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LTC1624 High Efficiency SO-8 N-Channel Switching Regulator Controller
FEATURES
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DESCRIPTION
The LTC(R)1624 is a current mode switching regulator controller that drives an external N-channel power MOSFET using a fixed frequency architecture. It can be operated in all standard switching configurations including boost, step-down, inverting and SEPIC. Burst ModeTM operation provides high efficiency at low load currents. A maximum high duty cycle limit of 95% provides low dropout operation which extends operating time in battery-operated systems. The operating frequency is internally set to 200kHz, allowing small inductor values and minimizing PC board space. The operating current level is user-programmable via an external current sense resistor. Wide input supply range allows operation from 3.5V to 36V (absolute maximum). A multifunction pin (I TH / RUN) allows external compensation for optimum load step response plus shutdown. Soft start can also be implemented with the ITH /RUN pin to properly sequence supplies.
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N-Channel MOSFET Drive Implements Boost, Step-Down, SEPIC and Inverting Regulators Wide VIN Range: 3.5V to 36V Operation Wide VOUT Range: 1.19V to 30V in Step-Down Configuration 1% 1.19V Reference Low Dropout Operation: 95% Duty Cycle 200kHz Fixed Frequency Low Standby Current Very High Efficiency Remote Output Voltage Sense Logic-Controlled Micropower Shutdown Internal Diode for Bootstrapped Gate Drive Current Mode Operation for Excellent Line and Load Transient Response Available in an 8-Lead SO Package
APPLICATIONS
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Notebook and Palmtop Computers, PDAs Cellular Telephones and Wireless Modems Battery-Operated Digital Devices DC Power Distribution Systems Battery Chargers
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
1000pF SENSE - ITH /RUN VFB 100pF GND SW VIN BOOST TG
VIN 4.8V TO 28V
RSENSE 0.05 M1 Si4412DY CB 0.1F D1 MBRS340T3 L1 10H
CC 470pF RC 6.8k
LTC1624
R2 35.7k R1 20k
Figure 1. High Efficiency Step-Down Converter
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CIN 22F 35V x2
VOUT 3.3V 2A
+
COUT 100F 10V x2
1624 F01
1
LTC1624
ABSOLUTE MAXIMUM RATINGS
Input Supply Voltage (VIN)......................... 36V to - 0.3V Topside Driver Supply Voltage (BOOST)....42V to - 0.3V Switch Voltage (SW).................................. 36V to - 0.6V Differential Boost Voltage (BOOST to SW) ....................................7.8V to - 0.3V SENSE - Voltage VIN < 15V .................................. (VIN + 0.3V) to - 0.3V VIN 15V .......................... (VIN +0.3V) to (VIN - 15V) ITH/RUN, VFB Voltages ............................ 2.7V to - 0.3V Peak Driver Output Current < 10s (TG) .................... 2A Operating Temperature Range LTC1624CS ............................................ 0C to 70C LTC1624IS ......................................... - 40C to 85C Junction Temperature (Note 1)............................. 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
PACKAGE/ORDER INFORMATION
TOP VIEW SENSE - 1 ITH /RUN 2 VFB 3 GND 4 8 7 6 5 VIN BOOST TG SW
ORDER PART NUMBER LTC1624CS8 LTC1624IS8 S8 PART MARKING 1624 1624I
S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 125C, JA = 110C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL IIN VFB VFB VLINE REG VLOAD REG PARAMETER Feedback Current Feedback Voltage Reference Voltage Line Regulation Output Voltage Load Regulation Main Control Loop
TA = 25C, VIN = 15V, unless otherwise noted.
MIN TYP 10
q
CONDITIONS (Note 2) (Note 2) VIN = 3.6V to 20V (Note 2) (Note 2) ITH Sinking 5A ITH Sourcing 5A (Note 3) VITH/RUN = 0V
q q
MAX 50 1.2019 0.01 0.8 - 0.8 1.32 900 30 - 5.0 - 350 185 150 150 225 5.5 5
UNITS nA V %/V % % V A A V A A mV ns ns kHz V %
1.1781
1.19 0.002 0.5 - 0.5
VOVL IQ
Output Overvoltage Lockout Input DC Supply Current Normal Mode Shutdown Run Threshold Run Current Source Run Pullup Current TG Transition Time Rise Time Fall Time Oscillator Frequency Boost Voltage Boost Load Regulation SW = 0V, IBOOST = 5mA, VIN = 8V SW = 0V, IBOOST = 2mA to 20mA VITH/RUN = 0.3V VITH/RUN = 1V VFB = 1.0V CLOAD = 3000pF CLOAD = 3000pF
q
1.24
1.28 550 16
VITH/RUN IITH/RUN
0.6 - 0.8 - 50 145
0.8 - 2.5 -160 160 50 50
VSENSE(MAX) Maximum Current Sense Threshold TG tr TG tf fOSC VBOOST VBOOST
175 4.8
200 5.15 3
The q denotes specifications which apply over the full operating temperature range. LTC1624CS: 0C TA 70C LTC1624IS: - 40C TA 85C Note 1: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula:
TJ = TA + (PD * 110C/W) Note 2: The LTC1624 is tested in a feedback loop which servos VFB to the midpoint for the error amplifier (VITH = 1.8V). Note 3: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information.
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LTC1624 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current VOUT = 3.3V
100 95 EFFICIENCY (%) 90 85 80 75 70 0.001 VOUT = 3.3V RSENSE = 0.033 VIN = 5V
EFFICIENCY (%)
ILOAD = 1A
85 ILOAD = 0.1A 80 75 70
EFFICIENCY (%)
VIN = 10V
0.01 0.1 1 LOAD CURRENT (A)
Efficiency vs Input Voltage VOUT = 5V
100 95 VOUT = 5V RSENSE = 0.033 ILOAD = 1A
0.7 0.6 0.5
SUPPLY CURRENT (A)
EFFICIENCY (%)
VIN - VOUT (V)
90 85 ILOAD = 0.1A 80 75 70
0
5
20 15 10 INPUT VOLTAGE (V)
Boost Line Regulation
6 5 6
BOOST VOLTAGE (V)
4 3 2 1 0
BOOST VOLTAGE (V)
4 3 2 1 VSW = 0V 0
BOOST VOLTAGE (V)
IBOOST = 1mA VSW = 0V 0 5 20 15 25 10 INPUT VOLTAGE (V) 30 35
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1624 G07
Efficiency vs Input Voltage VOUT = 3.3V
100 95 90 VOUT = 3.3V RSENSE = 0.033
100 95 90 85 80 75
Efficiency vs Load Current VOUT = 5V
VOUT = 5V VIN = 10V RSENSE = 0.033
0
5
20 15 10 INPUT VOLTAGE (V)
25
30
1624 G09
70 0.001
0.01 0.1 1 LOAD CURRENT (A)
10
1624 G08
VIN - VOUT Dropout Voltage vs Load Current
700
RSENSE = 0.033 VOUT DROP OF 5%
Input Supply Current vs Input Voltage
VFB = 1.21V 600 500 400 300 200 100 SHUTDOWN 0 SLEEP MODE
0.4 0.3 0.2 0.1 0
25
30
1624 G10
0
0.5
1.0 1.5 2.0 LOAD CURRENT (A)
2.5
3.0
1624 G11
0
5
20 15 25 10 INPUT VOLTAGE (V)
30
35
1624 G05
Boost Load Regulation
6.0 VIN = 15V 5 VIN = 5V 5.5
Boost Voltage vs Temperature
ILOAD = 1mA
5.0
4.5
0
5
20 15 25 10 BOOST LOAD CURRENT (mA)
30
1624 G06
4.0 -40 -15
60 35 85 10 TEMPERATURE (C)
110
135
1624 G04
1624 G15
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LTC1624 TYPICAL PERFORMANCE CHARACTERISTICS
ITH/RUN PIN SOURCE CURRENT WITH VITH = 1V (A)
VITH vs Output Current
200
2.4
VITH /RUN (V)
IITH (A)
ACTIVE MODE 1.2 0.8 SHUTDOWN 0
0 IOUT
IOUT(MAX)
(a)
Frequency vs Feedback Voltage
250
250
200
FREQUENCY (kHz)
200
CURRENT SENSE THRESHOLD (mV)
FREQUENCY (kHz)
150
100
50
0 0 0.25 0.75 1.00 0.50 FEEDBACK VOLTAGE 1.25
1624 G03
PIN FUNCTIONS
SENSE - (Pin 1): Connects to the (-) input for the current comparator. Built-in offsets between the SENSE - and VIN pins in conjunction with RSENSE set the current trip thresholds. Do not pull this pin more than 15V below VIN or more than 0.3V below ground. ITH/RUN (Pin 2): Combination of Error Amplifier Compensation Point and Run Control Inputs. The current comparator threshold increases with this control voltage. Nominal voltage range for this pin is 1.19V to 2.4V. Forcing this pin below 0.8V causes the device to be shut down. In shutdown all functions are disabled and TG pin is held low. VFB (Pin 3): Receives the feedback voltage from an external resistive divider across the output. GND (Pin 4): Ground. Connect to the (-) terminal of COUT, the Schottky diode and the (-) terminal of CIN. SW (Pin 5): Switch Node Connection to Inductor. In stepdown applications the voltage swing at this pin is from a Schottky diode (external) voltage drop below ground to VIN.
