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LT1935 1.2MHz Boost DC/DC Converter in ThinSOT with 2A Switch
FEATURES

DESCRIPTIO
1.2MHz Switching Frequency High Output Voltage: Up to 38V Wide Input Range: 2.3V to 16V Low VCESAT Switch: 180mV at 2A Soft-Start Uses Small Surface Mount Components 5V at 1A from 3.3V Input 12V at 600mA from 5V Input Low Shutdown Current: < 1A Pin-for-Pin Compatible with the LT1613 and LT1930 Low Profile (1mm) SOT-23 (ThinSOTTM) Package
The LT(R)1935 is the industry's highest power SOT-23 switching regulator. Its unprecedented 2A, 40V internal switch allows high output currents to be generated in a small footprint. Intended for space-conscious applications, the LT1935 switches at 1.2MHz, allowing the use of tiny, low profile inductors and capacitors 2mm or less in height. The NPN switch achieves a VCESAT of just 180mV at 2A independent of supply voltage, resulting in high efficiency even at maximum power levels from a 3V input. A constant frequency, internally compensated, current mode PWM architecture results in low, predictable output noise that is easy to filter. Low ESR ceramic capacitors can be used on the output, further reducing noise to the millivolt level. The high voltage switch on the LT1935 is rated at 40V, making the device ideal for boost converters up to 38V as well as for single-ended primary inductance converter (SEPIC) and flyback designs. The device can generate 5V at up to 1A from a 3.3V supply or 5V at 550mA from four alkaline cells in a SEPIC design. The LT1935 is available in a 5-lead SOT-23 package.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.
APPLICATIO S

Digital Cameras Battery Backup LCD Bias Local 5V or 12V Supply PC Cards xDSL Power Supply TFT-LCD Bias Supply
TYPICAL APPLICATIO
VIN 5V 4.7F VIN LT1935 L1 4.2H
D1
90
VOUT 12V 600mA
85 80
SW
EFFICIENCY (%)
84.5k 10F
75 70 65 60
ON OFF
SHDN GND
FB 10k
D1: ON SEMI MBRM120 L1: SUMIDA CDRH5D28-4R2
1935 F01
55 50 0 100 200 300 400 500 600 700 LOAD CURRENT (mA)
1935 F01b
Figure 1. 5V to 12V, 600mA Step-Up DC/DC Converter
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Efficiency, VOUT = 12V
VIN = 5V VIN = 3.3V
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LT1935
ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW SW 1 GND 2 FB 3 4 SHDN 5 VIN
VIN Voltage .............................................................. 16V SW Voltage ................................................- 0.4V to 40V FB Voltage ................................................................. 6V Current Into FB Pin .............................................. 1mA SHDN Voltage ......................................................... 16V Maximum Junction Temperature ......................... 125C Operating Ambient Temperature Range (Note 2) .............................................. - 40C to 85C Storage Temperature Range ................. - 65C to 150C Strict adherence to JDEC 020B solder attach and rework for assemblies containing lead is recommended.
ORDER PART NUMBER LT1935ES5
S5 PACKAGE 5-LEAD PLASTIC TSOT-23
S5 PART MARKING LTRX
TJMAX = 125C, JA = 113C/ W,
Consult LTC marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 3V, VSHDN = VIN unless otherwise noted. (Note 2)
PARAMETER Feedback Voltage Feedback Voltage Line Regulation FB Pin Bias Current Undervoltage Lockout Threshold Maximum Input Voltage Switching Frequency Maximum Duty Cycle Switch Current Limit Switch Saturating Voltage Switch Leakage Current SHDN Pin Input Current Operating Supply Current SHDN Supply Current SHDN Input High Voltage SHDN Input Low Voltage Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LT1935ES5 is guaranteed to meet performance specifications from 0C to 70C. Specifications over the - 40C to 85C operating (Note 3) ISW = 2A VSW = 5V VSHDN = 1.8V VSHDN = 0V VFB = 1.5V VSHDN = 0V 1.8 0.5 3 0.1 1

CONDITIONS Measured at the FB Pin 2.5V VIN 16V VFB = VREF

MIN 1.240
TYP 1.265 0.01 12 2.1
MAX 1.280 60 2.3 16 1.4
UNITS V %/V nA V V MHz % A
1 85 2
1.2 93 3.2 180 0.01 14
280 1 40 0.1
temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Current limit guaranteed by design and/or correlation to static test.
