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LTC1574 LTC1574-3.3/LTC1574-5 High Efficiency Step-Down DC/DC Converters with Internal Schottky Diode
FEATURES
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DESCRIPTIO
High Efficiency: Up to 94% Usable in Noise-Sensitive Products Peak Inductor Current Independent of Inductor Value Short-Circuit Protection Internal Low Forward Drop Schottky Diode Only Three External Components Required Wide VIN Range: 4V to 18.5V (Absolute Maximum) Low Dropout Operation Low-Battery Detector Pin Selectable Current Limit Internal 0.9 Power Switch: VIN < 11V Standby Current: 130A Active Low Micropower Shutdown
The LTC(R)1574 is a family of easy-to-use current mode DC/DC converters ideally suited for 9V to 5V, 5V to 3.3V and inverting operation. With an internal 0.9 switch (at a supply voltage of 12V) and a low forward drop Schottky diode (0.450V typ at 200mA, TA = 25C), the LTC1574 requires only three external components to construct a complete high efficiency DC/DC converter. Under no load condition, the LTC1574 draws only 130A. In shutdown, it draws a mere 2A making this converter ideal for battery-powered applications. In dropout, the internal P-channel MOSFET switch is turned on continuously allowing the user to maximize the life of the battery source. The maximum inductor current of the LTC1574 family is pin selectable to either 340mA or 600mA, optimizing efficiency for a wide range of applications. Operation up to 200kHz permits the use of small surface mount inductors and capacitors. For applications requiring higher output current or ultrahigh efficiency, see the LTC1148 or LTC1265 data sheets. For detailed applications information, see the LTC1174 data sheet.
APPLICATIO S
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Inverting Converters Step-Down Converters Memory Backup Supply Portable Instruments Battery-Powered Equipment Distributed Power Systems
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN 5.5V to 16V 12 11 6
High Efficiency Step-Down Converter
LTC1574-5 Efficiency
100 95 L = 100H VOUT = 5V IPGM = 0V VIN = 6V
5 VIN LBIN LTC1574-5 LBOUT IPGM GND 2, 4, 13, 15 VOUT SW SHDN 7 10 3, 14 100H
+
EFFICIENCY (%)
22F* 35V
90 VIN = 9V 85 80 75 70
+
5V 175mA 100F* 10V
1574 TA01
* AVX TPSD226K035 ** AVX TPSD107K010 COILTRONICS CTX100-4
1
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10 LOAD CURRENT (mA) 100 200
1574 TA02
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1
LTC1574 LTC1574-3.3/LTC1574-5
ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW NC 1 GND 2 SW 3 GND 4 VIN 5 IPGM 6 SHDN 7 NC 8 16 NC 15 GND 14 SW 13 GND 12 LBIN 11 LBOUT 10 VOUT (VFB*) 9 NC
(Voltage Referred to GND Pin) Input Supply Voltage (Pin 5) ................. - 0.3V to 18.5V Switch Current (Pin 3, 14) ........................................ 1A Switch Voltage (Pin 3, 14) .......................... VIN - 18.5V Operating Temperature Range .................... 0C to 70C Junction Temperature (Note 2) ............................ 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LTC1574CS LTC1574CS-3.3 LTC1574CS-5
S PACKAGE 16-LEAD PLASTIC SO *ADJUSTABLE OUTPUT VERSION TJMAX = 125C, JA = 110C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER IFB VFB VOUT VOUT Feedback Current into Pin 10 Feedback Voltage Regulated Output Voltage Output Voltage Line Regulation Output Voltage Load Regulation CONDITIONS LTC1574 LTC1574 LTC1574-3.3 LTC1574-5
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 9V, SHDN = VIN, IPGM = 0V, unless otherwise specified.