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1624 G01
IITH vs VITH
ITH/RUN Pin Source Current vs Temperature
300 250 200 150 100 50 0 -40 -15 1 ITH /RUN = 1V 3 ITH /RUN = 0V 2 5
ITH/RUN PIN SOURCE CURRENT WITH VITH = 0V (A)
4
150
50 ACTIVE MODE 3 SHUTDOWN 0 0 0.8 1.2 VITH (V) 2.4
60 35 85 10 TEMPERATURE (C)
110
0 135
(b)
1624 G02
1624 G14
Operating Frequency vs Temperature
170 VOUT IN REGULATION 168 166 164 162 160 158 156 154 152
Maximum Current Sense Threshold vs Temperature
150
100
50
VFB = 0V
0 -40 -15
60 10 85 35 TEMPERATURE (C)
110
135
150 -40 -15
60 35 10 85 TEMPERATURE (C)
110
135
1448 G12
1448 G13
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LTC1624
PIN FUNCTIONS
TG (Pin 6): High Current Gate Drive for Top N-Channel MOSFET. This is the output of a floating driver with a voltage swing equal to INTVCC superimposed on the switch node voltage SW. BOOST (Pin 7): Supply to Topside Floating Driver. The bootstrap capacitor CB is returned to this pin. Voltage swing at this pin is from INTVCC to VIN + INTVCC in stepdown applications. In non step-down topologies the voltage at this pin is constant and equal to INTVCC if SW = 0V. VIN (Pin 8): Main Supply Pin and the (+) Input to the Current Comparator. Must be closely decoupled to ground.
OPERATIO
Main Control Loop The LTC1624 uses a constant frequency, current mode architecture. During normal operation, the top MOSFET is turned on each cycle when the oscillator sets the RS latch and turned off when the main current comparator I1 resets the RS latch. The peak inductor current at which I1 resets the RS latch is controlled by the voltage on the ITH /RUN pin, which is the output of error amplifier EA. The VFB pin, described in the pin functions, allows EA to receive an output feedback voltage from an external resistive divider. When the load current increases, it causes a slight decrease in VFB relative to the 1.19V reference, which in turn causes the ITH /RUN voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the internal bottom MOSFET is turned on for approximately 300ns to 400ns to recharge the bootstrap capacitor CB. The top MOSFET driver is biased from the floating bootstrap capacitor CB that is recharged during each off cycle. The dropout detector counts the number of oscillator cycles that the top MOSFET remains on and periodically forces a brief off period to allow CB to recharge. The main control loop is shut down by pulling the ITH /RUN pin below its 1.19V clamp voltage. Releasing ITH /RUN allows an internal 2.5A current source to charge compensation capacitor CC. When the ITH /RUN pin voltage reaches 0.8V the main control loop is enabled with the ITH / RUN voltage pulled up by the error amp. Soft start can be
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(Refer to Functional Diagram)
implemented by ramping the voltage on the ITH /RUN pin from 1.19V to its 2.4V maximum (see Applications Information section). Comparator OV guards against transient output overshoots >7.5% by turning off the top MOSFET and keeping it off until the fault is removed. Low Current Operation The LTC1624 is capable of Burst Mode operation in which the external MOSFET operates intermittently based on load demand. The transition to low current operation begins when comparator B detects when the ITH /RUN voltage is below 1.5V. If the voltage across RSENSE does not exceed the offset of I2 (approximately 20mV) for one full cycle, then on following cycles the top and internal bottom drives are disabled. This continues until the ITH voltage exceeds 1.5V, which causes drive to be returned to the TG pin on the next cycle. INTVCC Power/Boost Supply Power for the top and internal bottom MOSFET drivers is derived from VIN. An internal regulator supplies INTVCC power. To power the top driver in step-down applications an internal high voltage diode recharges the bootstrap capacitor CB during each off cycle from the INTVCC supply. A small internal N-channel MOSFET pulls the switch node (SW) to ground each cycle after the top MOSFET has turned off ensuring the bootstrap capacitor is kept fully charged.
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+
RSENSE CIN VIN 8 1 SENSE -
LTC1624
VIN
2.5A
4k
1.19V REF
-
VIN I2 DB BOOST 7 3A CB RUN I1
-
4k
FUNCTIONAL DIAGRA
+
SLOPE COMP 3A
8k
5.6V INTVCC REG INTVCC
+
1.19V
180k
+ -
0.8V ST 6
- +
FLOATING DRIVER TG
N-CHANNEL MOSFET
30k 1.19V 1.5V B
8k
+
SW SWITCH LOGIC 5 D1
L1 VOUT
-
gm = 1m
+
COUT
(Shown in a step-down application)
ITH /RUN 1.19V R Q S EA
+ -
2
RC
CC 200kHz DROPOUT DET
1.28V OV OSC SLOPE COMP
+
INTVCC
VFB
3
-
VFB COSC
R1
R2
200kHz
1-SHOT 400ns
N-CHANNEL MOSFET 4
GND
1624 FD
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LTC1624
APPLICATIONS INFORMATION
The LTC1624 can be used in a wide variety of switching regulator applications, the most common being the stepdown converter. Other switching regulator architectures include step-up, SEPIC and positive-to-negative converters. The basic LTC1624 step-down application circuit is shown in Figure 1 on the first page. External component selection is driven by the load requirement and begins with the selection of RSENSE. Once RSENSE is known, the inductor can be chosen. Next, the power MOSFET and D1 are selected. Finally, CIN and COUT are selected. The circuit shown in Figure 1 can be configured for operation up to an input voltage of 28V (limited by the external MOSFETs). Step-Down Converter: RSENSE Selection for Output Current RSENSE is chosen based on the required output current. The LTC1624 current comparator has a maximum threshold of 160mV/RSENSE. The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current IMAX equal to the peak value less half the peak-to-peak ripple current, IL. Allowing a margin for variations in the LTC1624 and external component values yields: Accepting larger values of IL allows the use of low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is IL = 0.4(IMAX). Remember, the maximum IL occurs at the maximum input voltage. The inductor value also has an effect on low current operation. Lower inductor values (higher IL) will cause Burst Mode operation to begin at higher load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation lower inductance values will cause the burst frequency to decrease. In general, inductor values from 5H to 68H are typical depending on the maximum input voltage and output current. See also Modifying Burst Mode Operation section. Step-Down Converter: Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy or Kool M(R) cores. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and, therefore, copper losses will increase. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard," which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Molypermalloy (from Magnetics, Inc.) is a very good, low loss core material for toroids, but it is more expensive than ferrite. A reasonable compromise from the same manufacturer is Kool M. Toroids are very space efficient, especially when you can use several layers of wire. Because they generally lack a bobbin, mounting is more difficult. However, designs for surface mount that do not increase the height significantly are available.
Kool Mu is a registered trademark of Magnetics, Inc.
RSENSE =
100mV IMAX
The LTC1624 works well with values of RSENSE from 0.005 to 0.5. Step-Down Converter: Inductor Value Calculation With the operating frequency fixed at 200kHz smaller inductor values are favored. Operating at higher frequencies generally results in lower efficiency because of MOSFET gate charge losses. In addition to this basic trade-off, the effect of inductor value on ripple current and low current operation must also be considered. The inductor value has a direct effect on ripple current. The inductor ripple current IL decreases with higher inductance and increases with higher VIN or VOUT:
V +V V -V IL = IN OUT OUT D VIN + VD fL
( )( )
where VD is the output Schottky diode forward drop.