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mV A A A mA A V V
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LT1935 TYPICAL PERFOR A CE CHARACTERISTICS
FB Pin Voltage
1.28 1.6 1.4 1.27 1.2
FREQUENCY (MHz)
1.26
0.8 0.6 0.4 0.2
UVLO (V)
0 25 50 75 100 125
VFB (V)
1.25
1.24 -50 -25
0
25
50
75
TEMPERATURE (C)
1935 G01
Current Limit
4 400
SWITCH SATURATION VOLTAGE (mV)
TA = 25C
3
CURRENT LIMIT (A)
TYP MIN 2
TA = 85C 200 TA = 25C 100
CURRENT LIMIT (A)
1
0
0
20
60 40 DUTY CYCLE (%)
SHDN Pin Current
80 TA = 25C 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 2 10 12 4 8 6 SHDN PIN VOLTAGE (V) 14 16
60
40
20
0
SWITCHING FREQUENCY (MHz)
SHDN PIN CURRENT (A)
UW
100 80
1935 G04
Oscillator Frequency
2.4 2.3 2.2 2.1 2.0 1.9
Undervoltage Lockout
1.0
125
0 -50 -25
1.8 -50 -25
TEMPERATURE (C)
1935 G02
50 25 75 0 TEMPERATURE (C)
100
125
1935 G03
Switch Saturation Voltage
4
Peak Switch Current vs SHDN Pin Voltage (Soft-Start)
50% DUTY CYCLE TA = 25C
300
3
2
1
0 100
0
0.5
1.0 1.5 2.0 SWITCH CURRENT (A)
2.5
3.0
0
0
0.5
1.0 SHDN VOLTAGE (V)
1.5
2.0
1935 G06
1935 G05
Frequency Foldback
TA = 25C
0
0.2
1.0 0.4 0.6 0.8 FEEDBACK VOLTAGE (V)
1.2
1.4
1935 G07
1935 G08
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LT1935
PI FU CTIO S
SW (Pin 1): Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI. GND (Pin 2): Ground. Tie directly to local ground plane. FB (Pin 3): Feedback Pin. Reference voltage is 1.265V. Connect resistive divider tap here. Minimize trace area at FB. Set VOUT according to VOUT = 1.265V(1 + R1/R2). SHDN (Pin 4): Shutdown Pin. Tie to 1.8V or more to enable device. Ground to shut down. This pin also provides a softstart function; see Applications Information section. VIN (Pin 5): Input Supply Pin. Must be locally bypassed.
BLOCK DIAGRA
VIN 5
A1 FB 3
-
RC CC
SHDN 4
VOUT R1 (EXTERNAL) FB R2 (EXTERNAL) 1.2MHz OSCILLATOR RAMP GENERATOR
Figure 2. Block Diagram
OPERATIO
The LT1935 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Block Diagram in Figure 2. At the start of each oscillator cycle, the SR latch is set, turning on the power switch Q1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset, turning off the power switch. The level at the negative input of A2 is set by error amplifier A1, and
is simply an amplified version of the difference between the feedback voltage and the reference voltage of 1.265V. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier's output increases, more current is delivered to the output; if it decreases, less current is delivered. A clamp on the output of A1 (not shown) limits the switch current to 3A. A1's output is also clamped to the voltage on the SHDN pin, providing a soft-start function by controlling the peak switch current during start-up.
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1.265V REFERENCE
+
COMPARATOR DRIVER A2 R S Q
1 SW
Q1
+
x15 0.01
-
2 GND
1935 BD
LT1935
APPLICATIONS INFORMATION
Inductor Selection Use inductors that are intended for high frequency power applications. The saturation current rating should be at least 2A. The RMS current rating, which is usually based on heating of the inductor, should be higher than the average current in the inductor in your application. For best efficiency, the DC resistance should be less than 100m. A good first choice for the inductor value results in a ripple current that is 1/3 of the maximum switch current: L = 3 (VIN/VOUT) (VOUT - VIN)/(IMAX * f) IMAX is the maximum switch current of 2A and f is the switching frequency. At lower duty cycles (less than 70%), this value can be lowered somewhat in order to use a physically smaller inductor. Table 1 lists several inductor manufacturers, along with part numbers for inductors that are a good match to the LT1935.
Table 1. Inductor Suppliers
Supplier Sumida Coiltronics/Cooper Wurth Elektronik Coilcraft
..
