWITCH PEAK CURRENT (A) 25C 0.3 -20C 0.2
0.4 SWITCH VOLTAGE (V)
1.5
MINIMUM
FEEDBACK VOLTAGE (V)
0.1
0 0 0.25 0.50 0.75 1.00 SWITCH CURRENT (A) 1.25 1.50
UW
1576 G01
Switch Peak Current Limit
2.5 TYPICAL 2.0
1.22 1.23
Feedback Pin Voltage
1.21
1.0
0.5
1.20
0 0 20 60 40 DUTY CYCLE (%) 80 100
1576 G02
1.19 0 25 50 75 100 -50 -25 JUNCTION TEMPERATURE (C)
125
1576 G03
3
LT1578/LT1578-2.5 TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Pin Bias Current (VSHDN = Lockout Threshold)
4 AT 2.44V STANDBY THRESHOLD (CURRENT FLOWS OUT OF PIN)
SHDN PIN CURRENT (A)
SHDN PIN CURRENT (A)
3
140 120 100 80 60 40 20 CURRENT REQUIRED TO FORCE SHUTDOWN (FLOWS OUT OF PIN). AFTER SHUTDOWN, CURRENT DROPS TO A FEW A 125
SHUTDOWN PIN VOLTAGE (V)
2
1
0 0 25 50 75 100 -50 -25 JUNCTION TEMPERATURE (C)
Standby Thresholds
2.46
INPUT SUPPLY CURRENT (A)
SHUTDOWN PIN VOLTAGE (V)
20 VSHDN = 0V 15
INPUT SUPPLY CURRENT (A)
2.45 2.44 ON 2.43 STANDBY 2.42 2.41 2.40 50 100 -50 -25 25 75 0 JUNCTION TEMPERATURE (C)
Error Amplifier Transconductance
2000 PHASE 1500
GAIN (Mho)
TRANSCONDUCTANCE (Mho)
150 GAIN 100
VC
SWITCHING FREQUENCY (kHz) OR CURRENT (A)
1000
500
VFB 1 x 10-3
(
)
ROUT 570k
0
ERROR AMPLIFIER EQUIVALENT CIRCUIT
RLOAD = 50 -500 10 100 1k 10k FREQUENCY (Hz) 100k -50 1M
1576 G09
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1576 G04
Shutdown Pin Bias Current (VSHDN = Shutdown Threshold)
180 160
0.8 0.7 0.6 0.5 0.4
Shutdown Thresholds
START-UP SHUTDOWN
0.3 0.2 0.1 0 0 50 25 75 100 -50 -25 JUNCTION TEMPERATURE (C) 125
125
0 0 25 50 75 100 -50 -25 JUNCTION TEMPERATURE (C)
1576 G05
1576 G06
Shutdown Supply Current
25
30 25 20 15 10 5 0
Shutdown Supply Current
VIN = 10V
10
5
0
125
0
5 10 INPUT VOLTAGE (V)
15
1576 G08
0
0.1 0.2 0.3 SHUTDOWN VOLTAGE (V)
0.4
1576 G010
1576 G07
Error Amplifier Transconductance
200
1600 1400 1200 1000 800 600 400 200 0 25 75 100 0 50 -50 -25 JUNCTION TEMPERATURE (C) 200 250
Frequency Foldback
SWITCHING FREQUENCY
PHASE (DEG)
150
COUT 2.4pF
50
100
0
50 FEEDBACK PIN CURRENT
125
0
0
1.0 0.5 1.5 FEEDBACK VOLTAGE (V)
2.0
1576 G12
1576 G11
LT1578/LT1578-2.5 TYPICAL PERFORMANCE CHARACTERISTICS
Switching Frequency
240
SWITCH CURRENT LIMIT (A)
2.0 1.5 1.0 0.5
INPUT VOLTAGE (V)
220
FREQUENCY (kHz)
200
180
160 0 25 50 75 100 -50 -25 JUNCTION TEMPERATURE (C)
Maximum Output Current at VOUT = 5V
1.6 1.4 L = 60H L = 30H L = 15H 1.6 1.4
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
1.2 1.0 0.8 0.6 0.4 0.2 0 6 9
12
INPUT VOLTAGE (V)
1578 G15
BOOST Pin Current
30 25 BOOST PIN CURRENT (mA) 20 15 10 5 0
1.0
THRESHOLD VOLTAGE (V)
0
0.25
Kool M is a registered trademark of Magnetics, Inc. Metglas is a registered trademark of AlliedSignal, Inc.
UW
1576 G13
Switch Current Limit Foldback
3.0 2.5
4.25 4.50
Minimum Input Voltage to Start with 3.3V Output
4.00
3.75
0
125
0
1.0 0.4 0.6 0.8 0.2 FEEDBACK PIN VOLTAGE (V)
1.2
1578 G19
3.50 1 10 100 LOAD CURRENT (mA) 1000
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Maximum Output Current at VOUT = 3.3V
1.6 L = 60H 1.4 L = 30H L = 15H 1.2 1.0 0.8 0.6 0.4 0.2 0 4 6 8 10 12 14
1578 G16
Maximum Output Current at VOUT = 2.5V
L = 60H L = 30H L = 15H
1.2 1.0 0.8 0.6 0.4 0.2 0
15
4
6
8
10
12
14
1578 G17
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
VC Pin Shutdown Threshold
0.8
0.6
0.4
0.2
0.50 0.75 1.00 SWITCH CURRENT (A)
1.25
1.50
0 0 25 50 75 100 -50 -25 JUNCTION TEMPERATURE (C)
125
1576 G20
1576 G21
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LT1578/LT1578-2.5
PIN FUNCTIONS
VSW (Pin 1): The switch pin is the emitter of the on-chip power NPN switch. This pin is driven up to the input pin voltage during switch on time. Inductor current drives the switch pin negative during switch off time. Negative voltage is clamped with the external catch diode. Maximum negative switch voltage allowed is - 0.8V. VIN (Pin 2): This is the collector of the on-chip power NPN switch. This pin powers the internal circuitry and internal regulator. At NPN switch on and off, high dI/dt edges occur through this pin. Keep the external bypass and catch diode close to this pin. Trace inductance in this path will create a voltage spike at switch off, adding to the VCE voltage across the internal NPN. BOOST (Pin 3): The BOOST pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. Without this added voltage, the typical switch voltage loss would be about 1.5V. The additional boost voltage allows the switch to saturate with its voltage drop approximating that of a 0.2 FET structure. Efficiency improves from 75% for conventional bipolar designs to > 88% for the LT1578. GND (Pin 4): The GND pin connection needs consideration for two reasons. First, it acts as the reference for the regulated output, so load regulation will suffer if the "ground" end of the load is not at the same voltage as the GND pin of the IC. This condition will occur when load current or other currents flow through metal paths between the GND pin and the load ground point. Keep the ground path short between the GND pin and the load and use a ground plane when possible. The second consideration is EMI caused by GND pin current spikes. Internal capacitance between the VSW pin and the GND pin creates very narrow (<10ns) current spikes in the GND pin. If the GND pin is connected to system ground with a long metal trace, this trace may radiate EMI. Keep the path between the input bypass and the GND pin short. The GND pin of the SO-8 package is directly attached to the internal tab. This pin should be attached to a large copper area to improve thermal resistance. VC (Pin 5): The VC pin is the output of the error amplifier and the input to the peak switch current comparator. It is normally used for frequency compensation, but can do double duty as a current clamp or control loop override. This pin sits at about 1V for very light loads and 2V at maximum load. It can be driven to ground to shut off the regulator, but if driven high, current must be limited to 4mA. FB/SENSE (Pin 6): The feedback pin is used to set output voltage using an external voltage divider that generates 1.21V at the pin with the desired output voltage. The fixed voltage (2.5V) parts have the divider included on the chip and the FB pin is used as a sense pin, connected directly to the 2.5V output. Three additional functions are performed by the FB pin. When the pin voltage drops below 0.7V, the switch current limit and the switching frequency are reduced and the external sync function is disabled. See Feedback Pin Function section in Applications Information for details. SHDN (Pin 7): The shutdown pin is used to turn off the regulator and to reduce input drain current to a few microamperes. Actually, this pin has two separate thresholds, one at 2.42V to disable switching, and a second at 0.4V to force complete micropower shutdown. The 2.42V threshold functions as an accurate undervoltage lockout (UVLO). This can be used to prevent the regulator from operating until the input voltage has reached a predetermined level. SYNC (Pin 8): The SYNC pin is used to synchronize the internal oscillator to an external signal. It is directly logic compatible and can be driven with any signal between 10% and 90% duty cycle. The synchronizing range is equal to initial operating frequency, up to 400kHz. When not used, this pin should be grounded. See Synchronizing section in Applications Information for details.
