LINEAR LTRX

LT1935
1.2MHz Boost DC/DC
Converter in ThinSOT
with 2A Switch
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FEATURES
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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: < 1µA
Pin-for-Pin Compatible with the LT1613 and LT1930
Low Profile (1mm) SOT-23 (ThinSOTTM) Package
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APPLICATIO S
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Digital Cameras
Battery Backup
LCD Bias
Local 5V or 12V Supply
PC Cards
xDSL Power Supply
TFT-LCD Bias Supply
The LT®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.
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TYPICAL APPLICATIO
4.7µF
VIN
D1
SW
VOUT
12V
600mA
80
84.5k
LT1935
10µF
ON OFF
SHDN
FB
GND
VIN = 5V
85
10k
EFFICIENCY (%)
L1
4.2µH
VIN
5V
Efficiency, VOUT = 12V
90
VIN = 3.3V
75
70
65
60
D1: ON SEMI MBRM120
L1: SUMIDA CDRH5D28-4R2
1935 F01
Figure 1. 5V to 12V, 600mA Step-Up DC/DC Converter
55
50
0
100
200
300
400
500
600
700
LOAD CURRENT (mA)
1935 F01b
1935f
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LT1935
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ABSOLUTE
RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
VIN Voltage .............................................................. 16V
SW Voltage ................................................– 0.4V to 40V
FB Voltage ................................................................. 6V
Current Into FB Pin .............................................. ±1mA
SHDN Voltage ......................................................... 16V
Maximum Junction Temperature ......................... 125°C
Operating Ambient Temperature Range
(Note 2) .............................................. – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Strict adherence to JDEC 020B solder attach and rework
for assemblies containing lead is recommended.
ORDER PART
NUMBER
TOP VIEW
SW 1
LT1935ES5
5 VIN
GND 2
FB 3
4 SHDN
S5 PART MARKING
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
LTRX
TJMAX = 125°C, θJA = 113°C/ 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 = 25°C.
VIN = 3V, VSHDN = VIN unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
Feedback Voltage
Measured at the FB Pin
Feedback Voltage Line Regulation
2.5V ≤ VIN ≤ 16V
FB Pin Bias Current
VFB = VREF
●
MIN
TYP
MAX
UNITS
1.240
1.265
1.280
V
0.01
●
Undervoltage Lockout Threshold
12
60
nA
2.1
2.3
V
16
V
1.4
MHz
Maximum Input Voltage
Switching Frequency
●
Maximum Duty Cycle
Switch Current Limit
(Note 3)
Switch Saturating Voltage
ISW = 2A
Switch Leakage Current
VSW = 5V
SHDN Pin Input Current
VSHDN = 1.8V
1
1.2
85
93
%
2
3.2
A
●
180
280
mV
0.01
1
µA
14
40
µA
VSHDN = 0V
0.1
Operating Supply Current
VFB = 1.5V
3
SHDN Supply Current
VSHDN = 0V
0.1
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 0°C to 70°C. Specifications over the – 40°C to 85°C operating
%/V
µA
mA
1
1.8
µA
V
0.5
V
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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LT1935
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TYPICAL PERFOR A CE CHARACTERISTICS
FB Pin Voltage
Undervoltage Lockout
Oscillator Frequency
1.6
1.28
2.4
1.4
1.26
2.2
1.0
UVLO (V)
FREQUENCY (MHz)
VFB (V)
2.3
1.2
1.27
0.8
0.6
2.0
0.4
1.25
1.9
0.2
1.24
–50 –25
0
25
50
75
100
0
–50 –25
125
0
25
50
75
100
Current Limit
TYP
MIN
2
1
400
4
300
3
TA = 85°C
200
TA = 25°C
80
100
0.5
0
1.0
1.5
2.0
SWITCH CURRENT (A)
2.5
SHDN Pin Current
3.0
0
0
0.5
1.0
1.5
2.0
SHDN VOLTAGE (V)
1935 G06
Frequency Foldback
1.4
SWITCHING FREQUENCY (MHz)
TA = 25°C
60
40
20
0
2
1935 G05
1935 G04
80
50% DUTY CYCLE
TA = 25°C
1
100
0
60
40
DUTY CYCLE (%)
SHDN PIN CURRENT (µA)
CURRENT LIMIT (A)
3
125
Peak Switch Current vs SHDN Pin
Voltage (Soft-Start)
CURRENT LIMIT (A)
SWITCH SATURATION VOLTAGE (mV)
TA = 25°C
100
1935 G03
Switch Saturation Voltage
4
20
50
25
75
0
TEMPERATURE (°C)
1935 G02
1935 G01
0
1.8
–50 –25
125
TEMPERATURE (°C)
TEMPERATURE (°C)
0
2.1
TA = 25°C
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
1935 G07
0
0.2
1.0
0.4 0.6 0.8
FEEDBACK VOLTAGE (V)
1.2
1.4
1935 G08
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LT1935
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PI FU CTIO S
SW (Pin 1): Switch Pin. Connect inductor/diode here.
Minimize trace area at this pin to reduce EMI.
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.
GND (Pin 2): Ground. Tie directly to local ground plane.
VIN (Pin 5): Input Supply Pin. Must be locally bypassed.
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).