Model Prefix CDRH4D18, CDRH4D28, CDRH5D18, CDRH5D28, CR43 SD10, SD12, SD18, SD20 WE-PD2S, WE-PD3S, WE-PD4S MSS5131, MSS6132, DO1608
ILOAD 200mA/DIV VOUT 100mV/DIV
Diode Selection Use a Schottky rectifier with a 1A or higher current rating, such as the On Semiconductor MBRM120. Its 20V reverse voltage rating is adequate for most applications. Higher output voltages may require a 30V of 40V diode. Capacitor Selection Use capacitors with low ESR (equivalent series resistance). In most cases, multilayer ceramic capacitors are the best choice. They offer high performance (very low ESR) in a small package. Use only X5R or X7R types; they maintain their capacitance over temperature and applied voltage. Other suitable capacitor types include low-ESR tantalum capacitors that are specified for power applications, and newer types of capacitors such as Sanyo's POSCAP and Panasonic's SP CAP.
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Use a 4.7F ceramic capacitor to bypass the input of the LT1935. Be aware that the switching regulators require a low impedance input supply. Additional bulk capacitance may be required if the LT1935 circuit is more than a few inches away from the power source. If there are low ESR capacitors nearby, the input bypass capacitor can be reduced to 2.2F. The output capacitor supports the output under transient loads and stabilizes the control loop of the LT1935. Look at the typical application circuits as a starting point to choose a value. Generally, a higher output capacitance is required at higher load currents and lower input voltages. Figure 3 shows transient response of the circuit in Figure 1. The load is stepped from 200mA to 400mA and back to 200mA. The transient performance can be improved by increasing the output capacitance, but may require a phase lead capacitor between the output and the FB pin. Figure 4 shows the transient response with the output capacitor increased to 20F. Figure 5 shows the additional improvement resulting from the phase lead capacitor.
0 50s/DIV
1935 F03
Figure 3. Transient Response of the Circuit in Figure 1, COUT = 10F
VOUT 100mV/DIV
ILOAD 200mA/DIV
50s/DIV
1935 F04
Figure 4. Transient Response with COUT = 20F
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LT1935
APPLICATIONS INFORMATION
Soft Start
VOUT 100mV/DIV
ILOAD 200mA/DIV
50s/DIV OUT 84.5k FB 10k 68pF 20F
1935 F05
Figure 5. Transient Response with a 68pF Phase-Lead Capacitor
RUN 5V/DIV VOUT 2V/DIV
IIN 1A/DIV
20s/DIV
RUN
SHDN GND
Figure 6. Adding a Resistor and Capacitor to the SHDN Pin Reduces the Peak Input Current During Start-Up. VIN = 3.3V, VOUT = 5V, C2 = 20F, Output Load = 10.
D1 VOUT
L1
C1
+
C2 R2 SHDN
GND
R1
C3
1935 F03
Figure 7. Suggested Layout
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The SHDN pin can be used to soft start the LT1935, reducing the maximum input current during start up. The SHDN pin is driven through an external RC filter to create a ramp at this pin. Figure 6 shows the start-up waveforms with and without the soft start circuit. Without soft start, the input current peaks at ~3A. With soft start, the peak current is reduced to 1A. By choosing a large RC time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply 100A when the SHDN pin reaches 1.8V.
RUN 5V/DIV VOUT 2V/DIV
IIN 1A/DIV
1935 F06a
200s/DIV
1935 F06b
10k RUN 0.22F SHDN GND
Layout Hints The high speed operation of the LT1935 demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 7 shows the recommended component placement. Make the ground pin copper area large. This helps to lower the die temperature.