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LT1578/LT1578-2.5
BLOCK DIAGRAM
The LT1578 is a constant frequency, current mode buck converter. This means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. In addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. A switch cycle starts with an oscillator pulse which sets the RS flip-flop to turn the switch on. When switch current reaches a level set by the inverting input of the comparator, the flip-flop is reset and the switch turns off. Output voltage control is obtained by using the output of the error amplifier to set the switch current trip point. This technique means that the error amplifier commands current to be delivered to the output rather than voltage. A voltage fed system will have low phase shift up to the resonant frequency of the inductor
INPUT
2.9V BIAS REGULATOR
SYNC
SHUTDOWN COMPARATOR
0.4V SHDN 3.5A FOLDBACK CURRENT LIMIT CLAMP Q2 FREQUENCY SHIFT CIRCUIT
+ -
2.42V
VC
ERROR AMPLIFIER gm = 1000Mho
Figure 1. Block Diagram
+
LOCKOUT COMPARATOR
-
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and output capacitor, then an abrupt 180 shift will occur. The current fed system will have 90 phase shift at a much lower frequency, but will not have the additional 90 shift until well beyond the LC resonant frequency. This makes it much easier to frequency compensate the feedback loop and also gives much quicker transient response. High switch efficiency is attained by using the BOOST pin to provide a voltage to the switch driver which is higher than the input voltage, allowing the switch to saturate. This boosted voltage is generated with an external capacitor and diode. Two comparators are connected to the shutdown pin. One has a 2.42V threshold for undervoltage lockout and the second has a 0.4V threshold for complete shutdown.
0.025
+
INTERNAL VCC
-
CURRENT SENSE AMPLIFIER DC VOLTAGE GAIN = 35
SLOPE COMP
0.8V
BOOST
200kHz OSCILLATOR
S CURRENT COMPARATOR
RS FLIP-FLOP
DRIVER CIRCUITRY
+ - - +
R
Q1 POWER SWITCH
VSW
FB
1.21V GND
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LT1578/LT1578-2.5
APPLICATIONS INFORMATION
FEEDBACK PIN FUNCTIONS The feedback (FB) pin on the LT1578 is used to set output voltage and provide several overload protection features. The first part of this section deals with selecting resistors to set output voltage and the remaining part talks about foldback frequency and current limiting created by the FB pin. Please read both parts before committing to a final design. The fixed 2.5V LT1578-2.5 has internal divider resistors and the FB pin, renamed SENSE, is connected directly to the 2.5V output. The suggested value for the output divider resistor (see Figure 2) from FB to ground (R2) is 5k or less, and a formula for R1 is shown below. The output voltage error caused by ignoring the input bias current on the FB pin is less than 0.25% with R2 = 5k. Please read the following if divider resistors are increased above the suggested values.
R1 = R2 VOUT - 1.21 1.21
(
)
LT1578
TO FREQUENCY SHIFTING 1.4V ERROR AMPLIFIER Q1
+ -
R5 5k Q2 TO SYNC CIRCUIT
VC
GND
Figure 2. Frequency and Current Limit Foldback
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More Than Just Voltage Feedback The feedback pin is used for more than just output voltage sensing. It also reduces switching frequency and current limit when output voltage is very low (see the Frequency Foldback graph in Typical Performance Characteristics). This is done to control power dissipation in both the IC and the external diode and inductor during short-circuit conditions. A shorted output requires the switching regulator to operate at very low duty cycles, and the average current through the diode and inductor is equal to the short-circuit current limit of the switch (typically 2A for the LT1578, folding back to less than 0.77A). Minimum switch on time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 200kHz, so frequency is reduced by about 5:1 when the feedback pin voltage drops below 0.7V (see Frequency Foldback graph). This does not affect operation with normal load conditions; one simply sees a gear shift in switching frequency during start-up as the output voltage rises.
VSW OUTPUT 5V
1.21V R3 1k R4 1k FB
R1
+
R2 5k
1578 F02
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
In addition to lower switching frequency, the LT1578 also operates at lower switch current limit when the feedback pin voltage drops below 0.7V. Q2 in Figure 2 performs this function by clamping the VC pin to a voltage less than its normal 2.1V upper clamp level. This foldback current limit greatly reduces power dissipation in the IC, diode and inductor during short-circuit conditions. External synchronization is also disabled to prevent interference with foldback operation. Again, it is nearly transparent to the user under normal load conditions. The only loads that may be affected are current sources, such as lamps and motors, that maintain high load current with output voltage less than 50% of final value. In these rare situations the feedback pin can be clamped above 0.7V to defeat foldback current limit. Caution: clamping the feedback pin means that frequency shifting will also be defeated, so a combination of high input voltage and dead shorted output may cause the LT1578 to lose control of current limit. The internal circuitry which forces reduced switching frequency also causes current to flow out of the feedback pin when output voltage is low. The equivalent circuitry is shown in Figure 2. Q1 is completely off during normal operation. If the FB pin falls below 0.7V, Q1 begins to conduct current and reduces frequency at the rate of approximately 1kHz/A. To ensure adequate frequency foldback (under worst-case short-circuit conditions), the external divider Thevinin resistance must be low enough to pull 35A out of the FB pin with 0.5V on the pin (RDIV 14.3k). The net result is that reductions in frequency and current limit are affected by output voltage divider impedance. Although divider impedance is not critical, caution should be used if resistors are increased beyond the suggested values and short-circuit conditions will occur with high input voltage. High frequency pickup will increase and the protection accorded by frequency and current foldback will decrease. MAXIMUM OUTPUT LOAD CURRENT Maximum load current for a buck converter is limited by the maximum switch current rating (IP) of the LT1578. This current rating is 1.5A up to 50% duty cycle (DC), decreasing to 1.3A at 80% duty cycle. This is shown graphically in Typical Performance Characteristics and as shown in the formula below: IP = 1.5A for DC 50% IP = 1.67 - 0.18 (DC) - 0.32(DC)2 for 50% < DC < 90% DC = Duty cycle = VOUT/VIN Example: with VOUT = 5V, VIN = 8V; DC = 5/8 = 0.625, and; ISW(MAX) = 1.67 - 0.18 (0.625) - 0.32(0.625)2 = 1.43A Current rating decreases with duty cycle because the LT1578 has internal slope compensation to prevent current mode subharmonic switching. For more details, read Application Note 19. The LT1578 is a little unusual in this regard because it has nonlinear slope compensation which gives better compensation with less reduction in current limit. Maximum load current would be equal to maximum switch current for an infinitely large inductor, but with finite inductor size, maximum load current is reduced by one-half peak-to-peak inductor current. The following formula assumes continuous mode operation, implying that the term on the right is less than one-half of IP. VOUT VIN - VOUT IOUT(MAX) = IP - Continuous Mode 2 L f VIN For the conditions above and L = 15H, IOUT MAX = 1.43 -