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LTC1624
APPLICATIONS INFORMATION
Step-Down Converter: Power MOSFET Selection One external N-channel power MOSFET must be selected for use with the LTC1624 for the top (main) switch. The peak-to-peak gate drive levels are set by the INTVCC voltage. This voltage is typically 5V. Consequently, logic level threshold MOSFETs must be used in most LTC1624 applications. If low input voltage operation is expected (VIN < 5V) sublogic level threshold MOSFETs should be used. Pay close attention to the BVDSS specification for the MOSFETs as well; many of the logic level MOSFETs are limited to 30V or less. Selection criteria for the power MOSFET include the "ON" resistance RDS(ON), reverse transfer capacitance CRSS, input voltage and maximum output current. When the LTC1624 is operating in continuous mode the duty cycle for the top MOSFET is given by: characteristics. The constant k = 2.5 can be used to estimate the contributions of the two terms in the PMAIN dissipation equation. Step-Down Converter: Output Diode Selection (D1) The Schottky diode D1 shown in Figure 1 conducts during the off-time. It is important to adequately specify the diode peak current and average power dissipation so as not to exceed the diode ratings. The most stressful condition for the output diode is under short circuit (VOUT = 0V). Under this condition, the diode must safely handle ISC(PK) at close to 100% duty cycle. Under normal load conditions, the average current conducted by the diode is simply:
IDIODE AVG = ILOAD AVG VIN - VOUT V +V IN D
+V V Main Switch Duty Cycle = OUT D VIN + VD
The MOSFET power dissipation at maximum output current is given by:
2 V + VD PMAIN = OUT IMAX 1 + RDS ON + VIN + VD
( )( ) ( ) 1.85 k(VIN) (IMAX)(CRSS)(f)
where is the temperature dependency of RDS(ON) and k is a constant inversely related to the gate drive current. MOSFETs have I2R losses, plus the PMAIN equation includes an additional term for transition losses that are highest at high output voltages. For VIN < 20V the high current efficiency generally improves with larger MOSFETs, while for VIN > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CRSS actual provides higher efficiency. The diode losses are greatest at high input voltage or during a short circuit when the diode duty cycle is nearly 100%. The term (1+ ) is generally given for a MOSFET in the form of a normalized RDS(ON) vs Temperature curve, but = 0.005/C can be used as an approximation for low voltage MOSFETs. CRSS is usually specified in the MOSFET
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()
Remember to keep lead lengths short and observe proper grounding (see Board Layout Checklist) to avoid ringing and increased dissipation. The forward voltage drop allowable in the diode is calculated from the maximum short-circuit current as: VD VIN + VD ISC AVG VIN PD
()
where PD is the allowable diode power dissipation and will be determined by efficiency and/or thermal requirements (see Efficiency Considerations). Step-Down Converter: CIN and COUT Selection In continuous mode the source current of the top N-channel MOSFET is a square wave of approximate duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CIN Required IRMS IMAX
[V ( V
OUT IN - VOUT
)]
1/ 2
VIN
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com-
LTC1624
APPLICATIONS INFORMATION
monly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer's ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. Always consult the manufacturer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement is satisfied the capacitance is adequate for filtering. The output ripple (VOUT) is determined by: ratings that are ideal for input capacitor applications. Consult the manufacturer for other specific recommendations. INTVCC Regulator An internal regulator produces the 5V supply that powers the drivers and internal circuitry within the LTC1624. Good VIN bypassing is necessary to supply the high transient currents required by the MOSFET gate drivers. High input voltage applications in which large MOSFETs are being driven at high frequencies may cause the maximum junction temperature rating for the LTC1624 to be exceeded. The supply current is dominated by the gate charge supply current as discussed in the Efficiency Considerations section. The junction temperature can be estimated by using the equations given in Note 1 of the Electrical Characteristics table. For example, the LTC1624 is limited to less than 17mA from a 30V supply: TJ = 70C + (17mA)(30V)(110C/W) = 126C To prevent maximum junction temperature from being exceeded, the input supply current must be checked operating in continuous mode at maximum VIN. Step-Down Converter: Topside MOSFET Driver Supply (CB, DB) An external bootstrap capacitor CB connected to the BOOST pin supplies the gate drive voltage for the topside MOSFET. Capacitor CB in the functional diagram is charged through internal diode DB from INTVCC when the SW pin is low. When the topside MOSFET is to be turned on, the driver places the CB voltage across the gate to source of the MOSFET. This enhances the MOSFET and turns on the topside switch. The switch node voltage SW rises to VIN and the BOOST pin rises to VIN + INTVCC. The value of the boost capacitor CB needs to be 50 times greater than the total input capacitance of the topside MOSFET. In most applications 0.1F is adequate. Significant efficiency gains can be realized by supplying topside driver operating voltage from the output, since the VIN current resulting from the driver and control currents will be scaled by a factor of (Duty Cycle)/(Efficiency). For 5V regulators this simply means connecting the BOOST
1 VOUT IL ESR + 4 fCOUT
where f = operating frequency, COUT = output capacitance and IL = ripple current in the inductor. The output ripple is highest at maximum input voltage since IL increases with input voltage. With IL = 0.4IOUT(MAX) the output ripple will be less than 100mV at maximum VIN, assuming: COUT Required ESR < 2RSENSE Manufacturers such as Nichicon, United Chemicon and SANYO should be considered for high performance through-hole capacitors. The OS-CON semiconductor dielectric capacitor available from SANYO has the lowest ESR(size) product of any aluminum electrolytic at a somewhat higher price. Once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. In surface mount applications multiple capacitors may have to be paralleled to meet the ESR or RMS current handling requirements of the application. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. Other capacitor types include SANYO OS-CON, Nichicon WF series and Sprague 595D series and the new ceramics. Ceramic capacitors are now available in extremely low ESR and high ripple current
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LTC1624
APPLICATIONS INFORMATION
pin through a small Schottky diode (like a Central CMDSH-3) to VOUT as shown in Figure 10. However, for 3.3V and other lower voltage regulators, additional circuitry is required to derive boost supply power from the output. For low input voltage operation (VIN < 7V), a Schottky diode can be connected from VIN to BOOST to increase the external MOSFET gate drive voltage. Be careful not to exceed the maximum voltage on BOOST to SW pins of 7.8V. Output Voltage Programming The output voltage is set by a resistive divider according to the following formula:
R2 VOUT = 1.19V 1 + R1
3.3V OR 5V D1 CC RC CC RC ITH /RUN ITH /RUN
The external resistive divider is connected to the output as shown in Figure 2, allowing remote voltage sensing. When using remote sensing, a local 100 resistor should be connected from L1 to R2 to prevent VOUT from running away if the sense lead is disconnected.
L1 R2 VFB LTC1624 GND
1624 F02
VOUT
100pF
R1
Figure 2. Setting the LTC1624 Output Voltage
ITH /RUN Function The ITH /RUN pin is a dual purpose pin that provides the loop compensation and a means to shut down the LTC1624. Soft start can also be implemented with this pin. Soft start reduces surge currents from VIN by gradually increasing the internal current limit. Power supply sequencing can also be accomplished using this pin. An internal 2.5A current source charges up the external capacitor CC. When the voltage on ITH /RUN reaches 0.8V the LTC1624 begins operating. At this point the error amplifier pulls up the ITH /RUN pin to its maximum of 2.4V (assuming VOUT is starting low).
