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LT3462/LT3462A Inverting 1.2MHz/2.7MHz DC/DC Converters with Integrated Schottky in ThinSOT
FEATURES
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DESCRIPTIO
Integrated Schottky Rectifier Fixed Frequency 1.2MHz/2.7MHz Operation Very Low Noise: 1mVP-P Output Ripple Low VCESAT Switch: 270mV at 250mA -5V at 100mA from 5V Input -12V at 30mA from 3.3V Input Low Input Bias Current GND Based FB Input Low Impedance (40) 1.265V Reference Output High Output Voltage: Up to - 38V Wide Input Range: 2.5V to 16V Uses Tiny Surface Mount Components Low Shutdown Current: <10A Low Profile (1mm) SOT-23 (ThinSOTTM) Package
The LT(R)3462/LT3462A are general purpose fixed frequency current mode inverting DC/DC converters. Both devices feature an integrated Schottky and a low VCESAT switch allowing a small converter footprint and lower parts cost. The LT3462 switches at 1.2MHz while the LT3462A switches at 2.7MHz. These high speeds enable the use of tiny, low cost and low height capacitors and inductors. The LT3462/LT3462A operate in a dual inductor inverting topology that filters both the input and output currents. Very low output voltage ripple approaching 1mVP-P can be achieved when ceramic capacitors are used. Fixed frequency switching ensures a clean output free from low frequency noise typically present with charge pump solutions. The 40V switch allows a VIN to VOUT differential of up to 38V for dual inductor topologies. Both devices provide a low impedance 1.265V reference output to supply the feedback resistor network. A ground referenced, high impedance FB input allows high feedback resistor values without compromising output accuracy. The LT3462/LT3462A are available in a 6-lead SOT-23 package.
APPLICATIO S
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CCD Bias LCD Bias GaAs FET Bias General Purpose Negative Voltage Supply
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation
TYPICAL APPLICATIO
22H 1F
5V to -5V, 100mA Inverting DC/DC Converter
75 22H VIN 5V TA = 25C
1F
SW VIN
D FB
267k
22pF
EFFICIENCY (%)
VOUT -5V 100mA
70 VIN = 3.3V 65
LT3462A SDREF GND
68.1k
10F
60
3462 TA01
55
0
20
U
Efficiency
VIN = 5V 60 80 40 LOAD CURRENT (mA) 100
3462 TA01b
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U
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1
LT3462/LT3462A
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW SW 1 GND 2 FB 3 6 VIN 5D 4 SDREF
Input Voltage (VIN) .................................................. 16V SW Voltage .............................................................. 40V D Voltage ............................................................... -40V SDREF, FB Voltage ................................................. 2.5V Operating Ambient Temperature Range (Note 3) ...............-40C to 85C Maximum Junction Temperature .......................... 125C Storage Temperature Range ..................-65C to 150C Lead Temperature (Soldering, 10sec)................... 300C
ORDER PART NUMBER LT3462ES6 LT3462AES6 S6 PART MARKING LTBBV LTBGB
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 q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C, VIN = 3V, unless otherwise noted.
PARAMETER Minimum Operating Voltage Maximum Operating Voltage SDREF Voltage FB Pin Bias Current (Note 2) SDREF Minus FB Voltage Error Amp Offset Voltage SDREF Reference Source Current Supply Current SDREF Line Regulation Switching Frequency (LT3462) Switching Frequency (LT3462A) Maximum Duty Cycle (LT3462) Maximum Duty Cycle (LT3462A) Switch Current Limit Switch VCESAT Switch Leakage Current Rectifier Leakage Current Rectifier Forward Drop SDREF Voltage Low SDREF Off-State Pull-Up Current SDREF Turn-Off Current Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Current flows out of the pin. ISW = 250mA VSW = 5V VD = -40V ISCHOTTKY = 250mA
q q q q q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN 2.5
TYP
MAX 16
UNITS V V V nA V mV A mA A %/V MHz MHz % %
10A > ISDREF -80A 10A > ISDREF -80A SDREF >1.2V FB = -0.05V, Not Switching SDREF = 0V, FB = Open, VIN = 5V
q
1.245 1.235 -12 120
1.265 15 1.263 180 2.9 6.5 0.007
1.285 50 1.285 12 3.6 10 1.6 3.5
q
q
0.8 2.0 90 77 300
1.2 2.7
420 270 0.01 0.03 800 350 1 4 1100 0.20 3
1 -300
2 -200
Note 3: The LT3462E 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 A mV V A A