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( )( ) ( )( )( ) (5)(8 - 5)
()
2 15 * 10- 6 200 * 103 8 = 1.43 - 0.31 = 1.12A
()
At VIN = 15V, duty cycle is 33%, so IP is just equal to a fixed 1.5A, and IOUT(MAX) is equal to: 1.5 -
2 15 * 10- 6 200 * 103 15 = 1.5 - 0.56 = 0.94A
(5)(15 - 5)
()
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LT1578/LT1578-2.5
APPLICATIONS INFORMATION
Note that there is less load current available at the higher input voltage because inductor ripple current increases. This is not always the case. Certain combinations of inductor value and input voltage range may yield lower available load current at the lowest input voltage due to reduced peak switch current at high duty cycles. If load current is close to the maximum available, please check maximum available current at both input voltage extremes. To calculate actual peak switch current with a given set of conditions, use: ISW(PEAK ) = IOUT + VOUT VIN - VOUT
IN
( ) 2(L)(f)(V )
(IP) (f)(L)(VIN) 2(VOUT )(VIN - VOUT )
2
For lighter loads where discontinuous operation can be used, maximum load current is equal to: IOUT(MAX) = Discontinuous mode
Example: with L = 5H, VOUT = 5V, and VIN(MAX) = 15V,
(1.5) 200 * 103 5 *10-6 (15) IOUT(MAX ) = = 0.34A 2(5)(15 - 5)
2
The main reason for using such a tiny inductor is that it is physically very small, but keep in mind that peak-to-peak inductor current will be very high. This will increase output ripple voltage. If the output capacitor has to be made larger to reduce ripple voltage, the overall circuit could actually wind up larger. CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR For most applications the output inductor will fall in the range of 15H to 60H. Lower values are chosen to reduce
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physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1578 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage, and reduce core loss. Graphs in the Typical Performance Characteristics section show maximum output load current versus inductor size and input voltage. When choosing an inductor you might have to consider maximum load current, core and copper losses, allowable component height, output voltage ripple, EMI, fault current in the inductor, saturation, and of course, cost. The following procedure is suggested as a way of handling these somewhat complicated and conflicting requirements. 1. Choose a value in microhenries from the graphs of maximum load current and core loss. Choosing a small inductor may result in discontinuous mode operation at lighter loads, but the LT1578 is designed to work well in either mode. Keep in mind that lower core loss means higher cost, at least for closed core geometries like toroids. Assume that the average inductor current is equal to load current and decide whether or not the inductor must withstand continuous fault conditions. If maximum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 1.5A overload condition. Dead shorts will actually be more gentle on the inductor because the LT1578 has foldback current limiting. 2. Calculate peak inductor current at full load current to ensure that the inductor will not saturate. Peak current can be significantly higher than output current, especially with smaller inductors and lighter loads, so don't omit this step. Powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. Other core materials fall somewhere in between. The following formula assumes continuous mode of operation, but it errs only slightly on the high side for discontinuous mode, so it can be used for all conditions.
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
IPEAK = IOUT + VOUT VIN - VOUT
IN
( ) 2(f)(L)(V )
VIN = Maximum input voltage f = Switching frequency, 200kHz 3. Decide if the design can tolerate an "open" core geometry like a rod or barrel, with high magnetic field radiation, or whether it needs a closed core like a toroid to prevent EMI problems. One would not want an open core next to a magnetic storage media, for instance! This is a tough decision because the rods or barrels are temptingly cheap and small and there are no helpful guidelines to calculate when the magnetic field radiation will be a problem. 4. Start shopping for an inductor (see representative surface mount units in Table 1) which meets the requirements of core shape, peak current (to avoid saturation), average current (to limit heating), and fault current (if the inductor gets too hot, wire insulation will melt and cause turn-to-turn shorts). Keep in mind that all good things like high efficiency, low profile, and high temperature operation will increase cost, sometimes dramatically. Get a quote on the cheapest unit first to calibrate yourself on price, then ask for what you really want. 5. After making an initial choice, consider the secondary things like output voltage ripple, second sourcing, etc. Use the experts in the Linear Technology's applications department if you feel uncertain about the final choice. They have experience with a wide range of inductor types and can tell you about the latest developments in low profile, surface mounting, etc.
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Table 1
VENDOR/ PART NO. Coiltronics CTX15-2 CTX33-2 CTX68-4 CTX15-1P CTX33-2P CTX68-4P Sumida CDRH74-150 CDH115-330 CDRH125-680 CDH74-330 Coilcraft DO3308P-153 DO3316P-333 DO3316P-683 Pulse PE-53602 PE-53604 PE-53632 PE-53633 Gowanda SMP3316-152K SMP3316-332K SMP3316-682K 15 33 68 3.5 2.3 1.7 SC SC SC 0.041 0.092 0.178 Fer Fer Fer 6 6 6 35 73 22 40 1.4 1.3 2.7 2.7 Tor Tor Tor Tor 0.166 0.290 0.063 0.085 Fer Fer Fer Fer 9.1 9.1 9.1 10 15 33 68 2 2 1.4 SC SC SC 0.12 0.1 0.18 Fer Fer Fer 3 5.21 5.21 15 33 68 33 1.47 1.68 1.5 1.45 SC SC SC SC 0.081 0.082 0.12 0.17 Fer Fer Fer Fer 4.5 5.2 6 5.2 15 33 68 15 33 68 1.7 1.4 1.2 1.4 1.3 1.1 Tor Tor Tor Tor Tor Tor 0.059 0.106 0.158 0.087 0.126 0.238 KM KM KM 52 52 52 6.0 6.0 6.4 4.2 6.0 6.4 SERIES CORE VALUE DC CORE RESIS- MATER- HEIGHT IAL (mm) (H) (Amps) TYPE TANCE()
Tor = Toroid SC = Semi-closed geometry Fer = Ferrite core material 52 = Type 52 powdered iron core material KM = Kool M
11
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
Output Capacitor The output capacitor is normally chosen by its Effective Series Resistance (ESR), because this is what determines output ripple voltage. To get low ESR takes volume, so physically smaller capacitors have high ESR. The ESR range for typical LT1578 applications is 0.05 to 0.2. A typical output capacitor is an AVX type TPS, 100F at 10V, with a guaranteed ESR less than 0.1. This is a "D" size surface mount solid tantalum capacitor. TPS capacitors are specially constructed and tested for low ESR, so they give the lowest ESR for a given volume. The value in microfarads is not particularly critical, and values from 22F to greater than 500F work well, but you cannot cheat mother nature on ESR. If you find a tiny 22F solid tantalum capacitor, it will have high ESR, and output ripple voltage will be terrible. Table 2 shows some typical solid tantalum surface mount capacitors.
