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LTC1626 Low Voltage, High Efficiency Step-Down DC/DC Converter
FEATURES
s s s s
DESCRIPTION
The LTC (R)1626 is a monolithic, low voltage, step-down current mode DC/DC converter featuring Burst ModeTM operation at low output current. The input supply voltage range of 2.5V to 6V makes the LTC1626 ideal for single cell Li-Ion and 3- or 4-cell NiCd/ NiMH applications. A built-in 0.32 switch (VIN = 4.5V) allows up to 0.6A of output current. The LTC1626 incorporates automatic power saving Burst Mode operation to reduce gate charge losses when the load current drops below the level required for continuous operation. With no load, the converter draws only 165A. In shutdown, it draws a mere 0.5A--making it ideal for current sensitive applications. The inductor current is user-programmable via an external current sense resistor. In dropout, the internal P-channel MOSFET switch is turned on continuously, maximizing battery life.
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Wide Input Supply Voltage Range: 2.5V to 6V High Efficiency: Up to 95% Low RDS(ON) Internal Switch: 0.32 (VIN = 4.5V) Current Mode Operation for Excellent Line and Load Transient Response Short-Circuit Protected Low Dropout Operation: 100% Duty Cycle Built-In Low-Battery Detector Low Quiescent Current at Light Loads: IQ = 165A Ultralow Shutdown Current: IQ = 0.5A Peak Inductor Current Independent of Inductor Value Available in 14-Pin SO Package
APPLICATIONS
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Single Cell Li-Ion Step-Down Converters 3- or 4-Cell NiMH Step-Down Converters Cellular Telephones 5V to 3.3V Conversion 3.3V to 2.5V Conversion Inverting Converters Portable Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
VIN 2.7V TO 6V
+ CIN
47F 16V
0.1F PWR VIN SHDN LTC1626 VIN SW PGND 470 ITH CT SENSE + SENSE - SGND VFB 100pF 10k
1626 F01
100
L* 33H D1 MBRS130LT
RSENSE** 0.1
VOUT 2.5V 0.25A
95
3900pF
+
COUT 100F 6.3V
EFFICIENCY (%)
90 85 80 75 70 0.01
1000pF
10k
CT 270pF * COILTRONICS CTX33-4 ** IRC 1206-R100F AVX TPSD476KO16 AVX TPSC107M006R0150
Figure 1. High Efficiency 2.5V Step-Down Converter
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Efficiency
VIN = 3.5V L1 = 33H VOUT = 2.5V RSENSE = 0.1 CT = 270pF 0.1 OUTPUT CURRENT (A) 1
1626 F01a
1
LTC1626
ABSOLUTE MAXIMUM RATINGS
(Voltages Referred to GND Pin) Input Supply Voltage (Pins 1, 2, 13) ............- 0.3V to 7V Shutdown Input Voltage (Pin 10) ................- 0.3V to 7V Sense-, Sense+ (Pins 7, 8)........... - 0.3V to (VIN + 0.3V) LBO, LBI (Pins 3, 4) .................................... - 0.3V to 7V CT, ITH, VFB (Pins 5, 6, 9) ............. - 0.3V to (VIN + 0.3V) DC Switch Current (Pin 14) .................................... 1.2A Peak Switch Current (Pin 14) ................................. 1.6A Switch Voltage (Pin 14) .......(VIN - 7.5V) to (VIN + 0.3V) Operating Temperature Range ..................... 0C to 70C Extended Commercial Operating Temperature Range (Note 4) ............. - 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 PWR VIN 1 VIN 2 LBO 3 LBI 4 CT 5 ITH 6 SENSE - 7 14 SW 13 PWR VIN 12 PGND 11 SGND 10 SHDN 9 VFB 8 SENSE +
ORDER PART NUMBER LTC1626CS
S PACKAGE 14-LEAD PLASTIC SO
TJMAX = 125C, JA = 110C/ W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL IFB VFB VOUT PARAMETER Feedback Pin Current Feedback Voltage Output Voltage Line Regulation Output Voltage Load Regulation Burst Mode Output Ripple IQ Input DC Supply Current (Note 2) Active Mode Sleep Mode Shutdown Low-Battery Trip Point Low-Battery Input Bias Current Low-Battery Output Sink Current Current Sense Threshold Voltage VSENSE + - VSENSE- ON Resistance of Switch Switch Off-Time (Note 3) SHDN Pin High SHDN Pin Low SHDN Pin Input Current VLBO = 0.4V CONDITIONS
TA = 25C, VIN = 4.5V, VOUT = 2.5V, VSHDN = 0V, unless otherwise specified.