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(a)
ITH /RUN R1 D1
(b)
C1
CC RC
(c)
1624 F03
Figure 3. ITH / RUN Pin Interfacing
Soft start can be implemented by ramping the voltage on ITH /RUN during start-up as shown in Figure 3(c). As the voltage on ITH/RUN ramps from 1.19V to 2.4V the internal peak current limit is also ramped at a proportional linear rate. The peak current limit begins at approximately 10mV/RSENSE (at VITH/RUN = 1.4V) and ends at: 160mV/RSENSE (VITH/RUN = 2.4V) The output current thus ramps up slowly, charging the output capacitor. The peak inductor current and maximum output current are as follows: IL(PEAK) = (VITH/RUN - 1.3V)/(6.8RSENSE) IOUT(MAX) = ILPEAK - IL / 2 with IL = ripple current in the inductor. During normal operation the voltage on the ITH /RUN pin will vary from 1.19V to 2.4V depending on the load current. Pulling the ITH /RUN pin below 0.8V puts the LTC1624 into a low quiescent current shutdown (IQ < 30A). This pin can be driven directly from logic as shown in Figures 3(a) and 3(b). Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine
LTC1624
APPLICATIONS INFORMATION
what is limiting the efficiency and which change would produce the most improvement. Percent efficiency can be expressed as: %Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC1624 circuits: 1. LTC1624 VIN current 2. I2R losses 3. Topside MOSFET transition losses 4. Voltage drop of the Schottky diode 1. The VIN current is the sum of the DC supply current IQ, given in the Electrical Characteristics table, and the MOSFET driver and control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFET. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from INTVCC to ground. The resulting dQ/dt is a current out of VIN which is typically much larger than the control circuit current. In continuous mode, IGATECHG = f (QT + QB), where QT and QB are the gate charges of the topside and internal bottom side MOSFETs. By powering BOOST from an output-derived source (Figure 10 application), the additional VIN current resulting from the topside driver will be scaled by a factor of (Duty Cycle)/(Efficiency). For example, in a 20V to 5V application, 5mA of INTVCC current results in approximately 1.5mA of VIN current. This reduces the midcurrent loss from 5% or more (if the driver was powered directly from VIN) to only a few percent. 2. I2R losses are predicted from the DC resistances of the MOSFET, inductor and current shunt. In continuous mode the average output current flows through L but is "chopped" between the topside main MOSFET/current shunt and the Schottky diode. The resistances of the topside MOSFET and RSENSE multiplied by the duty cycle can simply be summed with the resistance of L to obtain I2R losses. (Power is dissipated in the sense resistor only when the topside MOSFET is on. The I2R loss is thus reduced by the duty cycle.) For example, at 50% DC, if RDS(ON) = 0.05, RL = 0.15 and RSENSE = 0.05, then the effective total resistance is 0.2. This results in losses ranging from 2% to 8% for VOUT = 5V as the output current increases from 0.5A to 2A. I2R losses cause the efficiency to drop at high output currents. 3. Transition losses apply only to the topside MOSFET(s), and only when operating at high input voltages (typically 20V or greater). Transition losses can be estimated from: Transition Loss = 2.5(VIN)1.85 (IMAX)(CRSS)(f) 4. The Schottky diode is a major source of power loss at high currents and gets worse at high input voltages. The diode loss is calculated by multiplying the forward voltage drop times the diode duty cycle multiplied by the load current. For example, assuming a duty cycle of 50% with a Schottky diode forward voltage drop of 0.5V, the loss is a relatively constant 5%. As expected, the I2R losses and Schottky diode loss dominate at high load currents. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT immediately shifts by an amount equal to (ILOAD * ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or discharge COUT which generates a feedback error signal. The regulator loop then acts to return VOUT to its steady-state value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The ITH external components shown in the Figure 1 circuit will provide adequate compensation for most applications. A second, more severe transient, is caused by switching in loads with large (>1F) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel
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LTC1624
APPLICATIONS INFORMATION
with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 * CLOAD). Thus a 10F capacitor would require a 250s rise time, limiting the charging current to about 200mA. Automotive Considerations: Plugging into the Cigarette Lighter As battery-powered devices go mobile there is a natural interest in plugging into the cigarette lighter in order to conserve or even recharge battery packs during operation. But before you connect, be advised: you are plugging into the supply from hell. The main battery line in an automobile is the source of a number of nasty potential transients, including load dump, reverse battery and double battery. Load dump is the result of a loose battery cable. When the cable breaks connection, the field collapse in the alternator can cause a positive spike as high as 60V which takes several hundred milliseconds to decay. Reverse battery is just what it says, while double battery is a consequence of tow-truck operators finding that a 24V jump start cranks cold engines faster than 12V. The network shown in Figure 4 is the most straightforward approach to protect a DC/DC converter from the ravages of an automotive battery line. The series diode prevents current from flowing during reverse battery, while the transient suppressor clamps the input voltage during load dump. Note that the transient suppressor should not conduct during double battery operation, but must still clamp the input voltage below breakdown of the converter. Although the LTC1624 has a maximum input voltage of
12V 50A IPK RATING VIN TRANSIENT VOLTAGE SUPPRESSOR GENERAL INSTRUMENT 1.5KA24A LTC1624
SW (VOUT - 0.7V) 180A D1 MBRS340T3 LTC1624 TG R* L1
1624 F04
Figure 4. Plugging into the Cigarette Lighter
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36V, most applications will be limited to 30V by the MOSFET BVDSS. Modifying Burst Mode Operation The LTC1624 automatically enters Burst Mode operation at low output currents to boost efficiency. The point when continuous mode operation changes to Burst Mode operation scales as a function of maximum output current. The output current when Burst Mode operation commences is approximately 8mV/RSENSE (8% of maximum output current). With the additional circuitry shown in Figure 5 the LTC1624 can be forced to stay in continuous mode longer at low output currents. Since the LTC1624 is not a fully synchronous architecture, it will eventually start to skip cycles as the load current drops low enough. The point when the minimum on-time (450ns) is reached determines the load current when cycle skipping begins at approximately 1% of maximum output current. Using the circuit in Figure 5 the LTC1624 will begin to skip cycles but stays in regulation when IOUT is less than IOUT(MIN):
2 tON MIN f V +V IOUT MIN = VIN - VOUT IN D 2L VOUT + VD
()
()
(
)
where tON(MIN) = 450ns, f = 200kHz. The transistor Q1 in the circuit of Figure 5 operates as a current source developing an 18mV offset across the
VIN
+
VIN SENSE - Q1 2N2222 1000pF 100 18mV RSENSE CIN
-
+
+
VOUT COUT
1624 F05
*R =
Figure 5. Modifying Burst Mode Operation
LTC1624
APPLICATIONS INFORMATION
100 resistor in series with the SENSE - pin. This offset cancels the internal offset in current comparator I2 (refer to Functional Diagram). This comparator in conjunction with the voltage on the ITH /RUN pin determines when to enter into Burst Mode operation (refer to Low Current Operation in Operation section). With the additional external offset present, the drive to the topside MOSFET is always enabled every cycle and constant frequency operation occurs for IOUT > IOUT(MIN). Step-Down Converter: Design Example As a design example, assume VIN = 12V(nominal), VIN = 22V(max), VOUT = 3.3V and IMAX = 2A. RSENSE can immediately be calculated: RSENSE = 100mV/2A = 0.05 Assume a 10H inductor. To check the actual value of the ripple current the following equation is used: With the 0.05 sense resistor ISC(AVG) = 2A will result, increasing the 0.5V Schottky diode dissipation to 0.98W. CIN is chosen for an RMS current rating of at least 1.0A at temperature. COUT is chosen with an ESR of 0.03 for low output ripple. The output ripple in continuous mode will be highest at the maximum input voltage. The output voltage ripple due to ESR is approximately: VORIPPLE = RESR(IL) = 0.03 (1.58AP-P) = 47mVP-P Step-Down Converter: Duty Cycle Limitations At high input to output differential voltages the on-time gets very small. Due to internal gate delays and response times of the internal circuitry the minimum recommended on-time is 450ns. Since the LTC1624's frequency is internally set to 200kHz a potential duty cycle limitation exists. When the duty cycle is less than 9%, cycle skipping may occur which increases the inductor ripple current but does not cause VOUT to lose regulation. Avoiding cycle skipping imposes a limit on the input voltage for a given output voltage only when VOUT < 2.2V using 30V MOSFETs. (Remember not to exceed the absolute maximum voltage of 36V.) VIN(MAX) = 11.1VOUT + 5V For DC > 9%
V +V V -V IL = IN OUT OUT D f L VIN + VD
( )( )
The highest value of the ripple current occurs at the maximum input voltage:
IL =
22V - 3.3V 3.3V + 0.5V = 1.58AP-P 200kHz 10H 22V + 0.5V
(
)
The power dissipation on the topside MOSFET can be easily estimated. Choosing a Siliconix Si4412DY results in: RDS(ON) = 0.042, CRSS = 100pF. At maximum input voltage with T(estimated) = 50C:
PMAIN = 2 3.3V + 0.5V 2A 1 + 0.005 50C - 25C 0.042 22V + 0.5V
( ) [ ( )( )]( 1.85 + 2.5 (22V) (2A)(100pF)(200kHz) = 62mW
VIN PD = ISC AVG VD VIN + VD
The most stringent requirement for the Schottky diode occurs when VOUT = 0V (i.e. short circuit) at maximum VIN. In this case the worst-case dissipation rises to:
( )( )
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Boost Converter Applications The LTC1624 is also well-suited to boost converter applications. A boost converter steps up the input voltage to a higher voltage as shown in Figure 6.