Table 2. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E Case Size ESR (Max., ) Ripple Current (A)
AVX TPS, Sprague 593D AVX TAJ D Case Size AVX TPS, Sprague 593D C Case Size AVX TPS 0.2 (typ) 0.5 (typ) 0.1 to 0.3 0.7 to 1.1 0.1 to 0.3 0.7 to 0.9 0.7 to 1.1 0.4
Many engineers have heard that solid tantalum capacitors are prone to failure if they undergo high surge currents. This is historically true, and type TPS capacitors are specially tested for surge capability, but surge ruggedness is not a critical issue with the output capacitor. Solid tantalum capacitors fail during very high turn-on surges, which do not occur at the output of regulators. High discharge surges, such as when the regulator output is dead shorted, do not harm the capacitors. Unlike the input capacitor, RMS ripple current in the output capacitor is normally low enough that ripple current rating is not an issue. The current waveform is triangular with a typical value of 200mARMS. The formula to calculate this is:
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Output Capacitor Ripple Current (RMS): IRIPPLE(RMS) = 0.29 VOUT VIN - VOUT
IN
( )( (L)(f)(V )
)
Ceramic Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempting for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop stability problems. Solid tantalum capacitor's ESR generates a loop "zero" at 5kHz to 50kHz that is instrumental in giving acceptable loop phase margin. Ceramic capacitors remain capacitive to beyond 300kHz and usually resonate with their ESL before their ESR provides any damping. They are appropriate for input bypassing because of their high ripple current ratings and tolerance of turn-on surges. OUTPUT RIPPLE VOLTAGE Figure 3 shows a typical output ripple voltage waveform for the LT1578. Ripple voltage is determined by the high frequency impedance of the output capacitor, and ripple current through the inductor. Peak-to-peak ripple current through the inductor into the output capacitor is: IP-P =
(V )(V - V ) (V )(L)(f)
OUT IN OUT IN
For high frequency switchers, the sum of ripple current slew rates may also be relevant and can be calculated from: dI VIN = dt L
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
Peak-to-peak output ripple voltage is the sum of a triwave created by peak-to-peak ripple current times ESR, and a square wave created by parasitic inductance (ESL) and ripple current slew rate. Capacitive reactance is assumed to be small compared to ESR or ESL. VRIPPLE = IP-P ESR + ESL
( )( ) ( )
(5)(10 - 5)
dI dt
Example: with VIN =10V, VOUT = 5V, L = 30H, ESR = 0.1, ESL = 10nH:
IP-P = = 0.42A
()
10 30 * 10- 6 200 * 103
dI 10 = = 0.33 * 10 6 -6 dt 30 * 10 VRIPPLE = 0.42A 0.1 + 10 * 10- 9 0.33 * 10 6 = 0.042 + 0.003 = 45mVP-P
(
)( )
20mV/DIV
VOUT AT IOUT = 1A
200mA/DIV
INDUCTOR CURRENT AT IOUT = 1A VOUT AT IOUT = 50mA INDUCTOR CURRENT AT IOUT = 50mA 2s/DIV
1578 F03
20mV/DIV
200mA/DIV
Figure 3. LT1578 Ripple Voltage Waveform
CATCH DIODE The suggested catch diode (D1) is a 1N5818 Schottky, or its Motorola equivalent, MBR130. It is rated at 1A average forward current and 30V reverse voltage. Typical forward voltage is 0.42V at 1A. The diode conducts current only during switch off time. Peak reverse voltage is equal to regulator input voltage. Average forward current in normal operation can be calculated from:
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ID(AVG) =
IOUT VIN - VOUT VIN
(
)
This formula will not yield values higher than 1A with maximum load current of 1.25A unless the ratio of input to output voltage exceeds 5:1. The only reason to consider a larger diode is the worst-case condition of a high input voltage and overloaded (not shorted) output. Under shortcircuit conditions, foldback current limit will reduce diode current to less than 1A, but if the output is overloaded and does not fall to less than 1/3 of nominal output voltage, foldback will not take effect. With the overloaded condition, output current will increase to a typical value of 1.8A, determined by peak switch current limit of 2A. With VIN = 15V, VOUT = 4V (5V overloaded) and IOUT = 1.8A:
ID AVG =
()
1.8 15 - 4 15
(
) = 1.32A
This is safe for short periods of time, but it would be prudent to check with the diode manufacturer if continuous operation under these conditions must be tolerated. BOOST PIN CONSIDERATIONS For most applications, the boost components are a 0.33F capacitor and a 1N914 or 1N4148 diode. The anode is connected to the regulated output voltage and this generates a voltage across the boost capacitor nearly identical to the regulated output. In certain applications, the anode may instead be connected to the unregulated input voltage. This could be necessary if the regulated output voltage is very low (< 3V) or if the input voltage is less than 6V. Efficiency is not affected by the capacitor value, but the capacitor should have an ESR of less than 1 to ensure that it can be recharged fully under the worst-case condition of minimum input voltage. Almost any type of film or ceramic capacitor will work fine.
WARNING! Peak voltage on the BOOST pin is the sum of unregulated input voltage plus the voltage across the
13
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
boost capacitor. This normally means that peak BOOST pin voltage is equal to input voltage plus output voltage, but when the boost diode is connected to the regulator input, peak BOOST pin voltage is equal to twice the input voltage. Be sure that BOOST pin voltage does not exceed its maximum rating. For nearly all applications, a 0.33F boost capacitor works just fine, but for the curious, more details are provided here. The size of the boost capacitor is determined by switch drive current requirements. During switch on time, drain current on the capacitor is approximately IOUT/ 50. At peak load current of 1.25A, this gives a total drain of 25mA. Capacitor ripple voltage is equal to the product of on time and drain current divided by capacitor value; V = (tON)(25mA/C). To keep capacitor ripple voltage to less than 0.5V (a slightly arbitrary number) at the worstcase condition of tON = 4.7s, the capacitor needs to be 0.24F. Boost capacitor ripple voltage is not a critical parameter, but if the minimum voltage across the capacitor drops to less than 3V, the power switch may not saturate fully and efficiency will drop. An approximate formula for absolute minimum capacitor value is: CMIN =
LT1578 INPUT IN 2.42V RHI 3.5A SHDN VSW OUTPUT
C1
RLO
Figure 4. Undervoltage Lockout
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(I
OUT / 50
( f )( V
)(V
OUT / VIN
OUT
- 3V
)
)
f = Switching frequency VOUT = Regulated output voltage VIN = Minimum input voltage This formula can yield capacitor values substantially less than 0.24F, but it should be used with caution since it does not take into account secondary factors such as capacitor series resistance, capacitance shift with temperature and output overload. SHUTDOWN FUNCTION AND UNDERVOLTAGE LOCKOUT Figure 4 shows how to add undervoltage lockout (UVLO) to the LT1578. Typically, UVLO is used in situations where the input supply is current limited, or has a relatively high source resistance. It is particularly useful for input supplies with foldback current limiting. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit and latch under low source voltage
RFB
-
STANDBY
+ +
TOTAL SHUTDOWN 0.4V
+
-
GND
1578 F04
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
conditions. UVLO helps prevent the regulator from operating at source voltages where these problems might occur. Threshold voltage for lockout is about 2.42V. A 3.5A bias current flows out of the pin at threshold. This internally generated current is used to force a default high state on the shutdown pin if the pin is left open. When low shutdown current is not an issue, the error due to this current can be minimized by making RLO 10k or less. If shutdown current is an issue, RLO can be raised to 100k, but the error due to initial bias current and changes with temperature should be considered.
R LO = 10k to 100k 25k suggested R HI = RLO VIN - 2.42V
(
(
2.42V - R LO 3.5A
(
)
)
)
VIN = Minimum input voltage Keep the connections from the resistors to the shutdown pin short and make sure that interplane or surface capacitance to the switching nodes are minimized. If high resistor values are used, the shutdown pin should be bypassed with a 1000pF capacitor to prevent coupling problems from the switch node. If hysteresis is desired in the undervoltage lockout point, a resistor RFB can be added to the output node. Resistor values can be calculated from:
R HI =
RLO VIN - 2.42 V / VOUT + 1 + V
[
(
R FB = RHI VOUT / V
( )(
2.42 - RLO 3.5A
)
(
)
)
]
25k suggested for RLO VIN = Input voltage at which switching stops as input voltage descends to trip level V = Hysteresis in input voltage level Example: output voltage is 5V, switching is to stop if input voltage drops below 12V and should not restart unless
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input rises back to 13.5V. V is therefore 1.5V and VIN = 12V. Let RLO = 25k.
R HI =
2.33 R FB = 111k 5 / 1.5 = 370 k
) 2.42 - 25k (3.5A ) 25k(10.35) = =111k ( )
25k 12 - 2.42 1.5 / 5 + 1 + 1.5
[
(
]
SWITCH NODE CONSIDERATIONS For maximum efficiency, switch rise and fall times are made as short as possible. To prevent radiated EMI and high frequency resonance problems, proper layout of the components connected to the switch node is essential. B field (magnetic) radiation is minimized by keeping catch diode, switch pin, and input bypass capacitor leads as short as possible. E field radiation is kept low by minimizing the length and area of all traces connected to the switch pin and BOOST pin. A ground plane should always be used under the switcher circuitry to prevent interplane coupling. A suggested layout for the critical components is shown in Figure 5. Note that the feedback resistors and compensation components are kept as far as possible from the switch node. Also note that the high current ground path of the catch diode and input capacitor are kept very short and separate from the analog ground line. The high speed switching current path is shown schematically in Figure 6. Minimum lead length in this path is essential to ensure clean switching and low EMI. The path including the switch, catch diode, and input capacitor is the only one containing nanosecond rise and fall times. If you follow this path on the PC layout, you will see that it is irreducibly short. If you move the diode or input capacitor away from the LT1578, get your resume in order. The other paths contain only some combination of DC and 200kHz triwave, so are much less critical.
