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LT3461/LT3461A 1.3MHz/3MHz Step-Up DC/DC Converters with Integrated Schottky in ThinSOT
FEATURES

DESCRIPTIO
Integrated Schottky Rectifier Fixed Frequency 1.3MHz/3MHz Operation High Output Voltage: Up to 38V Low VCESAT Switch: 260mV at 250mA 12V at 70mA from 5V Input 5V at 115mA from 3.3V Input Wide Input Range: 2.5V to 16V Uses Small Surface Mount Components Low Shutdown Current: <1A Soft-Start Low Profile (1mm) SOT-23 (ThinSOTTM) Package
APPLICATIO S

Digital Cameras CCD Bias Supply XDSL Power Supply TFT-LCD Bias Supply Local 5V or 12V Supply Medical Diagnostic Equipment Battery Backup
The LT(R)3461/LT3461A are general purpose fixed frequency current mode step-up DC/DC converters. Both devices feature an integrated Schottky and a low VCESAT switch allowing a small converter footprint and lower parts cost. The LT3461 switches at 1.3MHz while the LT3461A switches at 3MHz. These high switching frequencies enable the use of tiny, low cost and low height capacitors and inductors. The constant switching frequency results in predictable output noise that is easy to filter, and the inductor based topology ensures an input free from switching noise typically present with charge pump solutions. The high voltage switch in the LT3461/LT3461A is rated at 40V making the device ideal for boost converters up to 38V. The LT3461/LT3461A are available in a low profile (1mm) SOT-23 package.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
L1 10H 1 SW 6 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2
5V to 12V, 70mA Step-Up DC/DC Converter
85
VIN 5V C1 1F OFF ON
80
EFFICIENCY (%)
261k
15pF
VOUT 12V 70mA
75
30.1k
C2 1F
3461 TA01a
70
65
60 0 20 60 40 LOAD CURRENT (mA) 80
3461 TAO1b
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Efficiency
VIN = 5V VIN = 3.3V
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LT3461/LT3461A
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW SW 1 GND 2 FB 3 6 VIN 5 VOUT 4 SHDN
Input Voltage (VIN) .................................................. 16V VOUT, SW Voltage .................................................... 40V FB Voltage ................................................................. 5V SHDN Voltage .......................................................... 16V Operating Ambient Temperature Range (Note 2) .................. - 40C to 85C Maximum Junction Temperature .......................... 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LT3461AES6 LT3461ES6 S6 PART MARKING LTAHG LTAEB
S6 PACKAGE 6-LEAD PLASTIC TSOT-23 TJMAX = 125C, JA = 150C ON BOARD OVER GROUND PLANE, JC = 120C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C, VIN = 3V, VSHDN = 3V, unless otherwise noted.
PARAMETER Minimum Operating Voltage Maximum Operating Voltage Feedback Voltage
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN 2.5
TYP
MAX 16
UNITS V V V V %/V nA mA A MHz MHz % %
1.235 1.225
1.255 0.005
1.275 1.280 100 3.6 0.5 3.9 1.7
Feedback Line Regulation FB Pin Bias Current Supply Current Switching Frequency (LT3461A) Switching Frequency (LT3461) Maximum Duty Cycle (LT3461A) Maximum Duty Cycle (LT3461) Switch Current Limit Switch VCESAT Switch Leakage Current Schottky Forward Voltage Schottky Reverse Leakage SHDN Voltage High SHDN Voltage Low SHDN Pin Bias Current Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. ISW = 250mA VSW = 5V ISCHOTTKY = 250mA VOUT - SW = 40V 1.5 FB = 1.3V, Not Switching SHDN = 0V

40 2.8 0.1 2.1 1.0 82 92 300 420 260 0.01 800 0.03 3.0 1.3
600 350 1 1100 4 0.4
35
50
Note 2: The LT3461E/LT3461AE is guaranteed to meet specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
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mA mV A mV A V V A