MIN TYP MAX 1
q q q
UNITS A V V V mV mV mV mV mV A A A V A mA A mV A A s V
1.20 3.14 4.75
1.25 3.30 5.00 10 -5 - 45 -5 - 50 450 130 2 1.25
1.30 3.46 5.25 70 - 70 - 70 - 70 - 70 600 180 25 1.4 0.5
VIN = 6V to 12V, ILOAD = 100mA, IPGM = VIN (Note 3) LTC1574-3.3 (Note 3) LTC1574-5 (Note 3) 20mA < ILOAD < 175mA, IPGM = 0V 20mA < ILOAD < 400mA, IPGM = VIN 20mA < ILOAD < 175mA, IPGM = 0V 20mA < ILOAD < 400mA, IPGM = VIN
IQ
Input DC Supply Current (Note 4) Active Mode Sleep Mode Shutdown (Note 5) Low-Battery Trip Point Current into Pin 12 Current Sunk by Pin 11 Comparator Hysteresis Current Limit ON Resistance of Switch Switch Off Time SHDN Pin High SHDN Pin Low
4V < VIN < 16V, IPGM = 0V 4V < VIN < 16V SHDN = 0V, 4V < VIN < 16V
VLBTRIP ILBIN ILBOUT VHYST IPEAK RON tOFF VIH VIL
VLBOUT = 0.4V, VLBIN = 0V VLBOUT = 5V, VLBIN = 10V IPGM = VIN, VOUT = 0V IPGM = 0V, VOUT = 0V VOUT at Regulated Value Minimum Voltage at Pin 7 for Device to Be Active Maximum Voltage at Pin 7 for Device to Be in Shutdown
q q q
0.5 7.5 0.54 0.27
1.0 15 0.60 0.34 0.9
1.5 1.0 30 0.83 0.53 1.55 5
3 1.2
4
0.75
2
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LTC1574 LTC1574-3.3/LTC1574-5
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER IIH IIL VF IR SHDN Pin Input Current SHDN Pin Input Current Schottky Diode Forward Voltage Schottky Reverse Current CONDITIONS SHDN = 16V
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 9V, SHDN = VIN, IPGM = 0V, unless otherwise specified.
MIN TYP MAX 2 0.5 0.450 10 100 0.570 25 250 UNITS A A V A A
0 SHDN 0.8V Forward Current = 200mA Reverse Voltage = 5V Reverse Voltage = 18.5V
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formulas: TJ = TA + (PD * 110C/W)
Note 3: Guaranteed by design. Note 4: Does not include Schottky reverse current. Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 5: Current into Pin 5 only, measured without electrolytic input capacitor.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100 VIN = 5V 100 95
90
EFFICIENCY (%)
EFFICIENCY (%)
80
VIN = 9V
VIN = 9V 85 80 75 70 L = 50H VOUT = 5V IPGM = VIN COIL = CTX50-4 1 10 100 LOAD CURRENT (mA) 400
1574 * TPC02
EFFICIENCY (%)
70 L = 50H VOUT = 3.3V IPGM = VIN COIL = CTX50-4 1 10 100 LOAD CURRENT (mA) 500
1574 * TPC01
60
50
Efficiency Using Different Types of Inductor Core Material
100 CTX50-4 LEAKAGE CURRENT (nA) CTX50-4P 80
180 160
90
EFFICIENCY (%)
RDS(ON) ()
70
60
VIN = 5V VOUT = 3.3V IPGM = VIN 1 10 100 LOAD CURRENT (mA) 500
1574 * TPC04
50
UW
Efficiency vs Load Current
95 94
VIN = 6V
Efficiency vs Input Voltage
VOUT = 5V L = 100H COIL = CTX100-4
90
93 92 91 90 89 5 6 7 8 9 10 11 12 INPUT VOLTAGE (V) 13 14 ILOAD = 100mA IPGM = 0V
ILOAD = 300mA IPGM = VIN
1574 * TPC03
Switch Leakage Current vs Temperature
VIN = 13.5V 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0 20 40 60 TEMPERATURE (C) 80 100
1574 * TPC05
Switch Resistance vs Input Voltage
TA = 25C
140 120 100 80 60 40 20 0
0.7 4 6 8 10 12 14 16 INPUT VOLTAGE (V) 18 20
1574 * TPC06
3
LTC1574 LTC1574-3.3/LTC1574-5
PI FU CTIO S
NC (Pins 1, 8, 9, 16): No Connection. GND (Pins 2, 4, 13, 15): Ground. SW (Pins 3, 14): Drain of P-Channel MOSFET Switch and Cathode of Schottky Diode. VIN (Pin 5): Input Supply Voltage. It must be decoupled close to ground (Pin 4). IPGM (Pin 6): This pin selects the current limit of the P-channel switch. With IPGM = VIN, the current trip point is 600mA and with IPGM = 0V, the current trip point is reduced to 340mA. SHDN (Pin 7): Pulling this pin to ground keeps the internal switch off and puts the LTC1574 in micropower shutdown. VOUT or VFB (Pin 10): For the LTC1574, this pin connects to the main voltage comparator input. On the LTC1574-5 and LTC1574-3.3, this pin goes to an internal resistive divider which sets the output voltage. LBOUT (Pin 11): Open drain of an N-Channel Pull-Down. This pin will sink current when (Pin 12) LBIN goes below 1.25V. LBIN (Pin 12): The (-) Input of the Low-Battery Voltage Comparator. The (+) input is connected to a reference voltage of 1.25V.