15
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
CONNECT OUTPUT CAPACITOR DIRECTLY TO HEAVY GROUND TAKE OUTPUT DIRECTLY FROM END OF OUTPUT CAPACITOR TO AVOID PARASITIC RESISTANCE AND INDUCTANCE (KELVIN CONNECTION)
C1 MINIMIZE AREA OF CONNECTIONS TO SWITCH NODE AND BOOST NODE VOUT L1 D2 MINIMIZE SIZE OF FEEDBACK PIN CONNECTIONS TO AVOID PICKUP SYNC VIN BOOST R1 SHDN FB VC R2 CC RC TERMINATE FEEDBACK RESISTORS AND COMPENSATION COMPONENTS DIRECTLY TO SWITCHER GROUND PIN
C2 SW KEEP INPUT CAPACITOR AND CATCH DIODE CLOSE TO REGULATOR AND TERMINATE THEM TO THE SAME POINT D1 C3
GND
GROUND RING NEED NOT BE AS SHOWN (NORMALLY EXISTS AS INTERNAL PLANE)
1578 F05
Figure 5. Suggested Layout for LT1578
VIN
Figure 6. High Speed Switching Path
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GND
SWITCH NODE L1 5V
HIGH FREQUENCY CIRCULATING PATH
LOAD
1578 F06
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
PARASITIC RESONANCE Resonance or "ringing" may sometimes be seen on the switch node (see Figure 7). Very high frequency ringing following the switch voltage rise time is caused by switch/ diode/input capacitance lead inductance and diode capacitance. Schottky diodes have very high "Q" junction capacitance that can ring for many cycles when excited at high frequency. If total lead length for the input capacitor, diode and switch path is 1 inch, the inductance will be approximately 25nH. At switch off, this will produce a spike across the NPN output device in addition to the input voltage. At higher currents this spike can be in the order of 10V to 20V or higher with a poor layout, potentially exceeding the absolute max switch voltage. The path around switch, catch diode and input capacitor must be kept as short as possible to ensure reliable operation. When looking at this, a >100MHz oscilloscope must be used, and waveforms should be observed on the leads of the package. This switch off spike will also cause the SW node to go below ground. The LT1578 has special circuitry inside which mitigates this problem, but negative voltages over 1V lasting longer than 10ns should be avoided. Note that 100MHz oscilloscopes are barely fast enough to see the details of the falling edge overshoot in Figure 7. A second, much lower frequency ringing is seen during switch off time if load current is low enough to allow the inductor current to fall to zero during part of the switch off time (see Figure 8). Switch and diode capacitance resonate with the inductor to form damped ringing at 1MHz to 10 MHz. This ringing is not harmful to the regulator and it has not been shown to contribute significantly to EMI. Any attempt to damp it with an RC snubber will slightly degrade efficiency. INPUT BYPASSING AND VOLTAGE RANGE
RISE AND FALL WAVEFORMS ARE SUPERIMPOSED (PULSE WIDTH IS NOT 350ns)
5V/DIV
50ns/DIV
1578 F07
Figure 7. Switch Node Response
5V/DIV SWITCH NODE VOLTAGE
50mA/DIV
INDUCTOR CURRENT
1s/DIV
1578 F08
Figure 8. Discontinuous Mode Ringing
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Input Bypass Capacitor Step-down converters draw current from the input supply in pulses. The average height of these pulses is equal to load current, and the duty cycle is equal to VOUT/ VIN. Rise and fall times of the current are very fast. A local bypass capacitor across the input supply is necessary to ensure proper operation of the regulator and minimize the ripple current fed back into the input supply. The capacitor also forces switching current to flow in a tight local loop, minimizing EMI.
Do not cheat on the ripple current rating of the input bypass capacitor, but also do not be overly concerned with the value in microfarads. The input capacitor is intended to absorb all the switching current ripple, which can have an RMS value as high as one half of the load current. Ripple current ratings on the capacitor must be observed to ensure reliable operation. In many cases it is necessary to parallel two capacitors to obtain the required ripple rating. Both capacitors must be of the same value and manufacturer to guarantee power sharing. The actual value of the capacitor in microfarads is not particularly important
17
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
because at 200kHz, any value above 15F is essentially resistive. RMS ripple current rating is the critical parameter. Actual RMS current can be calculated from: series for instance, see Table 3), but even these units may fail if the input voltage surge approaches the maximum voltage rating of the capacitor. AVX recommends derating capacitor voltage by 2:1 for high surge applications. The highest voltage rating is 50V, so 25V may be a practical input voltage upper limit when using solid tantalum capacitors for input bypassing. Larger capacitors may be necessary when the input voltage is very close to the minimum specified on the data sheet. Small voltage dips during switch on time are not normally a problem, but at very low input voltage they may cause erratic operation because the input voltage drops below the minimum specification. Problems can also occur if the input-to-output voltage differential is near minimum. The amplitude of these dips is normally a function of capacitor ESR and ESL because the capacitive reactance is small compared to these terms. ESR tends to be the dominate term and is inversely related to physical capacitor size within a given capacitor type. SYNCHRONIZING The SYNC pin is used to synchronize the internal oscillator to an external signal. The SYNC input must pass from a logic level low, through the maximum synchronization threshold with a duty cycle between 10% and 90%. The input can be driven directly from a logic level output. The synchronizing range is equal to initial operating frequency up to 400kHz. This means that minimum practical sync frequency is equal to the worst-case high self-oscillating frequency (250kHz), not the typical operating frequency of 200kHz. Caution should be used when synchronizing above 280kHz because at higher sync frequencies the amplitude of the internal slope compensation used to prevent subharmonic switching is reduced. This type of subharmonic switching only occurs at input voltages less than twice output voltage. Higher inductor values will tend to eliminate this problem. See Frequency Compensation section for a discussion of an entirely different cause of subharmonic switching before assuming that the cause is insufficient slope compensation. Application Note 19 has more details on the theory of slope compensation.
IRIPPLE RMS = IOUT VOUT VIN - VOUT / VIN
()
(
)
The term inside the radical has a maximum value of 0.5 when input voltage is twice output, and stays near 0.5 for a relatively wide range of input voltages. It is common practice therefore to simply use the worst-case value and assume that RMS ripple current is one half of load current. At maximum output current of 1.5A for the LT1578, the input bypass capacitor should be rated at 0.75A ripple current. Note however, that there are many secondary considerations in choosing the final ripple current rating. These include ambient temperature, average versus peak load current, equipment operating schedule, and required product lifetime. For more details, see Application Notes 19 and 46, and Design Note 95. Input Capacitor Type Some caution must be used when selecting the type of capacitor used at the input to regulators. Aluminum electrolytics are lowest cost, but are physically large to achieve adequate ripple current rating, and size constraints (especially height) may preclude their use. Ceramic capacitors are now available in larger values, and their high ripple current and voltage rating make them ideal for input bypassing. Cost is fairly high and footprint may also be somewhat large. Solid tantalum capacitors would be a good choice, except that they have a history of occasional spectacular failures when they are subjected to large current surges during power-up. The capacitors can short and then burn with a brilliant white light and lots of nasty smoke. This phenomenon occurs in only a small percentage of units, but it has led some OEMs to forbid their use in high surge applications. The input bypass capacitors of regulators can see these high surges when a battery or high capacitance source is connected. Several manufacturers have developed a line of solid tantalum capacitors specially tested for surge capability (AVX TPS
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LT1578/LT1578-2.5
APPLICATIONS INFORMATION
At power-up, when VC is being clamped by the FB pin (see Figure 2, Q2), the sync function is disabled. This allows the frequency foldback to operate in the shorted output condition. During normal operation, switching frequency is controlled by the internal oscillator until the FB pin reaches 0.7V, after which the SYNC pin becomes operational. If no synchronization is required, this pin should be connected to ground. THERMAL CALCULATIONS Power dissipation in the LT1578 chip comes from four sources: switch DC loss, switch AC loss, boost circuit current, and input quiescent current. The following formulas show how to calculate each of these losses. These formulas assume continuous mode operation, so they should not be used for calculating efficiency at light load currents. Switch loss:
PSW =
RSW IOUT
( ) (VOUT) + 60ns(IOUT)(VIN)(f)
2
VIN
Boost current loss:
PBOOST = VOUT IOUT / 50 VIN
2
(
)
Quiescent current loss:
PQ = VIN 0.55 * 10-3 + VOUT 1.6 * 10-3 2 VOUT (0.004 + VIN
)
RSW = Switch resistance ( 0.2) 60ns = Equivalent switch current/voltage overlap time f = Switch frequency Example: with VIN = 10V, VOUT = 5V and IOUT = 1A:
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(0.2)(1) (5) + 60 * 10-9 (1)(10)2 00 * 10 3 PSW =
2
10 = 0.1 + 012 = 0.22W .