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LT3461/LT3461A TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency (LT3461)
1.6 1.5 480 TA = 25C 1.27
1.3 1.2 1.1 1.0 -40 -20
FB VOLTAGE (V)
1.4
CURRENT LIMIT (mA)
FREQUENCY (MHz)
40 20 60 0 TEMPERATURE (C)
Oscillator Frequency (LT3461A)
3.9 3.6 3.3 3.0 2.7 2.4 2.1 -60 -40 -20 0 20 40 60 TEMPERATURE (C) 480 420
CURRENT LIMIT (mA)
360 300 240 180 120 60
SHDN PIN CURRENT (A)
FREQUENCY (MHz)
Switching Waveform Circuit of Figure 4
ILOAD VSW 5V/DIV 70mA 35mA
VOUT 50mV/DIV
ILOAD = 60mA
0.2s/DIV
UW
80
3461a G01
Current Limit
1.28
FB Pin Voltage
360 1.26 1.25 1.24 1.23 0 10 1.22 -40 -20
240
120
100
20
30
40 50 60 70 DUTY CYCLE (%)
80
90
40 20 60 0 TEMPERATURE (C)
80
100
3461a G02
3461a G03
Current Limit in Soft-Start Mode
320 TA = 25C 280 240 200 160 120 80 40 1.5 2.1 1.9 SHDN PIN VOLTAGE (V) 1.7 0 2.3
3461a G05
SHDN Pin Current
TA = 25C
80 100
3461a G04
0 1.3
0
4
8
12
16
3461a G06
SHDN PIN VOLTAGE (V)
Load Transient Response Circuit of Figure 4
VOUT 100mV/DIV
3461a G08
50s/DIV
3461a G09
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LT3461/LT3461A
PI FU CTIO S
SW (Pin 1): Switch Pin. Connect inductor here. Minimize trace at this pin to reduce EMI. GND (Pin 2): Ground Pin. Tie directly to local ground plane. FB (Pin 3): Feedback Pin. Reference voltage is 1.255V. Connect resistor divider tap here. Minimize trace area at FB. Set VOUT according to VOUT = 1.255V (1 + R1/R2). SHDN (Pin 4): Shutdown Pin. Tie to 1.5V or higher to enable device; 0.4V or less to disable device. Also functions as soft-start. Use RC filter (47k, 47nF typ) as shown in Figure 1. VOUT (Pin 5): Output Pin. Connect to resistor divider. Put capacitor close to pin and close to GND plane. VIN (Pin 6): Input Supply Pin. Must be locally bypassed.
BLOCK DIAGRA
VOUT
R1 (EXTERNAL) FB R2 (EXTERNAL) 3 FB CC
SHUTDOWN
RS (EXTERNAL)
RS, CS OPTIONAL SOFT-START COMPONENTS
OPERATIO
The LT3461/LT3461A 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 1. At the start of each oscillator cycle, the SR latch is set, which turns 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 the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 1.255V. 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.
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VIN 6
1.255V REFERENCE
+
A1
COMPARATOR
1 DRIVER
SW 5 VOUT
-
RC A2 R S Q
-
Q1
+
+
0.1
-
4 SHDN CS (EXTERNAL) 3MHz* OSCILLATOR *LT3461 IS 1.3MHz 2 GND
3461a F02
RAMP GENERATOR
Figure 1. Block Diagram
Layout Hints The high speed operation of the LT3461/LT3461A demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 2 shows the recommended component placement.
C1
+
GND
L1
VIN
R2
+
R1
C2 VOUT SHUTDOWN
3461a F03
C3
Figure 2. Suggested Layout
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LT3461/LT3461A
APPLICATIO S I FOR ATIO
Inrush Current
The LT3461 has a built-in Schottky diode. When supply voltage is applied to the VIN pin, the voltage difference between VIN and VOUT generates inrush current flowing from input through the inductor and the Schottky diode to charge the output capacitor. The maximum nonrepetitive surge current the Schottky diode in the LT3461 can sustain is 1.5A. The selection of inductor and capacitor value should ensure the peak of the inrush current to be below 1.5A. In addition, turn-on of the LT3461 should be delayed until the inrush current is less than the maximum current limit. The peak inrush current can be calculated as follows:
IOUT (mA)
V - 0.6 * exp - IP = IN L L -1 - 1 2 C C
IOUT (mA)
where L is the inductance, r is the resistance of the inductor and C is the output capacitance. Table 3 gives inrush peak currents for some component selections.