VIN
RSENSE
+
CIN
)
VIN SENSE - L1 BOOST LTC1624 TG CB GND SW VFB R1 M1 R2 D1 VOUT
+
COUT
1624 F06
Figure 6. Boost Converter
13
LTC1624
APPLICATIONS INFORMATION
Boost Converters: Power MOSFET Selection One external N-channel power MOSFET must be selected for use with the LTC1624 for the switch. In boost applications the source of the power MOSFET is grounded along with the SW pin. The peak-to-peak gate drive levels are set by the INTVCC voltage. The gate drive voltage is equal to approximately 5V for VIN > 5.6V and a logic level MOSFET can be used. At VIN voltages below 5V the gate drive voltage is equal to VIN - 0.6V and a sublogic level MOSFET should be used. Selection criteria for the power MOSFET include the "ON" resistance RDS(ON), reverse transfer capacitance CRSS, input voltage and maximum output current. When the LTC1624 is operating in continuous mode the duty cycle for the MOSFET is given by: Main Switch Duty Cycle = 1- VIN VOUT + VD Boost Converter: Inductor Selection For most applications the inductor will fall in the range of 10H to 100H. Higher values reduce the input ripple voltage and reduce core loss. Lower inductor values are chosen to reduce physical size. The input current of the boost converter is calculated at full load current. Peak inductor current can be significantly higher than output current, especially with smaller inductors and lighter loads. The following formula assumes continuous mode operation and calculates maximum peak inductor current at minimum VIN: IL MAX V IL PEAK = IOUT MAX OUT + 2 VIN MIN
The MOSFET power dissipation at maximum output current is given by:
VIN MIN 2 1 + RDS ON + PMAIN = IIN MAX 1 - V + VD OUT
()
()
( ) () )( )
k VOUT
(
)
1.85
C I 200kHz IN MAX RSS
( )(
VOUT + VD where IIN MAX = IOUT MAX VIN MIN
()
()
()
is the temperature dependency of RDS(ON) and k is a constant inversely related to the gate drive current. MOSFETs have I2R losses, plus the PMAIN equation includes an additional term for transition losses that are highest at high output voltages. For VOUT < 20V the high current efficiency generally improves with larger MOSFETs, while for VOUT > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CRSS actual provides higher efficiency. For additional information refer to Step-Down Converter: Power MOSFET Selection in the Applications Information section.
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(
)
()
()
()
The ripple current in the inductor (IL) is typically 20% to 30% of the peak inductor current occuring at VIN(MIN) and IOUT(MAX).
IL P-P =
()
(200kHz)(L)(VOUT + VD)
VIN VOUT + VD - VIN
(
)
with IL(MAX) = IL(P-P) at VIN = VIN(MIN). Remember boost converters are not short-circuit protected, and that under output short conditions, inductor current is limited only by the available current of the input supply, IOUT(OVERLOAD). Specify the maximum inductor current to safely handle the greater of IL(PEAK) or IOUT(OVERLOAD). Make sure the inductor's saturation current rating (current when inductance begins to fall) exceeds the maximum current rating set by RSENSE. Boost Converter: RSENSE Selection for Maximum Output Current RSENSE is chosen based on the required output current. Remember the LTC1624 current comparator has a maximum threshold of 160mV/RSENSE. The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current IOUT(MAX) equal to IL(PEAK) less half the peak-to-peak ripple current (IL), divided by the output-input voltage ratio (see equation for IL(PEAK)).
LTC1624
APPLICATIONS INFORMATION
Allowing a margin for variations in the LTC1624 (without considering variation in RSENSE), assuming 30% ripple current in the inductor, yields: VIN MIN RSENSE = IOUT MAX VOUT + VD 100mV
()
()
Boost Converter: Output Diode The output diode conducts current only during the switch off-time. Peak reverse voltage for boost converters is equal to the regulator output voltage. Average forward current in normal operation is equal to output current. Remember boost converters are not short-circuit protected. Check to be sure the diode's current rating exceeds the maximum current set by RSENSE. Schottky diodes such as Motorola MBR130LT3 are recommended. Boost Converter: Output Capacitors The output capacitor is normally chosen by its effective series resistance (ESR), because this is what determines output ripple voltage. Since the output capacitor's ESR affects efficiency, use low ESR capacitors for best performance. Boost regulators have large RMS ripple current in the output capacitor that must be rated to handle the current. The output capacitor ripple current (RMS) is:
COUT IRIPPLE RMS IOUT
()
VOUT - VIN VIN
Output ripple is then simply: VOUT = RESR (IL(RMS)). Boost Converter: Input Capacitors The input capacitor of a boost converter is less critical due to the fact that the input current waveform is triangular, and does not contain large square wave currents as found in the output capacitor. The input voltage source impedance determines the size of the capacitor that is typically 10F to 100F. A low ESR is recommended although not as critical as the output capacitor and can be on the order of 0.3. Input capacitor ripple current for the LTC1624 used as a boost converter is:
RSENSE VIN SENSE - L1 BOOST LTC1624 TG CB GND SW C1
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CIN IRIPPLE
0.3 VIN VOUT - VIN
) (200kHz)(L)(VOUT)
( )(
The input capacitor can see a very high surge current when a battery is suddenly connected and solid tantalum capacitors can fail under this condition. Be sure to specify surge tested capacitors. Boost Converter: Duty Cycle Limitations The minimum on-time of 450ns sets a limit on how close VIN can approach VOUT without the output voltage overshooting and tripping the overvoltage comparator. Unless very low values of inductances are used, this should never be a problem. The maximum input voltage in continuous mode is: VIN(MAX) = 0.91VOUT + 0.5V For DC = 9%
SEPIC Converter Applications The LTC1624 is also well-suited to SEPIC (Single Ended Primary Inductance Converter) converter applications. The SEPIC converter shown in Figure 7 uses two inductors. The advantage of the SEPIC converter is the input voltage may be higher or lower than the output voltage. The first inductor L1 together with the main N-channel MOSFET switch resemble a boost converter. The second inductor L2 and output diode D1 resemble a flyback or buck-boost converter. The two inductors L1 and L2 can be independent but also can be wound on the same core since
VIN
+
CIN
D1 VOUT L2 VFB R1 R2
+
M1
+
COUT
1624 F07
Figure 7. SEPIC Converter
15
LTC1624
APPLICATIONS INFORMATION
identical voltages are applied to L1 and L2 throughout the switching cycle. By making L1 = L2 and wound on the same core the input ripple is reduced along with cost and size. All SEPIC applications information that follows assumes L1 = L2 = L. SEPIC Converter: Power MOSFET Selection One external N-channel power MOSFET must be selected for use with the LTC1624 for the switch. As in boost applications the source of the power MOSFET is grounded along with the SW pin. The peak-to-peak gate drive levels are set by the INTVCC voltage. This voltage is equal to approximately 5V for VIN > 5.6V and a logic level MOSFET can be used. At VIN voltages below 5V the INTVCC voltage is equal to VIN - 0.6V and a sublogic level MOSFET should be used. Selection criteria for the power MOSFET include the "ON" resistance RDS(ON), reverse transfer capacitance CRSS, input voltage and maximum output current. When the LTC1624 is operating in continuous mode the duty cycle for the MOSFET is given by: Main Switch Duty Cycle = VOUT + VD VIN + VOUT + VD highest at high total input plus output voltages. For (VIN + VOUT) < 20V the high current efficiency generally improves with larger MOSFETs, while for (VIN + VOUT) > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CRSS actual provides higher efficiency. For additional information refer to the Step-Down Converter: Power MOSFET Selection in the Applications Information section. SEPIC Converter: Inductor Selection For most applications the equal inductor values will fall in the range of 10H to 100H. Higher values reduce the input ripple voltage and reduce core loss. Lower inductor values are chosen to reduce physical size and improve transient response. Like the boost converter the input current of the SEPIC converter is calculated at full load current. Peak inductor current can be significantly higher than output current, especially with smaller inductors and lighter loads. The following formula assumes continuous mode operation and calculates maximum peak inductor current at minimum VIN: I VOUT + L1 IL1 PEAK = IOUT MAX 2 VIN MIN VIN MIN + VD I + L2 IL2 PEAK = IOUT MAX 2 VIN MIN
The MOSFET power dissipation and maximum switch current at maximum output current are given by:
PMAIN = 2 VOUT + VD I 1 + RDS ON + SW MAX V IN MIN + VOUT + VD
() ()
()
1.85
( ) () )( )
k VIN MIN + VOUT
I C 200kHz SW MAX RSS
( )(
+V V where ISW MAX = IOUT MAX OUT D + 1 VIN MIN
()
()
()
is the temperature dependency of RDS(ON) and k is a constant inversely related to the gate drive current. The peak switch current is ISW(MAX) + IL. MOSFETs have I2R losses plus the PMAIN equation includes an additional term for transition losses that are
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(
)
()
(
)
()
() () ()
The ripple current in the inductor (IL) is typically 20% to 30% of the peak current occuring at VIN(MIN) and IOUT(MAX), and IL1 = IL2. Maximum IL occurs at maximum VIN.