2
(5) (1/ 50) = 0.05W PBOOST = (5) (0.004) PQ = 10 0.55 * 10-3 + 5 1.6 * 10-3 +
2
10

10
= 0.02W
Total power dissipation is 0.22 + 0.05 + 0.02 = 0.29W. Thermal resistance for LT1578 package is influenced by the presence of internal or backside planes. With a full plane under the SO package, thermal resistance will be about 80C/W. No plane will increase resistance to about 120C/W. To calculate die temperature, add in worst-case ambient temperature: TJ = TA + JA (PTOT) With the SO-8 package (JA = 80C/W), at an ambient temperature of 50C, TJ = 50 + 80 (0.29) = 73.2C Die temperature is highest at low input voltage, so use lowest continuous input operating voltage for thermal calculations. FREQUENCY COMPENSATION Loop frequency compensation of switching regulators can be a rather complicated problem because the reactive components used to achieve high efficiency also introduce multiple poles into the feedback loop. The inductor and output capacitor on a conventional step-down converter actually form a resonant tank circuit that can exhibit peaking and a rapid 180 phase shift at the resonant frequency. By contrast, the LT1578 uses a "current mode" architecture to help alleviate the phase shift created by the inductor. The basic connections are shown in Figure 9. Figure 10 shows a Bode plot of the phase and gain of the power section of the LT1578, measured from the VC pin to
19
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
the output. Gain is set by the 1.5A/V transconductance of the LT1578 power section and the effective complex impedance from output to ground. Gain rolls off smoothly above the 160Hz pole frequency set by the 100F output capacitor. Phase drop is limited to about 85. Phase recovers and gain levels off at the zero frequency (16kHz) set by capacitor ESR (0.1). Error amplifier transconductance phase and gain are shown in Figure 11. The error amplifier can be modeled as a transconductance of 1000Mho, with an output impedance of 570k in parallel with 2.4pF. In all practical applications, the compensation network from the VC pin to ground has a much lower impedance than the output impedance of the amplifier at frequencies above 200Hz.
LT1578 CURRENT MODE POWER STAGE gm = 1.5A/V VSW ERROR AMPLIFIER FB ESR R1 GAIN (Mho) OUTPUT
1.21V
GND
VC
+
C1 R2
CF
RC CC
Figure 9. Model for Loop Response
40 VIN = 10V VOUT = 5V IOUT = 500mA 20 GAIN
PHASE (DEG) GAIN (dB)
40
0 LOOP GAIN (dB)
PHASE 0 -40
-20
-80
-40 10 100 1k 10k FREQUENCY (Hz)
-120 100k
1578 F07
Figure 10. Response from VC Pin to Output
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This means that the error amplifier characteristics themselves do not contribute excess phase shift to the loop, and the phase/gain characteristics of the error amplifier section are completely controlled by the external compensation network. In Figure 12, full loop phase/gain characteristics are shown with a compensation capacitor of 100pF, giving the error amplifier a pole at 2.8kHz, with phase rolling off to 90 and staying there. The overall loop has a gain of 66dB at low frequency, rolling off to unity-gain at 58kHz. The phase plot shows a two-pole characteristic until the ESR of the output capacitor brings it back to single pole above 16kHz. Phase margin is about 77 at unity-gain.
2000 PHASE 1500 GAIN
200
150
PHASE (DEG)
1000
100
500
VFB 1 x 10-3
(
)
ROUT 570k
VC COUT 2.4pF
50
0
ERROR AMPLIFIER EQUIVALENT CIRCUIT
0
RLOAD = 50 -500 10 100 1k 10k FREQUENCY (Hz) 100k -50 1M
1578 F11
1578 F09
Figure 11. Error Amplifier Gain and Phase
80 180
60 PHASE
135
LOOP PHASE (DEG)
40 VIN = 10V VOUT = 5V IOUT = 500mA COUT = 100F 10V, AVX TPS CC = 100pF L = 30H 10 100 1k 10k FREQUENCY (Hz)
90
20
45 GAIN 0
0
-20 100k
-45 1M
1578 F12
Figure 12. Overall Loop Characteristics
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
Analog experts will note that around 7kHz, phase dips close to the zero phase margin line. This is typical of switching regulators, especially those that operate over a wide range of loads. This region of low phase is not a problem as long as it does not occur near unity-gain. In practice, the variability of output capacitor ESR tends to dominate all other effects with respect to loop response. Variations in ESR will cause unity-gain to move around, but at the same time phase moves with it so that adequate phase margin is maintained over a very wide range of ESR ( 3:1). What About a Resistor in the Compensation Network? It is common practice in switching regulator design to add a "zero" to the error amplifier compensation to increase loop phase margin. This zero is created in the external network in the form of a resistor (RC) in series with the compensation capacitor. Increasing the size of this resistor generally creates better and better loop stability, but there are two limitations on its value. First, the combination of output capacitor ESR and a large value for RC may cause loop gain to stop rolling off altogether, creating a gain margin problem. An approximate formula for RC where gain margin falls to zero is:
R C Loop Gain = 1 =
(
) (GMP)(GMA)(ESR)(1.21)
VOUT
GMP = Transconductance of power stage = 1.5A/V GMA = Error amplifier transconductance = 1(10-3) ESR = Output capacitor ESR 1.21 = Reference voltage With VOUT = 5V and ESR = 0.1, a value of 27.5k for RC would yield zero gain margin, so this represents an upper limit. There is a second limitation however which has nothing to do with theoretical small signal dynamics. This resistor sets high frequency gain of the error amplifier, including the gain at the switching frequency. If the switching frequency gain is high enough, an excessive amout of output ripple voltage will appear at the VC pin resulting in improper operation of the regulator. In a marginal case, subharmonic switching occurs, as
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evidenced by alternating pulse widths seen at the switch node. In more severe cases, the regulator squeals or hisses audibly even though the output voltage is still roughly correct. None of this will show on a Bode plot since this is an amplitude insensitive measurement. Tests have shown that if ripple voltage on the VC is held to less than 100mVP-P, the LT1578 will generally be well behaved. The formula below will give an estimate of VC ripple voltage when RC is added to the loop, assuming that RC is large compared to the reactance of CC at 200kHz. VC RIPPLE =
(
(RC)(GMA)(VIN - VOUT)(ESR)(1.21) ) (VIN)(L)(f)
GMA = Error amplifier transconductance (1000Mho) If a series compensation resistor of 15k gave the best overall loop response, with adequate gain margin, the resulting VC pin ripple voltage with VIN = 10V, VOUT = 5V, ESR = 0.1, L = 30H, would be:
(15k)(1* 10-3 )(10 - 5)(0.1)(1.21) = 0.151V VC(RIPPLE ) = (10)(30 * 10-6 )(200 * 103 )
This ripple voltage is high enough to possibly create subharmonic switching. In most situations a compromise value (< 10k in this case) for the resistor gives acceptable phase margin and no subharmonic problems. In other cases, the resistor may have to be larger to get acceptable phase response, and some means must be used to control ripple voltage at the VC pin. The suggested way to do this is to add a capacitor (CF) in parallel with the RC /CC network on the VC pin. The pole frequency for this capacitor is typically set at one-fifth of the switching frequency so that it provides significant attenuation of the switching ripple, but does not add unacceptable phase shift at the loop unity-gain frequency. With RC = 15k,
CF =
(2)(f)(RC )
5
=
2 200 * 103 15k
(
5
)( )
= 265pF
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LT1578/LT1578-2.5
APPLICATIONS INFORMATION
How Do I Test Loop Stability? The "standard" compensation for LT1578 is a 100pF capacitor for CC, with RC = 0. While this compensation will work for most applications, the "optimum" value for loop compensation components depends, to various extents, on parameters which are not well controlled. These include inductor value (30% due to production tolerance, load current and ripple current variations), output capacitance (20% to 50% due to production tolerance, temperature, aging and changes at the load), output capacitor ESR (200% due to production tolerance, temperature and aging), and finally, DC input voltage and output load current . This makes it important for the designer to check out the final design to ensure that it is "robust" and tolerant of all these variations. One way to check switching regulator loop stability is by pulse loading the regulator output while observing the transient response at the output, using the circuit shown in Figure 13. The regulator loop is "hit" with a small transient AC load current at a relatively low frequency, 50Hz to 1kHz. This causes the output to jump a few millivolts, then settle back to the original value, as shown in Figure 14. A well behaved loop will settle back cleanly, whereas a loop with poor phase or gain margin will "ring" as it settles. The number of rings indicates the degree of stability, and the frequency of the ringing shows the approximate unity-gain frequency of the loop. Amplitude of the signal is not particularly important, as long as the amplitude is not so high that the loop behaves nonlinearly.