MIN
q q
TYP 0.1 1.25 0 25 50
MAX 1 1.28 1.3 40 50
UNITS A V V mV mV mVP-P mA A A V A mA mV mV s V V A
0C to 70C - 40C to 85C VIN = 3.5V to 5.5V, ILOAD = 250mA 10mA ILOAD 250mA ILOAD = 0
1.22 1.2 - 40
q q
VSHDN = VIN 1.15 0.4 130 4 VIN - 0.4
1.9 165 0.5 1.25 1.4 25 155 0.32 5
3.0 300 5 1.35 0.5
VLBTRIP ILBI ILBO VSENSE RON tOFF VIHSD VILSD IINSD
VSENSE - = 2.5V, VFB = VOUT/2 + 25mV (Forced) VSENSE - = 2.5V, VFB = VOUT/2 - 25mV (Forced) CT = 390pF, ILOAD = 400mA Minimum Voltage for Device to Be Shut Down Maximum Voltage for Device to Be Active 0V VSHDN 7V
q
180 0.45 6 0.4 1
The q denotes specifications that apply over the specified operating temperature range. Note 1: TJ is calculated from the ambient temperature TA and power dissipation according to the following formula: TJ = TA + (PD * 110C/W) Note 2: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency.
Note 3: In applications where RSENSE is placed at ground potential, the off-time increases by approximately 40%. Note 4: C grade device specifications are guaranteed over the 0C to 70C temperature range. In addition, C grade device specifications are assured over the - 40C to 85C temperature range by design or correlation, but are not production tested.
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LTC1626 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Input Voltage (VOUT = 2.5V)
100 98 96
EFFICIENCY (%)
L1 = 33H RSENSE = 0.1 CT = 270pF IOUT = 100mA
EFFICIENCY (%)
EFFICIENCY (%)
94 92
90 I OUT = 250mA 88 86 84 82 80 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 INPUT VOLTAGE (V)
1626 G01
Operating Frequency
2.0 1.8 FIGURE 1 CIRCUIT 1.0 0.9
NORMALIZED FREQUENCY
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 INPUT VOLTAGE (V)
1626 G04
0.7
LEAKAGE CURRENT (A)
1.6
RDS(ON) ()
DC Supply Current*
5.0 4.5 4.0 TJ = 25C * DOES NOT INCLUDE GATE CHARGE CURRENT 0.50 0.45 0.40
SUPPLY CURRENT (mA)
SUPPLY CURRENT (A)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 SLEEP MODE ACTIVE MODE
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 INPUT VOLTAGE (V)
1626 G08
OUTPUT VOLTAGE (V)
0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 INPUT VOLTAGE (V)
1626 G07
UW
Efficiency vs Output Current (VOUT = 3.3V)
100 95 90 85 80 75 L1 = 33H VIN = 5V VOUT = 3.3V RSENSE = 0.1 CT = 270pF 0.1 OUTPUT CURRENT (A) 1
1626 G02
Efficiency vs Input Voltage (VOUT = 3.3V)
100 98 96 94 92 90 88 86 84 82 L1 = 33H RSENSE = 0.1 CT = 270pF IOUT = 250mA IOUT = 100mA
70 0.01
80 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 INPUT VOLTAGE (V)
1626 G03
Switch Resistance
100 90 80 70 60 50 40 30 20 10 0
Switch Leakage Current
VIN = 4.5V
0.8
0.6 0.5 0.4 0.3 0.2 0.1 0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 INPUT VOLTAGE (V)
1626 G05
TJ = 70C
TJ = 25C
TJ = 0C
0
10 20 30 40 50 60 70 80 90 100 JUNCTION TEMPERATURE (C)
1626 G06
Supply Current in Shutdown
5.0
TJ = 25C SHUTDOWN = VIN
Low Voltage Behavior
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V)
1626 G09
L1 = 33H RSENSE = 0.1 CT = 270pF TJ = 25C ILOAD = 250mA
VOUT = 3.3V VOUT = 2.5V
3
LTC1626
PIN FUNCTIONS
PWR VIN (Pins 1, 13): Supply for the Power MOSFET and Its Driver. Decouple this pin properly to ground. VIN (Pin 2): Main Supply for All the Control Circuitry in the LTC1626. LBO (Pin 3): Open-Drain Output of the Low-Battery Comparator. This pin will sink current when Pin 4 (LBI) goes below 1.25V. During shutdown, this pin is high impedance. LBI (Pin 4): The (-) Input of the Low-Battery Comparator. The (+) input is connected to a reference voltage of 1.25V. If not used, connect to VIN. CT (Pin 5): External capacitor CT from Pin 5 to ground sets the switch off-time. The operating frequency is dependent on the input voltage and CT. ITH (Pin 6): Feedback Amplifier Decoupling Point. The current comparator threshold is proportional to Pin 6 voltage. SENSE - (Pin 7): Connects to the (-) Input of the Current Comparator. SENSE + (Pin 8): The (+) Input to the Current Comparator. A built-in offset between Pins 7 and 8 in conjunction with RSENSE sets the current trip threshold. VFB (Pin 9): This pin serves as the feedback pin from an external resistive divider used to set the output voltage. SHDN (Pin 10): Shutdown Pin. Pulling this pin to VIN keeps the internal switch off and puts the LTC1626 in micropower shutdown. If not used, connect to SGND. SGND (Pin 11): Small-Signal Ground. Must be routed separately from other grounds to the (-) terminal of COUT. PWR GND (Pin 12): Switch Driver Ground. Connects to the (-) terminal of CIN. SW (Pin 14): Drain of the P-Channel MOSFET Switch. Cathode of the Schottky diode must be connected closely to this pin.