IL P-P =
(VIN)(VOUT + VD) ( ) (200kHz)(L)(VIN + VOUT + VD)
By making L1 = L2 and wound on the same core the value of inductance in all the above equations are replaced by 2L due to their mutual inductance. Doing this maintains the same ripple current and inductive energy storage in the inductors. For example a Coiltronix CTX10-4 is a 10H inductor with two windings. With the windings in parallel
LTC1624
APPLICATIONS INFORMATION
10H inductance is obtained with a current rating of 4A. Splitting the two windings creates two 10H inductors with a current rating of 2A each. Therefore substitute (2)(10H) = 20H for L in the equations. Specify the maximum inductor current to safely handle IL(PEAK). Make sure the inductor's saturation current rating (current when inductance begins to fall) exceeds the maximum current rating set by RSENSE. SEPIC Converter: RSENSE Selection for Maximum Output Current RSENSE is chosen based on the required output current. Remember the LTC1624 current comparator has a maximum threshold of 160mV/RSENSE. The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current IOUT(MAX) equal to IL1(PEAK) less half the peak-to-peak ripple current, IL, divided by the output-input voltage ratio (see equation for IL1(PEAK)). Allowing a margin for variations in the LTC1624 (without considering variation in RSENSE), assuming 30% ripple current in the inductor, yields: VIN MIN RSENSE = IOUT MAX VOUT + VD 100mV output ripple voltage. The input capacitor needs to be sized to handle the ripple current safely. Since the output capacitor's ESR affects efficiency, use low ESR capacitors for best performance. SEPIC regulators, like step-down regulators, have a triangular current waveform but have maximum ripple at VIN(MAX). The input capacitor ripple current is:
IRIPPLE RMS =
()
()
SEPIC Converter: Output Diode The output diode conducts current only during the switch off-time. Peak reverse voltage for SEPIC converters is equal to VOUT + VIN. Average forward current in normal operation is equal to output current. Peak current is: VOUT + VD ID1 PEAK = I OUT MAX + 1 + IL VIN MIN
(
)
()
()
Schottky diodes such as MBR130LT3 are recommended. SEPIC Converter: Input and Output Capacitors The output capacitor is normally chosen by its effective series resistance (ESR), because this is what determines
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()
IL 12
The output capacitor ripple current is:
IRIPPLE RMS = IOUT VOUT
VIN
()
The output capacitor ripple voltage (RMS) is: VOUT(RIPPLE) = 2(IL)(ESR) The input capacitor can see a very high surge current when a battery is suddenly connected, and solid tantalum capacitors can fail under this condition. Be sure to specify surge tested capacitors. SEPIC Converter: Coupling Capacitor (C1) The coupling capacitor C1 in Figure 7 sees a nearly rectangular current waveform. During the off-time the current through C1 is IOUT(VOUT/VIN) while approximately - IOUT flows though C1 during the on-time. This current waveform creates a triangular ripple voltage on C1:
VOUT IOUT VC1 = 200kHz C1 VIN + VOUT + VD
(
)( )
The maximum voltage on C1 is then: VC1(MAX) = VIN + VC1 /2 (typically close to VIN(MAX)). The ripple current though C1 is:
IRIPPLE C1 = IOUT
()
VOUT VIN
The maximum ripple current occurs at IOUT(MAX) and VIN(MIN). The capacitance of C1 should be large enough so
17
LTC1624
APPLICATIONS INFORMATION
that the voltage across C1 is constant such that VC1 = VIN at full load over the entire VIN range. Assuming the enegry storage in the coupling capacitor C1 must be equal to the enegry stored in L1, the minimum capacitance of C1 is:
C1 MIN =
()
L1 IOUT
( ) (VOUT)
2
2
VIN MIN
()
4
SEPIC Converter: Duty Cycle Limitations The minimum on-time of 450ns sets a limit on how high an input-to-output ratio can be tolerated while not skipping cycles. This only impacts designs when very low output voltages (VOUT < 2.5V) are needed. Note that a SEPIC converter would not be appropriate at these low output voltages. The maximum input voltage is (remember not to exceed the absolute maximum limit of 36V): VIN(MAX) = 10.1VOUT + 5V For DC > 9%
Positive-to-Negative Converter Applications The LTC1624 can also be used as a positive-to-negative converter with a grounded inductor shown in Figure 8. Since the LTC1624 requires a positive feedback signal relative to device ground, Pin 4 must be tied to the regulated negative output. A resistive divider from the negative output to ground sets the output voltage. Remember not to exceed maximum VIN ratings VIN + VOUT 36V.
1000pF 1 RC CC 2 3 100pF 4 GND SW 5 BOOST ITH /RUN LTC1624 VFB TG 7 6 CB SENSE - VIN 8
VIN
RSENSE
M1 L1
D1
R1
R2 -VOUT
1624 F08
Figure 8. Positive-to-Negative Converter
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Positive-to-Negative Converter: Output Voltage Programming Setting the output voltage for a positive-to-negative converter is different from other architectures since the feedback voltage is referenced to the LTC1624 ground pin and the ground pin is referenced to - VOUT. The output voltage is set by a resistive divider according to the following formula:
R1 DC VOUT = 1.19V 1 + - VIN R2 1 - DC
The external resistive divider is connected to the output as shown in Figure 8. Positive-to-Negative Converter: Power MOSFET Selection One external N-channel power MOSFET must be selected for use with the LTC1624 for the switch. As in step-down applications the source of the power MOSFET is connected to the Schottky diode and inductor. The peak-topeak gate drive levels are set by the INTVCC voltage. The gate drive voltage is equal to approximately 5V for VIN > 5.6V and a logic level MOSFET can be used. At VIN voltages below 5V the INTVCC voltage is equal to VIN - 0.6V and a sublogic level MOSFET should be used. Selection criteria for the power MOSFET include the "ON" resistance RDS(ON), reverse transfer capacitance CRSS, input voltage and maximum output current. When the LTC1624 is operating in continuous mode the duty cycle for the MOSFET is given by:
Main Switch Duty Cycle =
CIN
VOUT + VD VIN + VOUT + VD
with VOUT being the absolute value of VOUT. The MOSFET power dissipation and maximum switch current are given by:
COUT
PMAIN = ISW(MAX) x
{I
OUT MAX I + RDS ON + k V IN MAX + VOUT 1.85 CRSS 200kHz
(
( )( ()
)
)(
()
)(
){
LTC1624
APPLICATIONS INFORMATION
V + V IN OUT + VD where: ISW MAX = IOUT MAX VIN
()
()
is the temperature dependency of RDS(ON) and k is a constant inversely related to the gate drive current. The maximum switch current occurs at VIN(MIN) and the peak switch current is ISW(MAX) + IL /2. The maximum voltage across the switch is VIN(MAX) + VOUT. MOSFETs have I2R losses plus the PMAIN equation includes an additional term for transition losses that are highest at high total input plus output voltages. For (VOUT+ VIN) < 20V the high current efficiency generally improves with larger MOSFETs, while for (VOUT+ VIN) > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CRSS actual provides higher efficiency. For additional information refer to the Step-Down Converter: Power MOSFET Selection in the Applications Information section. Positive-to-Negative Converter: Inductor Selection For most applications the inductor will fall in the range of 10H to 100H. Higher values reduce the input and output ripple voltage (although not as much as step-down converters) and also reduce core loss. Lower inductor values are chosen to reduce physical size and improve transient response but do increase output ripple. Like the boost converter, the input current of the positiveto-negative converter is calculated at full load current. Peak inductor current can be significantly higher than output current, especially with smaller inductors (with high IL values). The following formula assumes continuous mode operation and calculates maximum peak inductor current at minimum VIN:
V + V I IN OUT + VD IL PEAK = IOUT MAX + L VIN 2
(
)
()
The ripple current in the inductor (IL) is typically 20% to 50% of the peak inductor current occuring at VIN(MIN) and IOUT(MAX) to minimize output ripple. Maximum IL occurs at minimum VIN.
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IL P-P =
(VIN)( VOUT + VD) ( ) (200kHz)(L) V + V + V ( IN OUT D)
Specify the maximum inductor current to safely handle IL(PEAK). Make sure the inductor's saturation current rating (current when inductance begins to fall) exceeds the maximum current rating set by RSENSE. Positive-to-Negative Converter: RSENSE Selection for Maximum Output Current RSENSE is chosen based on the required output current. Remember the LTC1624 current comparator has a maximum threshold of 160mV/RSENSE. The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current IOUT(MAX) equal to IL(PEAK) less half the peak-to-peak ripple current with the remainder divided by the duty cycle. Allowing a margin for variations in the LTC1624 (without considering variation in RSENSE) and assuming 30% ripple current in the inductor, yields:
VIN MIN 100mV RSENSE = IOUT MAX VIN MIN + VOUT + VD
()
()
()
Positive-to-Negative Converter: Output Diode The output diode conducts current only during the switch off-time. Peak reverse voltage for positive-to-negative converters is equal to VOUT+ VIN. Average forward current in normal operation is equal to ID(PEAK) - IL /2. Peak diode current (occurring at VIN(MIN)) is:
V OUT + VD I ID PEAK = IOUT MAX + 1 + L VIN 2
(
)
()
(
)
Positive-to-Negative Converter: Input and Output Capacitors The output capacitor is normally chosen by its effective series resistance (ESR), because this is what determines output ripple voltage. Both input and output capacitors need to be sized to handle the ripple current safely.