SWITCHING REGULATOR
ADJUSTABLE INPUT SUPPLY
ADJUSTABLE DC LOAD
Figure 13. Loop Stability Test Circuit
10mV/DIV
5A/DIV 0.2ms/DIV
Figure 14. Loop Stability Check
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RIPPLE FILTER 470 4.7k 330pF TO X1 OSCILLOSCOPE PROBE
+
100F TO 1000F 50
3300pF
TO OSCILLOSCOPE SYNC 100Hz TO 1kHz 100mV TO 1VP-P
1578 F13
VOUT AT IOUT = 500mA BEFORE FILTER VOUT AT IOUT = 500mA AFTER FILTER VOUT AT IOUT = 50mA AFTER FILTER LOAD PULSE THROUGH 50 f 780Hz
1578 F14
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
The output of the regulator contains both the desired low frequency transient information and a reasonable amount of high frequency (200kHz) ripple. The ripple makes it difficult to observe the small transient, so a two-pole, 100kHz filter has been added. This filter is not particularly critical; even if it attenuated the transient signal slightly, this wouldn't matter because amplitude is not critical. After verifying that the setup is working correctly, start varying load current and input voltage to see if you can find any combination that makes the transient response look suspiciously "ringy." This procedure may lead to an adjustment for best loop stability or faster loop transient response. Nearly always you will find that loop response looks better if you add in several k for RC. Do this only if necessary, because as explained before, RC above 1k may require the addition of CF to control VC pin ripple. If everything looks OK, use a heat gun and cold spray on the circuit (especially the output capacitor) to bring out any temperature-dependent characteristics. Keep in mind that this procedure does not take initial component tolerance into account. You should see fairly clean response under all load and line conditions to ensure that component variations will not cause problems. One note here: according to Murphy, the component most likely to be changed in production is the output capacitor, because that is the component most likely to have manufacturer variations (in ESR) large enough to cause problems. It would be a wise move to lock down the sources of the output capacitor in production. Also, try varying component values by a factor of 2 and see if the behavior is still acceptable. Double and halve the values of RC and CC and output capacitors. If the regulator still works correctly, it will likely be good in production. A possible exception to the "clean response" rule is at very light loads, as evidenced in Figure 14 with ILOAD = 50mA. Switching regulators tend to have dramatic shifts in loop response at very light loads, mostly because the inductor current becomes discontinuous. One common result is very slow but stable characteristics. A second possibility is low phase margin, as evidenced by ringing at the output with transients. The good news is that the low phase margin at light loads is not particularly sensitive to component variation, so if it looks reasonable under a transient test, it will probably not be a problem in production. Note that frequency of the light load ringing may vary with component tolerance but phase margin generally hangs in there. POSITIVE-TO-NEGATIVE CONVERTER The circuit in Figure 15 is a classic positive-to-negative topology using a grounded inductor. It differs from the standard approach in the way the IC chip derives its feedback signal. Because the LT1578 accepts only positive feedback signals, the ground pin must be tied to the regulated negative output. A resistor divider to ground or, in this case, the sense pin, then provides the proper feedback voltage for the chip.
D1 1N4148 C2 L1* 0.33F 15H VSW R1 15.8k FB GND VC CC RC R2 4.99k D2 1N5818
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INPUT 5.5V TO 15V C3 10F TO 50F
BOOST VIN LT1578
+
+
C1 100F 10V TANT x2 OUTPUT** - 5V, 0.5A
* INCREASE L1 TO 30H OR 60H FOR HIGHER CURRENT APPLICATIONS. SEE APPLICATIONS INFORMATION ** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
1578 F15
Figure 15. Positive-to-Negative Converter
Inverting regulators differ from buck regulators in the basic switching network. Current is delivered to the output as square waves with a peak-to-peak amplitude much greater than load current. This means that maximum load current will be significantly less than the LT1578's 1.5A maximum switch current, even with large inductor values. The buck converter in comparison, delivers current to the output as a triangular wave superimposed on a DC level equal to load current, and load current can approach 1.5A
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LT1578/LT1578-2.5
APPLICATIONS INFORMATION
with large inductors. Output ripple voltage for the positiveto-negative converter will be much higher than a buck converter. Ripple current in the output capacitor will also be much higher. The following equations can be used to calculate operating conditions for the positive-to-negative converter. Maximum load current: VIN VOUT VOUT VIN - 0.35 IP - 2 VOUT + VIN f L IMAX = VOUT + VIN - 0.35 VOUT + VF This duty cycle is close enough to 50% that IP can be assumed to be 1.5A. OUTPUT DIVIDER If the adjustable part is used, the resistor connected to VOUT (R2) should be set to approximately 5k. R1 is calculated from:
( (
( )(
)( )( )( ) )(
)(
IP = Maximum rated switch current VIN = Minimum input voltage VOUT = Output voltage VF = Catch diode forward voltage 0.35 = Switch voltage drop at 1.5A Example: with VIN(MIN) = 5.5V, VOUT = 5V, L = 30H, VF = 0.5V, IP = 1.5A: IMAX = 0.6A. Note that this equation does not take into account that maximum rated switch current (IP) on the LT1578 is reduced slightly for duty cycles above 50%. If duty cycle is expected to exceed 50% (input voltage less than output voltage), use the actual IP value from the Electrical Characteristics table. Operating duty cycle:
OUTPUT RIPPLE VOLTAGE (mVP-P)
DC =
VOUT + VF VIN - 0.3 + VOUT + VF
(This formula uses an average value for switch loss, so it may be several percent in error.) With the conditions above: DC = 5 + 0.5 = 51% 5.5 - 0.3 + 5 + 0.5
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R1 =
R2 VOUT - 1.21 1.21
(
)
)
INDUCTOR VALUE Unlike buck converters, positive-to-negative converters cannot use large inductor values to reduce output ripple voltage. At 200kHz, values larger than 75H make almost no change in output ripple. The graph in Figure 16 shows peak-to-peak output ripple voltage for a 5V to - 5V converter versus inductor value. The criteria for choosing the
150 5V TO -5V CONVERTER OUTPUT CAPACITOR'S ESR = 0.1 DISCONTINUOUS ILOAD = 0.1A DISCONTINUOUS ILOAD = 0.25A
120
90
60
30 CONTINUOUS ILOAD > 0.38A 0 0 15 45 60 30 INDUCTOR SIZE (H) 75
1578 F16
Figure 16. Ripple Voltage on Positive-to-Negative Converter
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
inductor is therefore typically based on ensuring that peak switch current rating is not exceeded. This gives the lowest value of inductance that can be used, but in some cases (lower output load currents) it may give a value that creates unnecessarily high output ripple voltage. A compromise value is often chosen that reduces output ripple. As you can see from the graph, large inductors will not give arbitrarily low ripple, but small inductors can give high ripple. The difficulty in calculating the minimum inductor size needed is that you must first know whether the switcher will be in continuous or discontinuous mode at the critical point where switch current is 1.5A. The first step is to use the following formula to calculate the load current where the switcher must use continuous mode. If your load current is less than this, use the discontinuous mode formula to calculate the minimum inductor value needed. If the load current is higher, use the continuous mode formula. Output current where continuous mode is needed: ICONT = For the example above, with maximum load current of 0.25A: ICONT =