APPLICATIO S I FOR ATIO
Operating Frequency and Inductor
Since the LTC1574 utilizes a constant off-time architecture, its operating frequency is dependent on the value of VIN. The frequency of operation can be expressed as:
f=
1 VIN - VOUT t OFF VIN + VD
(Hz)
IPGM = VIN 100mA/DIV
where tOFF = 4s and VD is the voltage drop across the internal Schottky diode. Note that the operating frequency is a function of the input and output voltage. Although the size of the inductor does not affect the frequency or inductor peak current, it does affect the ripple current. The peak-to-peak ripple current is given by:
V + VD IRIPPLE = 4 * 10 -6 OUT L
(AP-P )
When choosing a small inductor, core loss will increase due to higher ripple current. Therefore, a low ESR output capacitor has to be used. Short-Circuit Protection The LTC1574 is protected from output short circuits by its internal current limit. Depending on the condition of the
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IPGM pin, the limit is either set to 340mA or 600mA. In addition, the off-time of the switch is increased to allow the inductor current to decay far enough to prevent any current build-up (see Figure 1).
IPGM = 0
GND
L = 100H VIN = 13.5V
20s/DIV
1574 * F01
Figure 1. Inductor Current with Output Shorted
Low-Battery Detector The low-battery indicator senses the input voltage through an external resistive divider. This divided voltage connects to the "-" input of a voltage comparator (Pin 12) which is compared with a 1.25V reference voltage. With the current
LTC1574 LTC1574-3.3/LTC1574-5
APPLICATIO S I FOR ATIO
R4 VLBTRIP = 1.251 + R3
VIN R4 12 R3 LTC1574
going into Pin 12 being negligible, the following expression is used for setting the trip limit:
- +
1.25V REFERENCE
1574 * F02
Figure 2. Low-Battery Comparator
LTC1574 Adjustable Applications The LTC1574 develops a 1.25V reference voltage between the feedback terminal (Pin 10) and ground (see Figure 3). By selecting resistor R1, a constant current is caused to flow through R1 and R2 to set the overall output voltage. The regulated output voltage is determined by:
VOUT
R2 = 1.25 1 + R1
For most applications, a 30k resistor is suggested for R1. To prevent stray pickup, a 100pF capacitor is suggested across R1 located close to the LTC1574.
VOUT LTC1574 VFB 10 100pF
1574 * F03
R2
R1
Figure 3. LTC1574 Adjustable Configuration
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Inverting Applications The LTC1574 can easily be set up for a negative output voltage. If - 5V is desired, the LTC1574-5 is ideal for this application as it requires the least components. Figure 4 shows the schematic for this application. Note that the output voltage is now taken off the GND pins. Therefore, the maximum input voltage is now determined by the
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difference between the absolute maximum voltage rating and the output voltage. A maximum of 12V is specified in Figure 4, giving the circuit 1.5V of headroom for VIN. Note that the circuit can operate from a minimum of 4V, making it ideal for a four NiCd cell application. For a higher output current circuit, please refer to the Typical Applications section.
INPUT VOLTAGE 4V TO 12V 5 VIN 12 11 6 LBIN LTC1574-5 7 SHDN VOUT SW GND 2, 4, 13, 15 * AVX TPSD476K016 ** COILTRONICS CTX50-4 10 3, 14 50H** 0.1F
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47F* 16V x2
LBOUT IPGM
+
47F* 16V x2
VOUT -5V 45mA
1574 * F04
Figure 4. Positive-to-Negative 5V Converter
Low Noise Regulators In some applications it is important not to introduce any switching noise within the audio frequency range. Due to the nature of the LTC1574 during Burst ModeTM operation, there is a possibility that the regulator will introduce audio noise at some load currents. To circumvent this problem, a feed-forward capacitor can be used to shift the noise spectrum up and out of the audio band. Figure 5 shows the low noise connection with C2 being the feed-forward capacitor. The peak-to-peak output ripple is reduced to 30mV over the entire load range. A toroidal surface mount
Burst Mode is a trademark of Linear Technology Corporation VIN 5V
5 VIN LTC1574 LBIN LBOUT IPGM GND 2, 4, 13, 15 SHDN SW VFB 7 3, 14 10 L1** 100H
+
100F* 10V
11 6
56k
C2 6.8nF
VOUT 3.3V 425mA 100F* 10V
+
33k
* AVX TPSD107K010 ** COILTRONICS CTX100-4
1574 * F05
Figure 5. Low Noise 5V to 3.3V Regulator
5
LTC1574 LTC1574-3.3/LTC1574-5
APPLICATIO S I FOR ATIO
inductor L1 is chosen for its excellent self-shielding properties. Open magnetic structures such as drum and rod cores are to be avoided since they inject high flux levels into their surroundings. This can become a major source of noise in any converter circuit. Design Example As a design example, assume VIN = 9V (nominal), VOUT = 5V and IOUT = 350mA maximum. The LTC1574-5 is used for this application with IPGM (Pin 6) connected to VIN. The minimum value of L is determined by assuming the LTC1574-5 is operating in continuous mode.