19
LTC1624
APPLICATIONS INFORMATION
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 rating of the capacitor. The following formula gives an approximate value for RMS ripple current. This formula assumes continuous mode and low current ripple. Small inductors will give somewhat higher ripple current, especially in discontinuous mode. For the exact formulas refer to Application Note 44, pages 28 to 30. The input and output capacitor ripple current (occurring at VIN(MIN)) is:
Capacitor IRMS = ff IOUT
( )( )
VOUT VIN
ff = Fudge factor (1.2 to 2.0) The output peak-to-peak ripple voltage is: VOUT(P-P) = RESR (ID(MAX)) The input capacitor can also see a very high surge current when a battery is suddenly connected, and solid tantalum capacitors can fail under this condition. Be sure to specify surge tested capacitors. Positive-to-Negative Converter: Duty Cycle Limitations The minimum on-time of 450ns sets a limit on how high of input-to-output ratio can be tolerated while not skipping cycles. This only impacts designs when very low output voltages (VOUT< 2.5V) are needed. The maximum input voltage is: VIN(MAX) < 10.1VOUT + 5V For DC > 9%
VIN(MAX) < 36V -VOUTFor absolute maximum ratings Positive-to-Negative Converter: Shutdown Considerations Since the ground pin on the LTC1624 is referenced to - VOUT, additional circuitry is needed to put the LTC1624 into shutdown. Shutdown is enabled by pulling the
20
U
W
U
U
ITH /RUN pin below 0.8V relative to the LTC1624 ground pin. With the LTC1624 ground pin referenced to - VOUT, the nonimal range on the ITH /RUN pin is - VOUT (in shutdown) to (- VOUT + 2.4V)(at Max IOUT). Referring to Figure 15, M2, M3 and R3 provide a level shift from typical TTL levels to the LTC1624 operating as positive-to-negative converter. MOSFET M3 supplies gate drive to M2 during shutdown, while M2 pulls the ITH/RUN pin voltage to - VOUT, shutting down the LTC1624. Step-Down Converters: PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1624. These items are also illustrated graphically in the layout diagram of Figure 9. Check the following in your layout: 1. Are the signal and power grounds segregated? The LTC1624 ground (Pin 4) must return to the (-) plate of COUT. 2. Does the VFB (Pin 3) connect directly to the feedback resistors? The resistive divider R1, R2 must be connected between the (+) plate of COUT and signal ground. The 100pF capacitor should be as close as possible to the LTC1624. 3. Does the VIN lead connect to the input voltage at the same point as RSENSE and are the SENSE - and VIN leads routed together with minimum PC trace spacing? The filter capacitor between VIN and SENSE - should be as close as possible to the LTC1624. 4. Does the (+) plate of CIN connect to RSENSE as closely as possible? This capacitor provides the AC current to the MOSFET(s). Also, does CIN connect as close as possible to the VIN and ground pin of the LTC1624? This capacitor also supplies the energy required to recharge the bootstrap capacitor. Adequate input decoupling is critical for proper operation. 5. Keep the switch node SW away from sensitive smallsignal nodes. Ideally, M1, L1 and D1 should be connected as closely as possible at the switch node.
LTC1624
TYPICAL APPLICATIONS
1000pF
1 RC CC 2 3 100pF 4
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 9. LTC1624 Layout Diagram (See Board Layout Checklist)
CC 560pF RC 4.7k 100pF
*COILTRONICS CTX10-4
Figure 10. 5V/3A Converter with Output Derived Boost Voltage
U
1 2 3 4
+
SENSE - VIN 8 7 6 5 CB 0.1F D1 RSENSE
+
CIN
VIN
BOOST ITH /RUN LTC1624 VFB GND TG SW
-
M1 L1
+
R2
+
COUT VOUT
R1
-
1624 F09
VIN 5.3V TO 28V
SENSE - ITH /RUN VFB GND
VIN BOOST TG SW
8 7 6 5
1000pF 0.1F
RSENSE 0.033
+
CIN 22F 35V x2
LTC1624
CB 0.1F D1 MBRS340T3
M1 Si4412DY
D2 CMDSH-3 VOUT 5V 3A
L1* 10H
R2 35.7k 1% R1 11k 1%
+
COUT 100F 10V x2
1624 F10
21
LTC1624
TYPICAL APPLICATIONS
1 2 CC 470pF RC 6.8k 100pF 4 GND SW 5 3 VFB SENSE - ITH /RUN VIN BOOST TG 8 7 6 CB 0.1F 1000pF 0.1F VIN 4.8V TO 22V RSENSE 0.068 M1 Si6436DY
*SUMIDA CDR105B-100
CC 470pF RC 6.8k 100pF
*SUMIDA CDRH125-470
1 2 CC 330pF RC 3.3k 100pF 4 3
*SUMIDA CDRH125-220
22
U
VFB
+
CIN 22F 35V x2
LTC1624
L1* 10H R2 35.7k 1% R1 69.8k 1%
D1 MBRS340T3
VOUT 1.8V 1.5A
+
COUT 100F 10V x2
Figure 11. Wide Input Range 1.8V/1.5A Converter
1624 F11
1 2 3
SENSE - ITH /RUN VFB
VIN BOOST TG SW
8 7 6 5
1000pF 0.1F
VIN 12.3V TO 28V
RSENSE 0.068 M1 Si4412DY
+
CIN 22F 35V x2
LTC1624
4
CB 0.1F
L1* 47H R2 35.7k 1% R1 3.92k 1%
GND
D1 MBRS140T3
VOUT 12V 1A
+
COUT 100F 16V x2
Figure 12. 12V/1A Low Dropout Converter
1624 F12
SENSE - ITH /RUN
VIN BOOST TG SW
8 7 6 5
1000pF 0.1F
VIN 5.2V TO 11V
+
RSENSE 0.04 L1* 22H
CIN 22F D1 35V x 2 MBRS130LT3 M1 Si4412DY R2 35.7k 1% R1 3.92k 1% VOUT 12V 0.75A
LTC1624
CB 0.1F
GND
+
COUT 100F 16V x2
Figure 13. 12V/0.75A Boost Converter
1624 F13
LTC1624
TYPICAL APPLICATIONS
VIN 5V TO 15V 1 2 CC 330pF RC 4.7k 100pF 4 GND SW 5 3 VFB SENSE - ITH /RUN 8 7 6 CB 0.1F 1000pF 0.1F RSENSE 0.068 VIN BOOST TG
*COILTRONICS CTX20-4
VCC SHUTDOWN
VCC M3 TP0610L
M2 VN2222
R3 100k
*COILCRAFT DO5022P-333
CC 470pF RC 6.8k 100pF
*COILTRONICS CTX20-4
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+
L1a*
CIN 22F 22F 35V 35V
D1 MBRS130LT3 VOUT 12V 0.5A R2 35.7k 1% R1 3.92k 1% COUT 100F 16V x2
LTC1624
M1 Si4412DY
+
L1b*
+
Figure 14. 12V/0.4A SEPIC Converter
1624 F14
VIN 5V TO 22V CC RC 1000pF 3.3k 1 2 3 VFB 100pF 4 GND SW 5 SENSE - ITH /RUN VIN BOOST TG 8 7 6 CB 0.1F 1000pF RSENSE 0.025 M1 Si4410DY L1* 33H
+
0.1F
CIN 22F 35V x2 D2 CMDSH-3
LTC1624
D1 MBRS340T3
R2 78.7k 1% R1 24.9k 1%
+
COUT 100F 10V x2 VOUT -5V 2A
1624 F15
Figure 15. Inverting - 5V/2A Converter
1 2 3
SENSE - ITH /RUN VFB
VIN BOOST TG SW
8 7 6 5
1000pF 0.1F
VIN 3.5V TO 18V
RSENSE 0.068
+
LTC1624
CIN 22F 35V x2
4
CB 0.1F
M1 Si6426DQ L1* 20H D1 MBRS340T3 R2 35.7k 1% R1 20k 1%
GND
VOUT 3.3V 1.5A
+
COUT 100F 10V x2
Figure 16. Low Dropout 3.3V/1.5A Converter
1624 F16
23
LTC1624
TYPICAL APPLICATIONS