BLOCK DIAGRAM
SLEEP
+
S
-
VTH2 VTH1
5 CT
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PWR VIN 1 13
SENSE + 8
SENSE - 7
PWR GND 12 14 SW
-
V
9
VFB
+
R Q S ITH 6
-
C
+
25mV TO 150mV
+
13k G
VOS
- + +
REFERENCE 10 SHDN 4 LBI
1626 BD
-
T
VIN 2 OFF-TIME CONTROL
LBO 3 A3
+
SENSE - VFB SGND 11
-
LTC1626
OPERATIO
The nominal off-time of the LTC1626 is set by an external timing capacitor connected between the CT pin and ground. The operating frequency is then determined by the offtime and the difference between VIN and VOUT. The output voltage is set by an external divider returned to the VFB pin. A voltage comparator V and a gain block G compare the divided output voltage with a reference voltage of 1.25V. To optimize efficiency, the LTC1626 automatically switches between continuous and Burst Mode operation. The voltage comparator is the primary control element when the device is in Burst Mode operation, while the gain block controls the output voltage in continuous mode. When the load is heavy, the LTC1626 is in continuous operation. During the switch "ON" time, current comparator C monitors the voltage between the SENSE + and SENSE - pins connected across an external shunt in series with the inductor. When the voltage across the shunt reaches the comparator's threshold value, its output signal changes state, resetting the flip-flop and turning the internal P-channel MOSFET off. The timing capacitor connected to the CT pin is now allowed to discharge at a rate determined by the off-time controller. When the voltage on the timing capacitor has discharged past VTH1, comparator T trips, sets the flip-flop and causes the switch to turn on. Also, the timing capacitor is recharged. The inductor current will again ramp up until the current comparator C trips. The cycle then repeats. When the load current increases, the output voltage
APPLICATIONS INFORMATION
The basic LTC1626 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of RSENSE. Once RSENSE is known, CT and L can be chosen. Next, the Schottky diode D1 is selected followed by CIN and COUT. RSENSE Selection for Output Current RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage developed across RSENSE, the threshold of the comparator determines the peak inductor current. Depending upon the load current condition, the threshold of the comparator lies between 25mV/RSENSE and 150mV/RSENSE. The maximum output current of the LTC1626 is: IOUT(MAX) = 150mV/RSENSE - IRIPPLE/2 (A) Where IRIPPLE is the peak-to-peak inductor ripple current. At a relatively light load, the LTC1626 is in Burst Mode operation. In this mode, the peak current is set at 25mV/ RSENSE. To fully benefit from Burst Mode operation, the
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decreases slightly. This causes the output of the gain stage (Pin 6) to increase the current comparator threshold, thus tracking the load current. When the load is relatively light, the LTC1626 automatically switches to Burst Mode operation. The current loop is interrupted when the output voltage reaches the desired regulated value. The hysteretic voltage comparator V trips when VOUT is above the desired output voltage, turning off the switch and causing the timing capacitor to discharge. This capacitor discharges past VTH1 until its voltage drops below VTH2. Comparator S then trips and a sleep signal is generated. The circuit now enters into sleep mode with the power MOSFET turned off. In sleep mode, the LTC1626 is in standby and the load current is supplied by the output capacitor. All unused circuitry is shut off, reducing quiescent current from 1.9mA to 165A. When the output capacitor discharges by the amount of the hysteresis of the comparator V, the P-channel switch turns on again and the process repeats itself. During Burst Mode operation, the peak inductor's current is set at 25mV/RSENSE. To avoid the operation of the current loop interfering with Burst Mode operation, a built-in offset VOS is incorporated in the gain stage. This prevents the current from increasing until the output voltage has dropped below a minimum threshold. In dropout, the P-channel MOSFET is turned on continuously (100% duty cycle) providing low dropout operation with VOUT VIN.