W
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W
LT3462/LT3462A TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency (LT3462)
1.6 1.5 CURRENT LIMIT (mA) 360 LT3462A FREQUENCY (MHz) 1.4 1.3 1.2 1.1 1.0 -40 -20 0 10 480 TA = 25C LT3462 SDREF MINUS FB (V) 1.28 1.27 1.26 1.25 1.24 1.23 -40 -20
40 20 60 0 TEMPERATURE (C)
Oscillator Frequency (LT3462A)
3.2 TA = 25C 3.0 FB BIAS CURRENT (nA) FREQUENCY (MHz) 2.8 2.6 2.4 2.2 2.0 -40 -20 0 -5
-15 -20 -25 -30 -35 -40 -45
QUIESCENT CURRENT (A)
40 20 60 0 TEMPERATURE (C)
PI FU CTIO S
SW (Pin 1): Switch Pin. Connect to external inductor L1 and positive terminal of transfer cap. GND (Pin 2): Ground. Tie directly to local ground plane. FB (Pin 3): Feedback Pin. Connect resistive divider tap here. Set R1 according to R1 = R2 * (VOUT/1.265V). In shutdown, a proprietary shutdown bias current cancellation circuit allows the internal 3A source to pull up the SDREF pin, even with residual negative voltage on VOUT. SDREF (Pin 4): Dual Function Shutdown and 1.265V Reference Output Pin. Pull to GND with external N-FET to turn regulator off. Turn-off pull-down and a 2A internal source will pull SDREF up to turn-on the regulator. At turnon, a 180A internal source pulls the pin to the regulation voltage. The SDREF pin can supply up to 80A at 1.265V to bias the feedback resistor divider. An optional soft-start circuit capacitor connects from this pin to -VOUT. D (Pin 5): Anode Terminal of Integrated Schottky Diode. Connect to negative terminal of transfer cap and external inductor L2. VIN (Pin 6): Input Supply Pin. Must be locally bypassed.
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UW
80 100
3462 G01
Current Limit
1.29
SDREF Minus FB Pin Voltage
240
120
20
30
40 50 60 70 DUTY CYCLE (%)
80
90
40 20 60 0 TEMPERATURE (C)
80
100
3462 G02
3462 G03
FB Bias Current
10
Quiescent Current in Shutdown Mode
TA = 25C FB = N/C 8
-10
6
4
2
80
100
-50 -40
0 -20 0 20 40 60 TEMPERATURE (C) 80 100
3462 G05
0
4
8
12
16
3462 G06
SUPPLY VOLTAGE (V)
3462 G04
U
U
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3
LT3462/LT3462A
BLOCK DIAGRA
FB 3
SDREF 4
SHUTDOWN BIAS CURRENT CANCELLATION OFF 3A ON 180A SHUTDOWN
VIN 6
1.265V REFERENCE
OPERATIO
The LT3462 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, 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. When this voltage exceeds the voltage at the output of the EAMP, the SR latch is reset, turning off the power switch. The level at the output of the EAMP is simply an amplified version of the difference between the feedback voltage and GND. 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 taken from the output; if it decreases, less current is taken. One function not shown in Figure 1 is the current limit. The switch current is constantly monitored and not allowed to exceed the nominal value of 400mA. If the switch current reaches 400mA, the SR latch is reset regardless of the
4
W
1 SW 5D
-
A1 E AMP
-
RC CC A2 COMP R S Q
DRIVER Q1 DO LG
+
+
+
0.1 ISRC RAMP GENERATOR VOUT 1.2MHz* OSCILLATOR *LT3462A IS 2.7MHz R1 (EXTERNAL) FB R2 (EXTERNAL) Q2 SDREF CS1, CS2 OPTIONAL SOFT-START COMPONENTS
3462 F02
-
2 GND
VOUT CS1 (EXTERNAL) SDREF CS2 (EXTERNAL)
- +
Figure 1. Block Diagram
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output state of the PWM comparator. This current limit cell protects the power switch as well as various external components connected to the LT3462. SDREF is a dual function input pin. When driven low it shuts the part down, reducing quiescent supply current to less than 10A. When not driven low, the SDREF pin has an internal pull-up current that turns the regulator on. Once the part is enabled, the SDREF pin sources up to 180A nominally at a fixed voltage of 1.265V through external resistor R2 to FB. If there is no fault condition present, FB will regulate to 0V, and VOUT will regulate to 1.265V * (-R1/R2). An optional soft-start circuit uses the fixed SDREF pull-up current and a capacitor from SDREF to VOUT to set the dV/dt on VOUT. In shutdown, an FB bias current cancellation circuit supplies up to 150A biasing current to external resistor R1 while VOUT is lower than FB. This function eliminates R2 loading of SDREF during shutdown. As a result, supply current in shutdown may exceed 10A by the amount of current flowing in R1.