Table 3. Inrush Peak Current
VIN (V) 5 5 L (H) 4.7 10 C (F) 1 1 IP (A) 1.1 0.9
Thermal Considerations Significant power dissipation can occur on the LT3461 and LT3461A, particularly at high input voltage. Device load, voltage drops in the power path components, and switching losses are the major contributors. It is important to measure device power dissipation in an application to ensure that the LT3461 does not exceed the absolute maximum operating junction temperature of 125C over the operating ambient temperature range. Generally, for supply voltages below 5V the integrated current limit function provides adequate protection for nonfault conditions. For supply voltages above 5V, Figures 3a and 3b show the recommended operating region of the LT3461 and LT3461A, respectively. These graphs are based on 250mW on-chip dissipation. Improvement of these Switching Frequency The key difference between the LT3461 and LT3461A is the faster switching frequency of the LT3461A. At 3MHz, the LT3461A switches at twice the rate of the LT3461. The higher switching frequency of the LT3461A allows physically smaller inductors and capacitors to be used in a given application, but with a slight decrease in efficiency and maximum output current when compared to the LT3461. Generally if efficiency and maximum output current are crucial, or a high output voltage is being generated, the LT3461 should be used. If application size and cost are more important, the LT3461A will be the better choice.
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numbers can be expected if the LT3461 is supplied from a separate low voltage rail.
160 VIN = 5V VIN = 8V VIN = 12V VIN >15V 80 120 40 0 6 14 22 VOUT (V)
3461 F01a
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30
38
Figure 3a. LT3461 Operating Region
160 VIN = 5V 120 VIN = 8V VIN = 12V VIN >15V
80
40
0
6
14
22 VOUT (V)
30
38
3461 F01b
Figure 3b. LT3461A Operating Region
5
LT3461/LT3461A
APPLICATIO S I FOR ATIO
Inductor Selection
The inductors used with the LT3461/LT3461A should have a saturation current rating of 0.3A or greater. If the device is used in an application where the input supply will be hot-plugged, then the saturation current rating should be equal to or greater than the peak inrush current. For the LT3461, an inductor value between 10H and 47H, depending upon output voltage, will usually be the best choice for most designs. For the LT3461A, inductor values between 4.7H and 15H inductor will suffice for most applications. For best loop stability results, the inductor value selected should provide a ripple current of 70mA or more. For a given VIN and VOUT the inductor value to use with LT3461A is estimated by the formula: L (in microhenries) =
D * VIN * VOUT * 1sec 1A * 1V
VOUT + 1V - VIN where D = VOUT + 1V Use twice this value for the LT3461.
Capacitor Selection Low ESR capacitors should be used at the output to minimize the output voltage ripple. Multilayer ceramic capacitors using X5R/X7R dielectrics are preferred as they
TYPICAL APPLICATIO S
VIN 5V C1 1F L1 10H CONTROL SIGNAL 47k 1 SW 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2 6
R1 261k R2 30.1k
15pF
47nF
C1, C2: TAIYO YUDEN EMK212BJ105 L1: MURATA LQH32CN100K53
Figure 4. 5V to 12V with Soft-Start Circuit (LT3461A)
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have a low ESR and maintain capacitance over wide voltage and temperature range. A 2.2F output capacitor is sufficient for most applications using the LT3461, while a 1F capacitor is sufficient for most applications using the LT3461A. High output voltages typically require less capacitance for loop stability. Always use a capacitor with sufficient voltage rating. Either ceramic or solid tantalum capacitors may be used for the input decoupling capacitor, which should be placed as close as possible to the LT3461/LT3461A. A 1F capacitor is sufficient for most applications. Phase Lead Capacitor A small value capacitor can be added across resistor R1 between the output and the FB pin to reduce output perturbation due to a load step and to improve transient response. This phase lead capacitor introduces a polezero pair to the feedback that boosts phase margin near the cross-over frequency. The following formula is useful to estimate the capacitor value needed: C PL = 500k * 1pF R2 For an application running 50A in the feedback divider, capacitor values from 10pF to 22pF work well.