VIN 3.6V TO 18V
1 2 CC 330pF RC 6.8k 100pF 4 3
SENSE - ITH /RUN VFB GND
LTC1624 TG SW
* COILTRONICS CTX20-4
Figure 17. 5V/1A SEPIC Converter with Output Derived Boost Voltage
VIN 13V TO 28V
+
CIN1, CIN2 1000F 35V x2
CC 100pF
1 2 3
RC 20k
R1 11k 1%
4
CIN1, CIN2 = SANYO 35MV1000GX C5, C7 = WIMA MKS2 COUT = SANYO 16MV2700GX D1 = MOTOROLA MBR2535CT L1 = PULSE ENGINEERING PO472
Figure 18. 24V to 12V/10A Buck Converter with Output-Derived Boost Voltage
24
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VIN BOOST
8 7 6 5
1000pF D2 CMDSH-3 0.1F
+
RSENSE 0.05 L1a* VOUT
CIN 22F 35V x 2 22F 35V
+
D1 MBRS130LT3
CB 0.1F
M1 Si6426DQ
L1b* R2 35.7k 1% R1 11k 1%
VOUT 5V 1A
+
COUT 100F 16V x2
1624 F17
RSENSE1, 0.015 RSENSE2, 0.015 C4, 0.1F LTC1624 SENSE - ITH /RUN VFB GND VIN BOOST TG SW 8 7 6 5 CB 0.1F M1* L1 R5 220 Z1 IN 755 D2 MBR0540 C9 0.1F VOUT 12V 10A R2 100k 1% C10 220pF C5 3.3F 50V C7 3.3F 50V
D1*
+
COUT 2700F 16V
1624 F18
M1 = INTERNATIONAL RECTIFIER IRL3803 RSENSE1, RSENSE2 = IRC LR2010-01-R015-F * BOTH D1 AND M1 MOUNTED TO SAME THERMALLOY #6399B HEAT SINK
LTC1624
TYPICAL APPLICATIONS
VIN 20V TO 32V
+
CC 820pF
RC 6.8k
R1 13.3k
CIN = KEMET T495X226M035AS COUT = SANYO 100MV100GX D1 = MOTOROLA MBRS1100
VIN 9V TO 15V
+
CC 4700pF
RC 27k
R1 52.3k
C3 100pF
CIN = KEMET T495X107M016AS COUT1, COUT2 = SANYO 35MV 1000GX D1 = MOTOROLA MBR2535CT
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CIN 22F 35V
RSENSE 0.025 C5 0.1F LTC1624 1 2 3 VFB 4 SENSE - ITH /RUN VIN BOOST TG SW 8 7 6 5 CB 0.1F
L1 47H D1 VOUT 90V 0.5A
+
M1
COUT 100F 100V
C3 100pF
GND
R2, 1M, 1% L1 = COILCRAFT D05022P-473 M1 = INTERNATIONAL RECTIFIER IRL 540NS RSENSE = IRC LR2010-01-R025-F
1624 F19
Figure 19. 24V to 90V at 0.5A Boost Converter
CIN 100F 16V
RSENSE 0.005, 5% C5 0.1F LTC1624 1 2 3 VFB 4 SENSE - ITH /RUN VIN BOOST TG SW 8 7 6 5 CB 0.1F
L1 10H D1* VOUT 24V 5A R5 750 0.5W
M1*
C4 1500pF
+
GND
COUT1 1000F 35V
+
COUT2 1000F 35V
Z1 IN755 7.5V
R2, 1M, 1% L1 = MAGNETICS CORE #55930AZ WINDING = 8T#14BIF M1 = INTERNATIONAL RECTIFIER IRL 3803 RSENSE = IRC OAR-3, 0.005, 5% *BOTH D1 AND Q1 MOUNTED ON THERMALLOY MODEL 6399 HEAT SINK
1624 F20
Figure 20. 12V to 24V/5A Boost Converter
25
LTC1624
TYPICAL APPLICATIONS
VIN 13V TO 28V
+
CIN1, CIN2 22F 35V x2
RSENSE 0.033 LTC1624
CC 330pF
1 2 3
SENSE - ITH /RUN VFB
RC 10k
R1 3.92k
C4 100pF
4
GND
Q2 1 2 3 4 SENSE IOUT
PROG LTC1620 GND VCC -IN +IN
CIN1, CIN2 = KEMET T495X226M035AS L1 = SUMIDA CDRH127-270 RSENSE = IRC LR2010-01-R033-F R4 = IRC LR2010-01-R025-F M1 = SILICONIX Si4412DY Q2 = MOTOROLA MMBT A14
Figure 21. 12V/3A Adjustable Current Power Supply for Battery Charger or Current Source Applications
26
U
+VIN
C5, 0.1F
VIN BOOST TG SW
8 7 6 5 CB 0.1F M1 L1 27H R4 0.025 VOUT 12V 3A
D1 MBRS340
+
COUT 100F 16V x2
R2 35.7k
C9 100pF
C10 0.1F AVE 8 7 6 5
C11 0.1F
R6 10k C12 1F R7 56k
1 2 3 4
OUT
IN
8 7
+VIN
NC/ADJ NC LT1121-5 6 GND NC NC SHDN 5
C13 0.1F
C14, 0.01F
CURRENT ADJ
R8 1M
1624 F21
LTC1624
TYPICAL APPLICATIONS
1 2 CC 680pF RC 3.3k 100pF 4 GND SW 5 3 VFB SENSE - ITH /RUN VIN BOOST TG 8 7 6 CB 0.1F D1 MBRD835L 1000pF 0.1F VIN 4.8V TO 28V
* PANASONIC 12TS-7ROLB ** SILICONIX SUD50N03-10
PACKAGE DESCRIPTION
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP
0.016 - 0.050 0.406 - 1.270
*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
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.
U
U
RSENSE 0.015
+
LTC1624
CIN 22F 35V x3
M1** L1* 8H R2 35.7k 1% R1 20k 1% VOUT 3.3V 6.5A
+
COUT 100F 10V x3
1624 F22
Figure 22. High Current 3.3V/6.5A Converter
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 0.053 - 0.069 (1.346 - 1.752)
2
3
4
0.004 - 0.010 (0.101 - 0.254)
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
SO8 0996
27
LTC1624
TYPICAL APPLICATION
R3, 10 RSENSE 0.0082 C3, 0.033F LTC1624 CC 820pF 1 2 3 VFB RC 6.8k R1 20k C2 100pF 4 GND SENSE - ITH /RUN VIN BOOST TG SW 8 7 6 5 D2 CB 0.1F M1 L1 1.68H COUT 470F 6.3V x2 R2 35.7k C8 100pF VOUT RTN Q2 10k
1624 F23
CIN (x 4) = KEMET T495D107M010AS COUT (x 2) = AVX TPSV477M006R0055 D1 = MOTOROLA MBRB2515L D2 = MOTOROLA MBR0520
RELATED PARTS
PART NUMBER LTC1147 LTC1148HV/LTC1148 LTC1149 LTC1159 LTC1174 LTC1265 LTC1266 LT (R)1375/LT1376 LTC1433/LTC1434 LTC1435 LTC1436/LTC1436-PLL LTC1474/LTC1475 DESCRIPTION High Efficiency Step-Down Controller High Efficiency Synchronous Step-Down Controllers High Efficiency Synchronous Step-Down Controller High Efficiency Synchronous Step-Down Controller Monolithic 0.6A Step-Down Switching Regulator 1.2A Monolithic High Efficiency Step-Down Switching Regulator High Efficiency Synchronous Step-Down Controller, N-Channel Drive 1.5A, 500kHz Step-Down Switching Regulators Monolithic 0.45A Low Noise Current Mode Step-Down Switching Regulators High Efficiency Low Noise Synchronous Step-Down Controller, N-Channel Drive High Efficiency Low Noise Synchronous Step-Down Controllers, N-Channel Drive Ultralow Quiesent Current Step-Down Monolithic Switching Regulators COMMENTS 100% DC, Burst Mode Operation, 8-Pin SO and PDIP 100% DC, Burst Mode Operation, VIN < 20V 100% DC,Std Threshold MOSFETs, VIN < 48V 100% DC, Logic Level MOSFETs, VIN < 40V 100% DC, Burst Mode Operation, 8-Pin SO 100% DC, Burst Mode Operation, 14-Pin SO 100% DC, Burst Mode Operation, VIN < 20V High Frequency 16- and 20-Pin Narrow SSOP Burst Mode Operation, 16-Pin Narrow SO Adaptive PowerTM Mode, 20- and 24-Pin SSOP 100% DC, 8-Pin MSOP, VIN < 20V
Adaptive Power is a trademark of Linear Technology Corporation.
28
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417q (408)432-1900 FAX: (408) 434-0507q TELEX: 499-3977 q www.linear-tech.com
U
+
C4, 0.1F
CIN 100F 10V x4
+VIN 4.5V TO 5.5V
+VOUT 3.3V 10A
+
D1
L1 = PULSE ENGINEERING PE53691 M1 = INTERNATIONAL RECTIFIER IRL3803S Q2 = MOTOROLA MMBTA14LT1 RSENSE = IRC OAR3-R0082
Figure 23. 5V to 3.3V/10A Converter (Surface Mount)
1624f LT/TP 0198 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1997

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