INDUCTOR CURRENT
IPEAK AVG CURRENT = IOUT I +I = PEAK V IV 2 = 350mA
TIME
Figure 6. Continuous Inductor Current
With IOUT = 350mA and IPEAK = 0.6A (IPGM = VIN), IV = 0.1A. The peak-to-peak ripple inductor current, IRIPPLE, is 0.5A and is also equal to:
V + VD IRIPPLE = 4 * 10 -6 OUT L
(AP-P )
Solving for L in the above equation and with VD = 0.5V, L = 44H. The next higher standard value of L is 50H (example: Coiltronics CTX50-4). The operating frequency, ignoring voltage across diode VD is:
V f 2.5 * 105 1 - OUT VIN = 111kHz With the value of L determined, the requirements for CIN and COUT are calculated. For CIN, its RMS current rating should be at least:
IRMS =
IOUT VOUT VIN - VOUT VIN
[
(
)]
1/ 2
(A RMS)
= 174mA
6
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For COUT, the RMS current rating should be at least: IRMS I PEAK 2 = 300mA
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(A RMS)
Absolute Maximum Ratings and Latchup Prevention The absolute maximum ratings specify that SW (Pins 3, 14) can never exceed VIN (Pin 5) by more than 0.3V. Normally this situation should never occur. It could, however, if the output is held up while the supply is pulled down. A condition where this could potentially occur is when a battery is supplying power to an LTC1574 regulator and also to one or more loads in parallel with the the regulator's VIN. If the battery is disconnected while the LTC1574 regulator is supplying a light load and one of the parallel circuits is a heavy load, the input capacitor of the LTC1574 regulator could be pulled down faster than the output capacitor, causing the absolute maximum ratings to be exceeded. The result is often a latchup which can be destructive if VIN is reapplied. Battery disconnect is possible as a result of mechanical stress, bad battery contacts or use of a lithium-ion battery with a built-in internal disconnect. The user needs to assess his/her application to determine whether this situation could occur. If so, additional protection is necessary. Prevention against latchup can be accomplished by simply connecting a Schottky diode across the SW and VIN pins as shown in Figure 7. The diode will normally be reverse biased unless VIN is pulled below VOUT at which time the diode will clamp the (VOUT - VIN) potential to less than the 0.6V required for latchup. Note that a low leakage Schottky should be used to minimize the effect
LATCHUP PROTECTION SCHOTTKY
1574 * F06
VIN
SW
VOUT
LTC1574
+
1574 F07
Figure 7. Preventing Absolute Maximum Ratings from Being Exceeded
LTC1574 LTC1574-3.3/LTC1574-5
APPLICATIO S I FOR ATIO
on no-load supply current. Schottky diodes such as MBR0530, BAS85 and BAT84 work well. Another more serious effect of the protection diode leakage is that at no load with nothing to provide a sink for this leakage current, the output voltage can potentially float above the
TYPICAL APPLICATIO S
Low Noise, High Efficiency 3.3V Regulator
VIN 4V TO 12.5V
VIN 6 12 11 IPGM LTC1574 LBIN LBOUT GND 2, 4, 13, 15 * AVX TPSD226K025 ** AVX TPSD107K010 COILTRONICS CTX50-4 100pF VFB SW SHDN 7 10 3, 14
PACKAGE DESCRIPTIO
Dimension in inches (millimeters) unless otherwise noted.
S Package 16-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.386 - 0.394* (9.804 - 10.008) 0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0.053 - 0.069 (1.346 - 1.752) 0 - 8 TYP 0.228 - 0.244 (5.791 - 6.197) 0.150 - 0.157** (3.810 - 3.988) 0.004 - 0.010 (0.101 - 0.254) 16 15 14 13 12 11 10 9
0.016 - 0.050 (0.406 - 1.270)
0.014 - 0.019 (0.355 - 0.483) TYP
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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maximum allowable tolerance. To prevent this from occuring, a resistor must be connected between VOUT and ground with a value low enough to sink the maximum possible leakage current.