Input Current and Output Voltage
VOUT 12V 70mA
CONTROL SIGNAL 5V/DIV IIN 50mA/DIV VOUT 5V/DIV 1ms/DIV
3461a TA02b
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C2 1F
3461a TA02a
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LT3461/LT3461A
TYPICAL APPLICATIO S
3.3V to 5V Step-Up Converter Efficiency 3.3V to 5V Step-Up Converter (LT3461A)
VIN 3.3V C1 1F OFF ON L1 4.7H
1 SW 6 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2
R1 45.3k R2 15k
VOUT 5V 115mA 15pF C2 1F
3461a TA03a
EFFICIENCY (%)
C1, C2: TAIYO YUDEN X7R LMK212BJ105 L1: MURATA LQH32CN4R7M33 OR EQUIVALENT
PACKAGE DESCRIPTIO
0.62 MAX
0.95 REF
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 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
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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80
75
70
65
60
0
30 60 90 LOAD CURRENT (mA)
120
3461a TA03b
S6 Package 6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
2.90 BSC (NOTE 4)
1.22 REF
1.4 MIN
2.80 BSC
1.50 - 1.75 (NOTE 4) PIN ONE ID
0.95 BSC 0.80 - 0.90
0.30 - 0.45 6 PLCS (NOTE 3)
0.01 - 0.10
0.09 - 0.20 (NOTE 3)
1.90 BSC
S6 TSOT-23 0302
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LT3461/LT3461A
TYPICAL APPLICATIO S
Low Profile (1mm) 3.3V to 15V Step-Up Converter
VIN 3.3V C1 1F OFF ON L1 10H 1 SW 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2 6
332k
EFFICIENCY (%)
C1: TAIYO YUDEN LMK107BJ105KA C2: TAIYO YUDEN EMK316BJ225KD (X5R) L1: MURATA LQH2MCN100K02
5V to 36V Step-Up Converter (LT3461)
80
VIN 5V C1 1F OFF ON
L1 47H 1 SW 6 5 VIN VOUT LT3461 4 3 SHDN FB GND 2
280k
22pF
EFFICIENCY (%)
10k
C1: TAIYO YUDEN X7R LMK212BJ105 C2: MURATA GRM42-6X7R474K50 L1: MURATA LQH32CN470
RELATED PARTS
PART NUMBER LT1615/LT1615-1 LT1944/LT1944-1 LTC3400/LTC3400B LT3460 LT3465/LT3465A DESCRIPTION 300mA/80mA (ISW) Constant Off-Time, High Efficiency Step-Up DC/DC Converter Dual Output 350mA/100mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converter 600mA (ISW), 1.2MHz, Synchronous Step-Up DC/DC Converter 0.32A (ISW), 1.3MHz, High Efficiency Step-Up DC/DC Converter Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode COMMENTS VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20A, ISD <1A, ThinSOT Package VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 20A, ISD <1A, MS Package VIN: 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19A/300A, ISD <1A, ThinSOT VIN: 2.5V to 16V, VOUT(MAX) = 36V, IQ = 2mA, ISD <1A, SC70, ThinSOT Packages VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD <1A, ThinSOT Package
3461af LT/TP 1004 1K * PRINTED IN USA
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
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3.3V to 15V Efficiency
75
VOUT 15V 25mA 22pF C2 2.2F
3461a TA04a
70
65
30.1k
60
55
50
0
5
10 15 20 LOAD CURRENT (mA)
25
30
3461a TA04b
5V to 36V Efficiency
75
VOUT 36V 18mA C2 0.47F 50V
3461 TA05a
70 65 60 55 50 0 2 4 6 8 10 12 14 16 18 LOAD CURRENT (mA)
3461 TA05b
3.3V to 5V Dual Output Converter
VIN 3.3V C1 1F OFF ON L1 4.7H 1 SW 5 VIN VOUT LT3461A 4 3 SHDN FB GND 2 6 C3 1F
45.3k
VOUT 5V 100mA 15pF C2 1F
15k
D1 C1, C2, C3, C4: TAIYO YUDEN JMK107BJ105 D1, D2: PHILIPS PMEG2005EB L1: MURATA LQH2MCN4R7M02 D2 C4 1F 10 -5V 15mA
3461 TA06
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2003

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