5
LTC1626
APPLICATIONS INFORMATION
inductor current should be continuous during burst periods. Hence, the peak-to-peak inductor ripple current must not exceed 25mV/RSENSE. To account for light load conditions, the IOUT(MAX) is then given by: IOUT(MAX) = 150mV/RSENSE - 25mV/2RSENSE (A) = 137.5mV/RSENSE (A) Solving for RSENSE and allowing a margin of variations in the LTC1626 and external component values yields: RSENSE = 100mV/IOUT(MAX) () The LTC1626 switch is capable of supplying a maximum of 1.2A of output current. Therefore, the minimum value of RSENSE that can be used is 0.083. A graph for selecting RSENSE versus maximum output current is given in Figure 2.
0.5
0.4
RSENSE ()
0.3
0.2
0.1
0 0 0.2 0.4 0.6 0.8 MAXIMUM OUTPUT CURRENT (A) 1.0
1626 F02
Figure 2. Selecting RSENSE
During a short circuit of the regulator output to ground, the peak current is determined by: ISC = 150mV/RSENSE (A) In this condition, the LTC1626 automatically extends the off-time period of the P-channel MOSFET switch to allow the inductor current to decay far enough to prevent any current buildup. The resulting ripple current causes the average current to be approximately IOUT(MAX).
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Operating Frequency Considerations For most applications, the LTC1626 should be operated in the 100kHz to 300kHz range. This range can be extended, however, up to 600kHz, to accommodate smaller size/ valued inductors, such as low profile types, with a slight decrease in efficiency due to gate charge losses. Some experimentation may be required to determine the optimum operating frequency for a particular set of external components and operating conditions. CT and L Selection The value of CT is calculated from the desired continuous mode operating frequency:
CT =
(VIN - VOUT) F (VIN + VD)(3300)(VIN - VBE)(fO) ( )
where VD is the drop across the Schottky diode D1 and VBE is a base-emitter voltage drop (0.6V). The complete expression for operating frequency is given by:
1 VIN - VOUT fO t OFF VIN + VD
(Hz)
) (sec)
where:
t OFF = 3300 CT VIN - VBE
(
)( )(
Figure 3 is a graph of operating frequency versus power supply voltage for the 2.5V regulator circuit shown in Figure 1 (CT = 270pF). Note that the frequency is relatively constant with supply voltage but drops as the supply voltage approaches the regulated output voltage. To maintain continuous inductor current at light load, the inductor must be chosen to provide no more than 25mV/ RSENSE of peak-to-peak ripple current. This results in the following expression for L: L 5.2 105 RSENSE CT VREG
()
(
)( )( ) (H)
Using an inductance smaller than the above value will result in inductor current being discontinuous. As a con-
LTC1626
APPLICATIONS INFORMATION
sequence, the LTC1626 will delay entering Burst Mode operation and efficiency will be degraded at low currents.