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LT3462/LT3462A
APPLICATIO S I FOR ATIO
Inrush Current
The LT3462 has a built-in Schottky diode. When supply voltage is applied to the VIN pin, the voltage difference between VIN and VD generates inrush current flowing from input through the inductor and the Schottky diode to charge the flying capacitor to VIN. The maximum nonrepetitive surge current the Schottky diode in the LT3462 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. The peak inrush current can be calculated as follows:
VIN - O.6 - exp IP = L L -1 - 1 2 C C
where L is the inductance between supply and SW, and C is the capacitance between SW and D. Table 3 gives inrush peak currents for some component selections.
Table 3. Inrush Peak Current
VIN (V) 5 5 12 L (H) 22 33 47 C (F) 1 1 1 IP (A) 0.70 0.60 1.40
Inductor Selection Each of the two inductors used with LT3462 should have a saturation current rating (where inductance is approximately 70% of zero current inductance ) of approximately 0.25A or greater. If the device is used in the charge pump mode, where there is only one inductor, then its rating should be 0.35A or greater. DCR of the inductors should be less than 1. For LT3462, a value of 22H is suitable if using a coupled inductor such as Sumida CLS62-220. If using two separate inductors, increasing the value to 47H will result in the same ripple current. For LT3462A, a value of 10H for the coupled inductor and 22H for two inductors will be acceptable for most applications.
C3 R1
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Capacitor Selection Ceramic capacitors are recommended. An X7R or X5R dielectric should be used to avoid capacitance decreasing severely with applied voltage and at temperature limits. The "flying" capacitor between the SW and D pins should be a ceramic type of value 1F or more. When used in the dual inductor or coupled inductor topologies the flying capacitor should have a voltage rating that is more than the difference between the input and output voltages. For the charge pump inverter topology, the voltage rating should be more than the output voltage. The output capacitor should be a ceramic type. Acceptable output capacitance varies from 1F for high VOUT (-36V), to 10F for low VOUT (-5V). The input capacitor should be a 1F ceramic type and be placed as close as possible to the LT3462/LT3462A. Layout Hints The high speed operation of the LT3462 demands careful attention to board layout. You will not get advertised performance with careless layout. Figure 2 shows the recommended component placement. A ceramic capacitor of 1F or more must be placed close to the IC for input supply bypassing.
C1
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+
GND
L1 VIN 1 2 3 R2 6 5 4 L2
C2
C4 VOUT
3462 F03
Figure 2. Suggested Layout
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LT3462/LT3462A
TYPICAL APPLICATIO S
3.3V to -12V with Soft-Start Circuit
L1 47H C2 1F L2 47H
80 75 TA = 25C
VIN 3.3V
C1 4.7F
SW VIN
D FB
R1 267k R2 27.4k
C4 15pF CS1 100nF
EFFICIENCY (%)
LT3462 SDREF GND
OFF
C1: TAIYO YUDEN X5R JMK212BJ475MG C2: TAIYO YUDEN X5R EMK212BJ105MG C3: TAIYO YUDEN EMK316BJ225 L1, L2: MURATA LQH32CN470
VOUT Reaches -12V in 750s; Input Current Peaks at 300mA without CS1
OFF OFF
VOUT 10V/DIV
IIN 100mA/DIV 2ms/DIV
3462 TA02c
Li+ to -8V Supply
VIN 2.7V TO 4.2V L1A 22H C2 1F L1B 22H
80 75 TA = 25C
C1 4.7F
SW VIN LT3462
D FB
R1 267k R2 42.2k
C4 15pF C3 4.7F
EFFICIENCY (%)
SDREF GND
C1: TAIYO YUDEN X5R JMK212BJ475MG C2: TAIYO YUDEN X5R EMK212BJ105MG C3: TAIYO YUDEN LMK316BJ475 L1: SUMIDA CLS62-220 OR 2X MURATA LQH32CN330
6
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-12V Efficiency
VOUT -12V 30mA C3 2.2F
VIN = 3.3V 70 65 60 55
M1
22nF
50
3462 TA02a
0
5
10 15 20 25 LOAD CURRENT (mA)
30
35
3462 TA02b
VOUT Reaches -12V in 7.5ms; Input Current Peaks at 125mA with CS1 = 100nF
VOUT 10V/DIV
IIN 50mA/DIV 2ms/DIV
3462 TA02d
-8V Efficiency
VIN = 3.3V
VOUT -8V
70 65 60 55 50
3462 TA03a
0
10
20 30 40 LOAD CURRENT (mA)
50
3462 TA03b
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LT3462/LT3462A
TYPICAL APPLICATIO S
3.3V to -8V (LT3462A)
VIN 2.7V TO 4.2V L1A 10H C2 1F L1B 10H VIN 5V VOUT -8V 35mA C1 1F SW VIN D FB
C1 1F
SW VIN
D FB
LT3462A SDREF GND
C1: TAIYO YUDEN JMK107BJ105MA C2: TAIYO YUDEN EMK212BJ105MA C3: TAIYO YUDEN LMK316BJ475 .. L1: WURTH 50310057-100
PACKAGE DESCRIPTIO
0.62 MAX 0.95 REF
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC DATUM `A'
0.30 - 0.50 REF
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.
