LT1935 [Linear]
1.2MHz Boost DC/DC Converter in ThinSOT with 2A Switch; 采用ThinSOT封装具有2A开关的1.2MHz升压型DC / DC转换器
| 型号： | LT1935 |
| 厂家： | 
Linear |
| 描述： | 1.2MHz Boost DC/DC Converter in ThinSOT with 2A Switch |
| 文件： | 总8页 (文件大小：186K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT1935
1.2MHz Boost DC/DC
Converter in ThinSOT
with 2A Switch
U
FEATURES
DESCRIPTIO
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 applica-
tions, 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.
■
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
A constant frequency, internally compensated, current
mode PWM architecture results in low, predictable output
noisethatiseasytofilter.LowESRceramiccapacitorscan
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
generate5Vatupto1Afroma3.3Vsupplyor5Vat550mA
from four alkaline cells in a SEPIC design.
Low Profile (1mm) SOT-23 (ThinSOTTM) Package
■
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APPLICATIO S
■
Digital Cameras
■
Battery Backup
■
LCD Bias
■
Local 5V or 12V Supply
■
PC Cards
■
xDSL Power Supply
■
TFT-LCD Bias Supply
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.
U
TYPICAL APPLICATIO
Efficiency, VOUT = 12V
90
L1
D1
4.2µH
V
OUT
V
IN
V
= 5V
85
80
75
70
65
60
55
50
IN
12V
5V
600mA
V
= 3.3V
IN
V
SW
IN
84.5k
10k
4.7µF
LT1935
10µF
ON OFF
SHDN
FB
GND
D1: ON SEMI MBRM120
L1: SUMIDA CDRH5D28-4R2
1935 F01
Figure 1. 5V to 12V, 600mA Step-Up DC/DC Converter
100 200 300 400 500
LOAD CURRENT (mA)
700
0
600
1935 F01b
1935f
1
LT1935
W W U W
U W
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ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
(Note 1)
ORDER PART
NUMBER
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
TOP VIEW
SW 1
GND 2
FB 3
5 V
LT1935ES5
IN
4 SHDN
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
S5 PART MARKING
LTRX
TJMAX = 125°C, θJA = 113°C/ W,
(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.
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
MIN
TYP
1.265
0.01
12
MAX
UNITS
V
Feedback Voltage
Measured at the FB Pin
●
●
1.240
1.280
Feedback Voltage Line Regulation
FB Pin Bias Current
2.5V ≤ V ≤ 16V
%/V
nA
V
IN
V
= V
60
2.3
16
FB
REF
Undervoltage Lockout Threshold
Maximum Input Voltage
Switching Frequency
2.1
V
1
85
2
1.2
93
1.4
MHz
%
Maximum Duty Cycle
Switch Current Limit
●
●
(Note 3)
3.2
180
0.01
14
A
Switch Saturating Voltage
Switch Leakage Current
SHDN Pin Input Current
I
= 2A
= 5V
280
1
mV
µA
µA
µA
mA
µA
V
SW
V
V
V
V
V
SW
= 1.8V
= 0V
40
0.1
SHDN
SHDN
Operating Supply Current
SHDN Supply Current
= 1.5V
3
FB
= 0V
0.1
1
SHDN
SHDN Input High Voltage
SHDN Input Low Voltage
1.8
0.5
V
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
temperature range are assured by design, characterization and correlation
with statistical process controls.
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
Note 3: Current limit guaranteed by design and/or correlation to static test.