200 180 160 FIGURE 1 CIRCUIT
FREQUENCY (kHz)
140 120 100 80 60 40 20 0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 INPUT VOLTAGE (V)
1626 F03
Figure 3. Operating Frequency vs Supply Voltage for Circuit Shown in Figure 1
Inductor Core Selection With the value of L selected, the type of inductor must be chosen. Basically, there are two kinds of losses in an inductor--core and copper losses. Core losses are dependent on the peak-to-peak ripple current and core material. However, they are independent of the physical size of the core. By increasing inductance, the peak-to-peak inductor ripple current will decrease, therefore reducing core loss. Utilizing low core loss material, such as molypermalloy or Kool M(R) will allow the user to concentrate on reducing copper loss and preventing saturation. Although higher inductance reduces core loss, it increases copper loss as it requires more windings. When space is not a premium, larger wire can be used to reduce the wire resistance. This also prevents excessive heat dissipation in the inductor. Catch Diode Selection Losses in the catch diode depend on forward drop and switching times. Therefore, Schottky diodes are a good choice for low drop and fast switching times. The catch diode carries the load current during the offtime. The average diode current is therefore dependent on
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the P-channel switch duty cycle. At high input voltages, the diode conducts most of the time. As VIN approaches VOUT, the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the regulator output is shorted to ground. Under short-circuit conditions, the diode must safely handle ISC(PK) at close to 100% duty cycle. Most LTC1626 circuits will be well served by either an MBRM5819 or an MBRS130LT3. An MBR0520LT1 is a good choice for IOUT(MAX) 500mA. Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low effective series resistance (ESR) input capacitor must be used. In addition, the capacitor must handle a high RMS current. The CIN RMS current is given by:
IOUT VOUT VIN - VOUT VIN
IRMS
[(
)]
1/ 2
( A)
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst case is commonly used to 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 lifetime. This make it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Do not underspecify this component. An additional 0.1F ceramic capacitor is also required on PWR VIN for high frequency decoupling. Output Capacitor (COUT) Selection The selection of COUT is driven by the ESR for proper operation of the LTC1626. The required ESR of COUT is: ESRCOUT < 50mV/IRIPPLE where IRIPPLE is the ripple current of the inductor. For the case where the IRIPPLE is 25mV/RSENSE, the required ESR of COUT is:
Kool M is a registered trademark of Magnetics, Inc.
7
LTC1626
APPLICATIONS INFORMATION
ESRCOUT < 2RSENSE To avoid overheating, the output capacitor must be sized to handle the ripple current generated by the inductor. The worst-case RMS ripple current in the output capacitor is given by: IRMS < 150mV/2RSENSE (ARMS) Generally, once the ESR requirements for COUT have been met, the RMS current rating far exceeds the IRIPPLE requirement. In some surface mount applications, multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS current handling requirement of the application. Aluminum electrolyte 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 PL series and Sprague 595D series. Consult the manufacturer for other specific recommendations. When the capacitance of COUT is made too small, the output ripple at low frequencies will be large enough to trip the voltage comparator. This causes Burst Mode operation to be activated when the LTC1626 would normally be in continuous mode operation. The effect will be most pronounced with low RSENSE values and can be improved at higher frequencies. Low-Battery Detection The low-battery detector senses the input voltage through an external resistive divider. This divided voltage connects to the (-) input of a voltage comparator (LBI) and is compared to an internal 1.25V reference voltage. Neglecting LBI input bias current, the following expression is used for setting the trip voltage threshold:
R4 VLB _ TRIP = 1.25 1 + R3
1.25V LBI
CFILTER 0.01F
R3 1%
Figure 4. Low-Battery Comparator
Setting the Output Voltage The LTC1626 develops a 1.25V reference voltage between the feedback pin VFB and the signal ground as shown in Figure 5. By selecting resistor R1, a constant current is caused to flow through R1 and R2 which sets the desired output voltage. The regulated output voltage is determined by:
R2 VOUT = 1.25 1 + R1
R1 should be 10k to ensure that sufficient current flows through the divider to maintain accuracy and to provide a minimum load for the regulator output at elevated temperatures. (See Switch Leakage Current curve in Typical Performance Characteristics section.)
To prevent stray pickup, a 100pF capacitor is suggested across R1, located close to the LTC1626.
VOUT R2 1% LTC1626 SGND VFB 100pF R1 10k 1%
1626 F05
Figure 5. Setting the Output Voltage
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The LBO is an N-channel open drain that goes low when the battery voltage drops below the threshold voltage. In shutdown, the comparator is disabled and LBO is in the high impedance state. Figure 4 is a schematic diagram detailing the low-battery comparator connection and operation.