U
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5V to -5V Supply (LT3462A)
L1 22H C2 1F L2 22H
R1 267k R2 42.2k
C4 22pF
R1 267k R2 68.1k
C4 22pF
VOUT -5V 100mA C3 10F
C3 4.7F
LT3462A SDREF GND
3462 TA04a
C1: TAIYO YUDEN JMK107BJ105MA C2: TAIYO YUDEN EMK212BJ105MA C3: MURATA GRM219R60J106KE19B L1, L2: MURATA LQH32CN220
3462 TA05a
Switching Waveform
INDUCTOR 50mA/DIV
VSW 10V/DIV
VOUT 1mV/DIV AC COUPLED 200ns/DIV
3462 TA05b
S6 Package 6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
2.90 BSC (NOTE 4)
1.22 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
1.4 MIN
2.80 BSC
1.50 - 1.75 (NOTE 4) PIN ONE ID
0.95 BSC 0.80 - 0.90 0.09 - 0.20 (NOTE 3) 1.00 MAX
0.30 - 0.45 6 PLCS (NOTE 3)
0.01 - 0.10
1.90 BSC
S6 TSOT-23 0302
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LT3462/LT3462A
TYPICAL APPLICATIO S
12V to -36V DC/DC Converter
L1 47H C2 0.47F
VIN 12V
C1 1F
SW VIN LT3462
D FB
R1 432k R2 15k 100nF
EFFICIENCY (%)
SDREF GND
C1: TAIYO YUDEN X5R EMK212BJ105 C2: MURATA GRM42-6X7R474K50 C3: MURATA GRM42-6X7R474K50 x2 D1: CENTRAL CMSH5-4-LTN L1: MURATA LQH32CN470
RELATED PARTS
PART NUMBER LT1617/LT1617-1 LT1931/LT1931A LT1945 LT1946/LT1946A LT3463 DESCRIPTION 350mA/100mA (ISW) High Efficiency Micropower Inverting DC/DC Converter 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Micropower Inverting DC/DC Converter Dual Output, Boost/Inverter, 350mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converter 1.5A (ISW), 1.2MHz/2.7MHz, High Efficiency Step-Up DC/DC Converter Dual Output, Boost/Inverter, 250mA (ISW), Constant Off-Time, High Efficiency Step-Up DC/DC Converter with Integrated Schottky Diodes 85mA (ISW), High Efficiency Step-Up DC/DC Converter with Integrated Schottky and PNP Disconnect COMMENTS VIN: 1.2V to 15V, VOUT(MAX) = -34V, IQ = 20A, ISD <1A ThinSOT Package VIN: 2.6V to 16V, VOUT(MAX) = -34V, IQ = 5.8mA, ISD <1A ThinSOT Package VIN: 1.2V to 15V, VOUT(MAX) = 34V, IQ = 40A, ISD <1A, MS10 Package VIN: 2.45V to 16V, VOUT(MAX) = 34V, IQ = 3.2mA, ISD <1A MS8 Package VIN: 2.3V to 15V, VOUT(MAX) = 40V, IQ = 40A, ISD <1A DFN Package VIN: 2.3V to 10V, VOUT(MAX) = 34V, IQ = 25A, ISD <1A ThinSOT Package
LT3464
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
q
FAX: (408) 434-0507 q www.linear.com
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-36V Efficiency
85
D1
TA = 25C VIN = 12V
80
VOUT -36V 36mA
C4 5pF
75
C3 1F 50V
70
65
3462 TA06a
60
0
10
20
30
40
3462 TA06b
LOAD CURRENT (mA)
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(c) LINEAR TECHNOLOGY CORPORATION 2004

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