1935f
2
LT1935
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TYPICAL PERFOR A CE CHARACTERISTICS
Undervoltage Lockout
FB Pin Voltage
Oscillator Frequency
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1.28
1.27
1.26
1.25
1.24
2.4
2.3
2.2
2.1
2.0
1.9
1.8
–25
0
25
50
75
125
–25
0
25
50
75
125
–50
100
–50
100
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
TEMPERATURE (°C)
TEMPERATURE (°C)
1935 G02
1935 G01
1935 G03
Peak Switch Current vs SHDN Pin
Voltage (Soft-Start)
Current Limit
Switch Saturation Voltage
4
3
2
1
0
400
300
200
100
0
4
3
2
1
0
50% DUTY CYCLE
A
T
= 25°C
A
T
= 25°C
TYP
MIN
T
= 85°C
A
T
= 25°C
A
0
0.5
1.0
1.5
2.0
2.5
3.0
1.0
1.5
0
2.0
0.5
0
20
40
60
80
100
SWITCH CURRENT (A)
SHDN VOLTAGE (V)
DUTY CYCLE (%)
1935 G05
1935 G06
1935 G04
SHDN Pin Current
Frequency Foldback
80
60
40
20
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
T
= 25°C
T
= 25°C
A
A
4
8
10 12 14
SHDN PIN VOLTAGE (V)
0
0.2
1.2 1.4
0
2
6
16
0.4 0.6 0.8 1.0
FEEDBACK VOLTAGE (V)
1935 G07
1935 G08
1935f
3
LT1935
U
U
U
PI FU CTIO S
SW (Pin 1): Switch Pin. Connect inductor/diode here.
SHDN(Pin4):ShutdownPin.Tieto1.8Vormoretoenable
device.Groundtoshutdown.Thispinalsoprovidesasoft-
start function; see Applications Information section.
Minimize trace area at this pin to reduce EMI.
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).
W
BLOCK DIAGRA
1
SW
1.265V
REFERENCE
COMPARATOR
V
5
+
–
IN
–
DRIVER
A1
FB
3
4
Q1
A2
R
Q
R
C
+
C
S
C
SHDN
+
–
0.01Ω
x15
Σ
V
RAMP
GENERATOR
OUT
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 regula-
tion. 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 con-
trolling the peak switch current during start-up.
1935f
4
LT1935
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APPLICATIONS INFORMATION
Inductor Selection
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.
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.
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.
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
switchingfrequency.Atlowerdutycycles(lessthan70%),
this value can be lowered somewhat in order to use a
physically smaller inductor.
Figure 3 shows transient response of the circuit in Fig-
ure 1.Theloadissteppedfrom200mAto400mAandback
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.
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
V
OUT
100mV/DIV
Coiltronics/Cooper
SD10, SD12, SD18, SD20
..
Wurth Elektronik
WE-PD2S, WE-PD3S, WE-PD4S
MSS5131, MSS6132, DO1608
Coilcraft
I
LOAD
200mA/DIV
Diode Selection
0
Use a Schottky rectifier with a 1A or higher current rating,
suchastheOnSemiconductorMBRM120.Its20Vreverse
voltage rating is adequate for most applications. Higher
output voltages may require a 30V of 40V diode.
1935 F03
50µs/DIV
Figure 3. Transient Response of the Circuit in Figure 1,
COUT = 10µF
Capacitor Selection
V
OUT
100mV/DIV
Use capacitors with low ESR (equivalent series resis-
tance). 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 applica-
tions, and newer types of capacitors such as Sanyo’s
POSCAP and Panasonic’s SP CAP.
I
LOAD
200mA/DIV
1935 F04
50µs/DIV
Figure 4. Transient Response with COUT = 20µF
1935f
5
LT1935
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W U U
APPLICATIONS INFORMATION
Soft Start
V
OUT
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.
100mV/DIV
I
LOAD
200mA/DIV
1935 F05
50µs/DIV
84.5k 68pF
OUT
20µF
FB
10k
Figure 5. Transient Response with a 68pF Phase-Lead Capacitor
RUN
5V/DIV
RUN
5V/DIV
V
V
OUT
2V/DIV
OUT
2V/DIV
I
IN
1A/DIV
I
IN
1A/DIV
1935 F06a
1935 F06b
20µs/DIV
200µs/DIV
10k
SHDN
GND
SHDN
GND
RUN
RUN
0.22µF
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
ThehighspeedoperationoftheLT1935demandscareful
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.