VIN R4 1% LTC1626 LBO
1626 F04
LTC1626
APPLICATIONS INFORMATION
Thermal Considerations In a majority of applications, the LTC1626 does not dissipate much heat due to its high efficiency. However, in applications where the switching regulator is running at high duty cycles or the part is in dropout with the switch turned on continuously (DC), some thermal analysis is required. The goal of the thermal analysis is to determine whether the power dissipated by the regulator exceeds the maximum junction temperature. The temperature rise is given by: TRISE = PD * JA where PD is the power dissipated by the regulator and JA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature is given by: TJ = TRISE + TAMBIENT As an example, consider the case when the LTC1626 is in dropout at an input voltage of 3V with a load current of 0.5A. From the Typical Performance Characteristics graph of Switch Resistance, the ON resistance of the P-channel switch is 0.45. Therefore, power dissipated by the part is: PD = I2 * RDS(ON) = 113mW The SO package junction-to-ambient thermal resistance JA is 110C/W. Therefore, the junction temperature of the regulator when it is operating in a 25C ambient temperature is: TJ = (0.113 * 110) + 25 = 38C Remembering that the above junction temperature is obtained from an RDS(ON) at 25C, we might recalculate the junction temperature based on a higher RDS(ON) since it increases with temperature. However, we can safely assume that the actual junction temperature will not exceed the absolute maximum junction temperature of 125C. Board Layout Considerations When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1626. These items are also illustrated graphically in the layout diagram of Figure 6. Check the following in your layout: 1. Are the signal and power grounds separated? The LTC1626 signal ground (Pin 11) must return to the (-) plate of COUT. The power ground (Pin 12) returns to the anode of the Schottky diode and the (-) plate of CIN. 2. Does the (+) plate of CIN connect to the power VIN (Pins 1, 13) as close as possible? This capacitor provides the AC current to the internal P-channel MOSFET and its driver.
VIN 1 PWR VIN 2 VIN 14 SW PWR VIN 13 0.1F PGND SGND SHDN VFB 12 11 10 9 R2 SHUTDOWN R1 COUT RSENSE VOUT BOLD LINES INDICATE HIGH CURRENT PATHS
LTC1626 3 1k 3900pF CT 4 5 6 7 LBO LBI CT ITH SENSE -
8 SENSE + 1000pF
Figure 6. LTC1626 Layout Diagram (See Board Layout Checklist)
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CIN
D1 L
1626 F06
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LTC1626
APPLICATIONS INFORMATION
3. Is the input decoupling capacitor (0.1F) connected closely between power VIN (Pins 1, 13) and power ground (Pin 12)? This capacitor carries the high frequency peak currents. 4. Is the Schottky diode closely connected between the power ground (Pin 12) and switch output (Pin 14)? 5. Does the LTC1626 SENSE - (Pin 7) connect to a point close to RSENSE and the (+) plate of COUT? The resistor divider R1-R2 must be connected between the (+) plate of COUT and the signal ground. 6. Are the SENSE - and SENSE + leads routed together with minimum PC trace spacing? The 1000pF capacitor between Pin 7 and Pin 8 should be as close as possible to the LTC1626. 7. Is SHDN (Pin 10) actively pulled to ground during normal operation? The shutdown pin is high impedance and must not be allowed to float.
TYPICAL APPLICATIONS
Single Cell Li-Ion to 2.5V Converter
(VIN = 2.7V TO 4.5V) SINGLE Li-ION CELL
+
PWR VIN LBI LBO VIN SW 0.1F
SHUTDOWN
SHDN LTC1626 ITH
PGND SENSE + 1000pF SENSE - 10k
1k 3900pF
CT 270pF CT SGND
VFB 100pF 10k
1626 TA01
* SUMIDA CDRH62-220 ** IRC 1206-R100F AVX TPSD476K016 AVX TPSD107K010
3- to 4-Cell NiCd/NiMH to 2.5V Converter
(VIN = 2.7V TO 6V)
+
3- OR 4-CELL NiCd OR NiMH R4 PWR VIN LBI R3 SHUTDOWN LBO SHDN LTC1626 ITH 1k 3900pF CT 270pF CT SGND SENSE + 1000pF SENSE - VFB 100pF PGND VIN SW 0.1F
* SUMIDA CDRH62-220 ** IRC 1206-R100F AVX TPSD476K016 AVX TPSD107K010
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CIN 47F 16V
L1* 22H D1 MBR0520LT1
RSENSE** 0.1
VOUT 2.5V 0.25A
+
COUT 100F 10V
+
CIN 47F 16V
L1* 22H D1 MBR0520LT1
RSENSE** 0.1
VOUT 2.5V 0.25A
+
R1 10k R2 10k
COUT 100F 10V
1626 TA02
FOR 3.3V: R1 = 15k, 1% R2 = 9.09k, 1%
LTC1626
TYPICAL APPLICATIONS
Low Profile (3mm Maximum Height) 2.8V Converter
VIN 3V TO 6V PWR VIN LBI LBO SHUTDOWN SHDN LTC1626 ITH 1k 3900pF CT 56pF CT SGND SENSE + 1000pF SENSE - VFB 100pF R2 12.1k 1% R1 15k 1% PGND VIN SW
* COILCRAFT DO3308-153 ** IRC 1206-R100F AVX TPSC226M016R0375 AVX TPSC107M006R0150 MURATA GRM230Y5V475Z16
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
4.7F CERAMIC
+
CIN 22F 16V TANT
L1* 15H D1 MBR0520LT1
RSENSE** 0.1
VOUT 2.8V 0.25A
+
COUT 100F 6.3V
1626 TA03
Dimensions in inches (millimeters) unless otherwise noted.