+
VIN
LT1935
TYPICAL APPLICATIO S
Efficiency, VOUT = 5V 5V Boost Converter
VIN 2.3V TO 4.8V C1 4.7F ON OFF VIN LT1935 SHDN GND FB R2 10k L1 1.8H D1 VOUT 5V 1A, VIN = 3.3V 0.6A, VIN = 2.5V 90 85 80
EFFICIENCY (%)
C1, C2: X5R OR X7R 6.3V D1: ON SEMI MBRM120 L1: SUMIDA CR43-1R8
PACKAGE DESCRIPTIO
0.62 MAX
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 1.00 MAX DATUM `A'
0.30 - 0.50 REF
NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING
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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VIN = 3.3V VIN = 2.5V
R1 29.4k
C3 150pF
75 70 65 60 55
C2 20F
1935 TA01
50 0 200 400 600 800 1000 1200 LOAD CURRENT (mA)
3.3V to 12V Boost Converter
VIN 3.3V C1 4.7F ON OFF VIN LT1935 SHDN GND FB R2 10k L1 4.2H D1 VOUT 12V 320mA 47pF R1 84.5k
SW C2 22F
D1: ON SEMI MBRM120 L1: SUMIDA CDRH5D28-4R2
1935 TA02
S5 Package 5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
0.95 REF 2.90 BSC (NOTE 4)
1.22 REF
1.4 MIN
2.80 BSC
1.50 - 1.75 (NOTE 4)
PIN ONE 0.30 - 0.45 TYP 5 PLCS (NOTE 3)
0.95 BSC
0.80 - 0.90 0.01 - 0.10
1.90 BSC 0.09 - 0.20 (NOTE 3) 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193
S5 TSOT-23 0302
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LT1935
TYPICAL APPLICATIO S
8V, 16V and -8V TFT LCD Power Supply
C5 0.1F D2B D2A C3 1F 16V 10mA
VIN 3.3V C1 4.7F ON OFF VIN
VIN 3.2V TO 9V C1 4.7F ON OFF VIN
RELATED PARTS
PART NUMBER LT1618 LT1930/LT1930A LT1943 LT1946/LT1946A DESCRIPTION 1.5A (ISW), 1.25MHz, High Efficiency Step-Up DC/DC Converter 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up DC/DC Converter Quad Output, 2.6A Buck, 2.6A Boost, 0.3A Boost, 0.4A Inverter 1.2MHz TFT DC/DC Converter 1.5A (ISW), 1.2MHz/2.7MHz, High Efficiency Step-Up DC/DC Converter COMMENTS VIN: 1.6V to 18V, VOUT(MAX): 35V, IQ: 1.8mA, ISD: <1A, MS, DFN Packages VIN: 2.6V to 16V, VOUT(MAX): 34V, IQ: 4.2mA/5.5mA ISD: <1A, ThinSOT Package VIN: 4.5V to 22V, VOUT(MAX): 40V, IQ: 10mA, ISD: <35A, TSSOP-28E Package VIN: 2.45V to 16V, VOUT(MAX): 34V, IQ: 3.2mA, ISD: <1A, MS8 Package VIN: 0.85V to 5V, VOUT(MAX): 5V, IQ: 19A/300A ISD: <1A, ThinSOT Package VIN: 0.5V to 5V, VOUT(MAX): 6V, IQ: 38A, ISD: <1A, MS Package VIN: 0.5V to 4.5V, VOUT(MAX): 5.25V, IQ: 12A, ISD: <1A, QFN Package VIN: 3V to 25V, VOUT(MAX): 34V, IQ: 0.9mA, ISD: <6A, TSSOP-16E Package VIN: 2.6V to 16V, VOUT(MAX): 40V, IQ: 1.2mA, ISD: <1A, ThinSOT Package
1935f LT/TP 0604 1K * PRINTED IN USA
LTC3400/LTC3400B 600mA (ISW), 1.2MHz, Synchronous Step-Up DC/DC Converter LTC3401/LT3402 LTC3425 LT3436 LT3467/LT3467A 1A/2A(ISW), 3MHz, Synchronous Step-Up DC/DC Converter 5A (ISW), 8MHz, Multi-Phase Synchronous Step-Up DC/DC Converter 3A (ISW), 1MHz, 34V Step-Up DC/DC Converter 1.1A (ISW), 1.3MHz/2.1MHz, High Efficiency Step-Up DC/DC Converter
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
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L1 2.2H
8V 450mA D1 SW R1 100k C2 10F R2 18.7k C6 0.1F D3A C4 1F -8V 10mA
1935 TA03
LT1935 SHDN GND FB
C1: X5R OR X7R 6.3V C2, C4, C5, C6: X5R OR X7R 10V C3: X5R OR X7R 25V D1: MBRM120 OR EQUIVALENT D2, D3: BAT-54S OR EQUIVALENT L1: SUMIDA CDRH4D28-2R2
D3B
5V SEPIC Converter
L1 4.7H C3 2.2F D1 VOUT 5V 425mA, VIN >3.2V 500mA, VIN >3.6V 550mA, VIN >4V C2 47F R2 10k L2 4.7H
SW LT1935
R1 29.4k
47pF
SHDN GND
FB
C1, C3: X5R OR X7R 10V C2: X5R OR X7R 6.3V
D1: ON SEMI MBRM120 L1, L2: SUMIDA CDRH4D18-4R7
1935 TA04
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(c) LINEAR TECHNOLOGY CORPORATION 2004

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