5
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+
22F* 25V x2
0.1F
50H
6.8nF
+
100F** 10V x2
56k
VOUT 3.3V 450mA
33k
1574 TA03
0.050 (1.270) BSC
S16 1098
1
2
3
4
5
6
7
8
7
LTC1574 LTC1574-3.3/LTC1574-5
TYPICAL APPLICATIO S
Low Dropout 5V Step-Down Regulator with Low-Battery Detection
VIN 5.5V to 12.5V
VIN 4V TO 12.5V
4.7k *LOWBATTERY INDICATOR 162k 6 11 12 47.5k IPGM
5 VIN SHDN LTC1574-5 LBOUT LBIN GND 2, 4, 13, 15 VOUT SW 7 10 3, 14 L1 100H 0.1F
* LOW-BATTERY INDICATOR IS SET UP TO TRIP AT VIN = 5.5V ** AVX TPSD476K016
SELECTION
MANUFACTURER COILTRONICS SUMIDA GOWANDA
PART NO. CTX100-4 CD75-101 GA10-103K
TYPE SURFACE MOUNT SURFACE MOUNT THROUGH HOLE
* LOW-BATTERY INDICATOR IS SET TO TRIP AT VIN = 4.4V ** AVX TPSD106K035 *** AVX TPSD107K010
SELECTION
MANUFACTURER COILTRONICS COILCRAFT SUMIDA GOWANDA
PART NO. CTX50-3 DT3316-473 CD54-470 GA10-472K
TYPE SURFACE MOUNT SURFACE MOUNT SURFACE MOUNT THROUGH HOLE
RELATED PARTS
PART NUMBER LT(R)1074/LT1076 LTC1147 LTC1174 LTC1265 LT1375/LT1376 LT1611 LTC1701 LTC1707 DESCRIPTION Step-Down Switching Regulator High Efficiency Step-Down DC/DC Controller 1.2A High Efficiency Step-Down DC/DC Regulator 1.5A 500kHz Step-Down Switching Regulator Inverting 1.4MHz Switching Regulator in SOT-23 1MHz Step-Down DC/DC Converter in SOT-23 High Efficiency Synchronous Step-Down Regulator COMMENTS 100kHz, 5A (LT1074) or 2A (LT1076) Monolithic 8-Pin Controller Burst Mode Operation, Monolithic High Frequency Small Inductor - 5V at 150mA from 5V Input, 1mVP-P Output Ripple, SOT-23 Package VIN = 2.5V to 5.5V, IQ = 135A, VOUT = 5V to 1.25V VIN = 2.85V to 8.5V, Selectable Burst Mode Operation, 600mA Output Current, SO-8 Package 600mA at VIN = 5V, 2.65V to 10V = VIN, IQ = 10A
High Efficiency Step-Down and Inverting DC/DC Converter 0.5A, Burst Mode Operation, SO-8 Package, VIN to 18V
LTC1877/LTC1878 High Efficiency Synchronous Step-Down Regulator
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 FAX: (408) 434-0507 www.linear-tech.com
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High Efficiency 3.3V Regulator
+
47F** 16V x2 VOUT 5V 365mA
5 VIN 6 12 11 IPGM SHDN LTC1574-3.3 10 LBIN VOUT LBOUT GND SW 3, 14 7
+
22F* 25V x2
0.1F
50H
VOUT 3.3V 425mA
+
47F** 16V x2
+
1574 TA05
1574 TA04
* AVX TPSD226K025 ** AVX TPSD476K016 COILTRONICS CTX50-4
2, 4, 13, 15
47F* 16V x2
Positive to - 5V Converter
VIN 4V TO 12.5V
4.7k *LOWBATTERY INDICATOR 280k 6 11 12 43k IPGM
5 VIN SHDN LTC1574-5 LBOUT LBIN GND 2, 4, 13, 15 VOUT SW 7 10 3, 14 L1 50H 0.1F
+
10F** 35V x2
VIN (V) 4 6 8 10 12.5
IOUT (mA) 110 140 170 200 235
+
100F*** 10V VOUT -5V
1574 TA06
sn1574 1574fas LT/TP 1000 2K REV A * PRINTED IN USA (c) LINEAR TECHNOLOGY CORPORATION 1995

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