S Package 14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.337 - 0.344* (8.560 - 8.738) 14 13 12 11 10 9 8
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
1
2
3
4
5
6
7
0.053 - 0.069 (1.346 - 1.752)
0.004 - 0.010 (0.101 - 0.254)
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
S14 0695
11
LTC1626
TYPICAL APPLICATIONS
Single Li-Ion to 3.3V Buck-Boost Converter
L1B 3 2 TOP VIEW 4 L1B 1 L1A SHUTDOWN L1A SINGLE Li-ION CELL (VIN = 2.5V TO 4.2V)
+
PWR VIN LBI LBO SHDN LTC1626 ITH PGND VIN SW 0.1F
MANUFACTURER COILTRONICS DALE VIN (V) 2.5 3.0 3.5 4.0 4.2
PART NO. CTX33-4 LPT4545-330LA I OUT (mA) 200 350 500* 500* 500*
1k 3900pF
*DESIGN LIMIT
AVX TPSE107M016R0100 AVX TPSD107M010R0065
* IRC 1206-R100F
VIN 5V PWR VIN LBI LBO SHUTDOWN SHDN LTC1626 ITH 1k 3900pF CT 270pF CT SGND SENSE + 1000pF SENSE - VFB 100pF PGND VIN SW 0.1F
* COILCRAFT DO3316-473 ** IRC 1206-R100F AVX TPSD107K010 AVX TPSE227K010
RELATED PARTS
PART NUMBER LTC1174/LTC1174-3.3 LTC1174-5 LTC1265 LT1375/LT1376 LTC1435 LTC1436/LTC1436-PLL LTC1438/LTC1439 LTC1474/LTC1475 DESCRIPTION High Efficiency Step-Down and Inverting DC/DC Converters 1.2A, High Efficiency Step-Down DC/DC Converter 1.5A, 500kHz Step-Down Switching Regulators High Efficiency, Low Noise, Synchronous Step-Down Converter Dual, Low Noise, Synchronous Step-Down Converters Low Quiescent Current Step-Down DC/DC Converters COMMENTS Monolithic Switching Regulators, Burst Mode Operation Constant Off-Time Monolithic, Burst Mode Operation High Frequency, Small Inductor, High Efficiency 16-Pin Narrow SO and SSOP Multiple Output Capability Monolithic, IQ = 10A, 8-Pin MSOP
1626f LT/TP 0398 4K * PRINTED IN USA
High Efficiency, Low Noise, Synchronous Step-Down Converters 24-Pin Narrow and 28-Pin SSOP
12
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
+
CIN 100F 100F 16V 16V +
L1A 33H 1 D1 MBRS130LT1 2
VOUT 3.3V
4 L1B 33H
15k 1% 9.09k 1%
+
COUT 100F 10V
CT 75pF CT SENSE
-
VFB SGND SENSE + 1000pF
3
100pF
1626 TA05
RSENSE* 0.1
5V to 3.3V Converter
+
CIN 100F 10V
L1* 47H D1 MBRS130LT1
RSENSE** 0.1
VOUT 3.3V 0.5A
+
15k 1%
COUT 220F 10V
9.09k 1%
1626 TA04
(c) LINEAR TECHNOLOGY CORPORATION 1997

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