(V ) (I ) 4(V + V )(V + V
2 2 IN P IN OUT IN
OUT + VF
)
Minimum inductor discontinuous mode: L MIN = 2 VOUT IOUT
2 P
( )( ) (f)(I ) (V )(V )
IN OUT
Minimum inductor continuous mode: L MIN =
VOUT + VF 2 f VIN + VOUT IP - IOUT 1 + VIN
( )(
)
(
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(5.5) (1.5) = 0.38A 4(5.5 + 5)(5.5 + 5 + 0.5)
2 2
This says that discontinuous mode can be used and the minimum inductor needed is found from:
L MIN =
( )( ) = 5.6H 200 * 103 1.5 2 ( )
2 5 0.25
In practice, the inductor should be increased by about 30% over the calculated minimum to handle losses and variations in value. This suggests a minimum inductor of 7.3H for this application, but looking at the ripple voltage chart shows that output ripple voltage could be reduced by a factor of two by using a 30H inductor. There is no rule of thumb here to make a final decision. If modest ripple is needed and the larger inductor does the trick, this is probably the best solution. If ripple is noncritical use the smaller inductor. If ripple is extremely critical, a second stage filter may have to be added in any case, and the lower value of inductance can be used. Keep in mind that the output capacitor is the other critical factor in determining output ripple voltage. Ripple shown on the graph (Figure 16) is with a capacitor's ESR of 0.1. This is reasonable for AVX type TPS "D" or "E" size surface mount solid tantalum capacitors, but the final capacitor chosen must be looked at carefully for ESR characteristics.
)

25
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
Ripple Current in the Input and Output Capacitors Positive-to-negative converters have high ripple current in both the input and output capacitors. For long capacitor lifetime, the RMS value of this current must be less than the high frequency ripple current rating of the capacitor. The following formula will give an approximate value for RMS ripple current. This formula assumes continuous conduction mode and a large inductor value. Small inductors will give somewhat higher ripple current, especially in discontinuous mode. The exact formulas are very complex and appear in Application Note 44, pages 30 and 31. For our purposes here, a simple fudge factor (ff) is added. The value for ff is about 1.2 for load currents above 0.38A (in continuous conduction mode) and L 10H. It increases to about 2.0 for smaller inductors at lower load currents (in discontinuous conduction mode). Capacitor IRMS = ff IOUT ff = Fudge factor (1.2 to 2.0) Diode Current
( )( )
VOUT VIN
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Average diode current is equal to load current. Peak diode current will be considerably higher.
Peak diode current: Continuous Mode = IOUT
(VIN + VOUT ) + (VIN)(VOUT ) VIN 2(L)( f)( VIN + VOUT )
2 IOUT VOUT
Discontinuous Mode =
( )( ) (L)(f)
Keep in mind that during start-up and output overloads, the average diode current may be much higher than with normal loads. Care should be used if diodes rated less than 1A are used, especially if continuous overload conditions must be tolerated.
LT1578/LT1578-2.5
PACKAGE DESCRIPTION
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP
0.014 - 0.019 (0.355 - 0.483) TYP *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
0.016 - 0.050 (0.406 - 1.270)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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Dimensions in inches (millimeters) unless otherwise noted.
S8 Package 8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 - 0.197* (4.801 - 5.004) 8 7 6 5
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
1
2
3
4
0.053 - 0.069 (1.346 - 1.752)
0.004 - 0.010 (0.101 - 0.254)
0.050 (1.270) BSC
SO8 1298
27
LT1578/LT1578-2.5
TYPICAL APPLICATION
Dual Output SEPIC Converter The circuit in Figure 17 generates both positive and negative 5V outputs with a single piece of magnetics. The inductor L1 is a 33H surface mount inductor from Coiltronics. It is manufactured with two identical windings that can be connected in series or parallel. The topology for the 5V output is a standard buck converter. The - 5V topology would be a simple flyback winding coupled to the buck converter if C4 were not present. C4 creates the SEPIC (Single-Ended Primary Inductance Converter) topology which improves regulation and reduces ripple current in L1. Without C4, the voltage swing on L1B compared to L1A would vary due to relative loading and coupling losses. C4 provides a low impedance path to maintain an equal voltage swing in L1B, improving regulation. In a flyback converter, during switch on time, all the converter's energy is stored in L1A only, since no current flows in L1B. At switch off, energy is transferred by magnetic coupling into L1B, powering the - 5V rail. C4 pulls L1B positive during switch on time, causing current to flow, and energy to build in L1B and C4. At switch off, the energy stored in both L1B and C4 supply the - 5V rail. This reduces the current in L1A and changes L1B current waveform from square to triangular. For details on this circuit see Design Note 100.
INPUT 6V TO 15V
GND * L1 IS A SINGLE CORE WITH TWO WINDINGS COILTRONICS CTX33-2 ** AVX TSPD107M010 IF LOAD CAN GO TO ZERO, AN OPTIONAL PRELOAD OF 1k TO 5k MAY BE USED TO IMPROVE LOAD REGULATION C4** 100F
RELATED PARTS
PART NUMBER LT1074/LT1076 LTC1174 LT1370 LT1371 LT1372/LT1377 LT1376 LT1507 LT1676/LT1776 LTC1772 LTC1735 LT1777 DESCRIPTION Step-Down Switching Regulators High Efficiency Step-Down and Inverting DC/DC Converter High Efficiency DC/DC Converter High Efficiency DC/DC Converter High Efficiency Step-Down Switching Regulator High Efficiency Step-Down Switching Regulator High Efficiency Step-Down Switching Regulators SOT-23 Low Voltage Step-Down DC/DC Controller High Efficiency Step-Down Converter Low Noise Step-Down Switching Regulator COMMENTS 40V Input, 100kHz, 5A and 2A 0.5A, 150kHz Burst ModeTM Operation 42V, 6A, 500kHz Switch 35V, 3A, 500kHz Switch 25V, 1.5A, 500kHz Switch 15V, 1.5A, 500kHz Switch 7.4V to 60V Input, 100kHz/200kHz 550kHz, Drives PFET, 6-Lead SOT-23 Package; up to 4.5A Output Current Synchronous Buck Controller Drives External MOSFETs 48V Input, Internally Limited dV/dt, Programmable di/dt
500kHz and 1MHz High Efficiency 1.5A Switching Regulators Boost Topology
Burst Mode is a trademark of Linear Technology Corporation.
28
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
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VIN
BOOST LT1578
C2 0.33F VSW
D2 1N914 L1A* 33H
OUTPUT 5V R1 15.8k
SHDN GND C3 22F 35V TANT
FB VC CC 100pF
+
D1 1N5818 R2 4.99k
C1** 100F 10V TANT
+
L1B* D3 1N5818
+
C5** 100F 10V TANT OUTPUT -5V
1578 F17
Figure 17. Dual Output SEPIC Converter
1578f LT/TP 0100 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1999

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