L1
D1
C1
V
OUT
V
IN
+
C2
SHDN
R2
R1
C3
GND
1935 F03
Figure 7. Suggested Layout
1935f
6
LT1935
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TYPICAL APPLICATIO S
Efficiency, VOUT = 5V
90
85
80
75
70
65
60
55
50
5V Boost Converter
L1
1.8µH
D1
V
V
= 3.3V
OUT
5V
1A, V = 3.3V
IN
= 2.5V
V
IN
2.3V TO 4.8V
V
IN
IN
0.6A, V = 2.5V
IN
C3
150pF
V
SW
C1
R1
IN
4.7µF
29.4k
LT1935
SHDN
C2
20µF
ON OFF
FB
R2
10k
GND
C1, C2: X5R OR X7R 6.3V
D1: ON SEMI MBRM120
L1: SUMIDA CR43-1R8
1935 TA01
0
200
400
600
800 1000 1200
LOAD CURRENT (mA)
3.3V to 12V Boost Converter
L1
4.2µH
D1
V
OUT
V
IN
3.3V
12V
320mA
V
SW
C1
IN
R1
84.5k
4.7µF
47pF
LT1935
SHDN
C2
22µF
ON OFF
FB
R2
10k
GND
D1: ON SEMI MBRM120
L1: SUMIDA CDRH5D28-4R2
1935 TA02
U
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
1.50 – 1.75
(NOTE 4)
2.80 BSC
1.4 MIN
3.85 MAX 2.62 REF
PIN ONE
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.30 – 0.45 TYP
5 PLCS (NOTE 3)
0.95 BSC
0.80 – 0.90
0.20 BSC
DATUM ‘A’
0.01 – 0.10
1.00 MAX
0.30 – 0.50 REF
1.90 BSC
0.09 – 0.20
(NOTE 3)
NOTE:
S5 TSOT-23 0302
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
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
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 represen-
tation that the interconnection ofits circuits as described herein willnotinfringe on existing patentrights.
7
LT1935
TYPICAL APPLICATIO S
U
8V, 16V and –8V TFT LCD Power Supply
16V
10mA
C3
1µF
D2B
D2A
C5
0.1µF
L1
2.2µH
V
IN
3.3V
8V
450mA
D1
V
SW
C1
4.7µF
R1
100k
IN
LT1935
C2
10µF
ON OFF
SHDN
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
C4
D3A
1µF
D3B
–8V
10mA
1935 TA03
5V SEPIC Converter
C3
2.2µF
L1
4.7µH
D1
V
V
OUT
IN
3.2V TO 9V
5V
425mA, V >3.2V
IN
500mA, V >3.6V
IN
V
SW
C1
IN
R1
29.4k
550mA, V >4V
IN
47pF
4.7µF
LT1935
C2
47µF
ON OFF
SHDN
FB
L2
4.7µH
R2
10k
GND
C1, C3: X5R OR X7R 10V
C2: X5R OR X7R 6.3V
D1: ON SEMI MBRM120
L1, L2: SUMIDA CDRH4D18-4R7
1935 TA04
RELATED PARTS
PART NUMBER
DESCRIPTION
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COMMENTS
LT1618
V : 1.6V to 18V, V
IN
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V : 2.45V to 16V, V
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IN
OUT(MAX) Q
I
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V : 0.85V to 5V, V
: 5V, I : 19µA/300µA
SW
IN
OUT(MAX) Q
I
: <1µA, ThinSOT Package
SD
LTC3401/LT3402
LTC3425
1A/2A(I ), 3MHz, Synchronous Step-Up DC/DC Converter
V : 0.5V to 5V, V
: 6V, I : 38µA,
SW
IN
OUT(MAX) Q
I
: <1µA, MS Package
SD
5A (I ), 8MHz, Multi-Phase Synchronous Step-Up DC/DC Converter
V : 0.5V to 4.5V, V
: 5.25V, I : 12µA,
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SW
IN
I
: <1µA, QFN Package
SD
LT3436
3A (I ), 1MHz, 34V Step-Up DC/DC Converter
V : 3V to 25V, V
: 34V, I : 0.9mA,
SW
IN
OUT(MAX) Q
I
: <6µA, TSSOP-16E Package
SD
LT3467/LT3467A
1.1A (I ), 1.3MHz/2.1MHz, High Efficiency Step-Up DC/DC Converter
V : 2.6V to 16V, V
: 40V, I : 1.2mA,
SW
IN
OUT(MAX) Q
I
: <1µA, ThinSOT Package
SD
1935f
LT/TP 0604 1K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
8
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004
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