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BLOCK DIAGRA
1.265V
REFERENCE
VIN 5
–
A1
–
FB 3
1 SW
COMPARATOR
+
RC
+
DRIVER
A2
R
Q1
Q
S
CC
SHDN 4
+
Σ
0.01Ω
x15
–
VOUT
RAMP
GENERATOR
R1 (EXTERNAL)
2 GND
FB
1.2MHz
OSCILLATOR
R2 (EXTERNAL)
1935 BD
Figure 2. Block Diagram
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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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LT1935
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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
100mΩ.
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
Model Prefix
Sumida
CDRH4D18, CDRH4D28,
CDRH5D18, CDRH5D28, CR43
Coiltronics/Cooper
SD10, SD12, SD18, SD20
..
Wurth Elektronik
WE-PD2S, WE-PD3S, WE-PD4S
Coilcraft
MSS5131, MSS6132, DO1608
Use a 4.7µF 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 2.2µF.
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 20µF. Figure 5 shows the additional
improvement resulting from the phase lead capacitor.
VOUT
100mV/DIV
ILOAD
200mA/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.
0
50µs/DIV
1935 F03
Figure 3. Transient Response of the Circuit in Figure 1,
COUT = 10µF
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.
VOUT
100mV/DIV
ILOAD
200mA/DIV
50µs/DIV
1935 F04
Figure 4. Transient Response with COUT = 20µF
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LT1935
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APPLICATIONS INFORMATION
Soft Start
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 100µA when the SHDN pin reaches 1.8V.
VOUT
100mV/DIV
ILOAD
200mA/DIV
1935 F05
50µs/DIV
OUT
84.5k
68pF
20µF
FB
10k
Figure 5. Transient Response with a 68pF Phase-Lead Capacitor
RUN
5V/DIV
RUN
5V/DIV
VOUT
2V/DIV
VOUT
2V/DIV
IIN
1A/DIV
IIN
1A/DIV
1935 F06a
20µs/DIV
200µs/DIV
1935 F06b
10k
RUN
SHDN
SHDN
RUN
GND
0.22µF
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 = 20µF, Output Load = 10Ω.
Layout Hints
L1
D1
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.
C1
+
VIN
VOUT
+
C2
SHDN
R2
GND
R1
C3
1935 F03
Figure 7. Suggested Layout
1935f
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LT1935
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TYPICAL APPLICATIO S
Efficiency, VOUT = 5V
5V Boost Converter
C1
4.7µF
ON OFF
VIN
SW
LT1935
SHDN
C3
150pF
R1
29.4k
85
VOUT
5V
1A, VIN = 3.3V
0.6A, VIN = 2.5V
D1
C2
20µF
FB
R2
10k
GND
VIN = 3.3V
80
EFFICIENCY (%)
L1
1.8µH
VIN
2.3V TO 4.8V
90
VIN = 2.5V
75
70
65
60
55
C1, C2: X5R OR X7R 6.3V
D1: ON SEMI MBRM120
L1: SUMIDA CR43-1R8
1935 TA01
50
0
200
400
600
800
1000
1200
LOAD CURRENT (mA)
3.3V to 12V Boost Converter
L1
4.2µH
VIN
3.3V
VIN
C1
4.7µF
D1
SW
47pF
LT1935
ON OFF
VOUT
12V
320mA
SHDN
R1
84.5k
FB
C2
22µF
R2
10k
GND
D1: ON SEMI MBRM120
L1: SUMIDA CDRH5D28-4R2
1935 TA02
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PACKAGE DESCRIPTIO
S5 Package
5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
0.62
MAX
0.95
REF
2.90 BSC
(NOTE 4)
1.22 REF
1.4 MIN
3.85 MAX 2.62 REF
2.80 BSC
1.50 – 1.75
(NOTE 4)
PIN ONE
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.95 BSC
0.30 – 0.45 TYP
5 PLCS (NOTE 3)
0.80 – 0.90
0.20 BSC
0.01 – 0.10
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
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
1935f
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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LT1935
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TYPICAL APPLICATIO S
8V, 16V and –8V TFT LCD Power Supply
VIN
3.3V
D2B
D2A
C5
0.1µF
L1
2.2µH
C3
1µF
16V
10mA
8V
450mA
D1
VIN
C1
4.7µF
SW
R1
100k
LT1935
ON OFF
SHDN
C2
10µF
FB
R2
18.7k
GND
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
C6
0.1µF D3A
D3B
C4
1µF
–8V
10mA
1935 TA03
5V SEPIC Converter
C3
2.2µF
L1
4.7µH
VIN
3.2V TO 9V
C1
4.7µF
ON OFF
SW
VIN
LT1935
SHDN
R1
29.4k
D1
VOUT
5V
425mA, VIN >3.2V
500mA, VIN >3.6V
550mA, VIN >4V
47pF
C2
47µF
FB
GND
C1, C3: X5R OR X7R 10V
C2: X5R OR X7R 6.3V
R2
10k
L2
4.7µH
D1: ON SEMI MBRM120
L1, L2: SUMIDA CDRH4D18-4R7
1935 TA04
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DESCRIPTION
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LTC3425
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LT3436
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1935f
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Linear Technology Corporation
LT/TP 0604 1K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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