Document
LT1737 High Power Isolated Flyback Controller
FEATURES
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DESCRIPTIO
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Drives External Power MOSFET Supply Voltage Range: 4.5V to 20V Flyback Voltage Limited Only by External Components Senses Output Voltage Directly from Primary Side Winding—No Optoisolator Required Switching Frequency from 50kHz to 250kHz with External Capacitor Moderate Accuracy Regulation Without User Trims Regulation Maintained Well into Discontinuous Mode External ISENSE Resistor Optional Load Compensation Optional Undervoltage Lockout Shutdown Feature Reduces IQ to 50µA Typ Available in 16-Pin GN and SO Packages
The LT®1737 is a monolithic switching regulator controller specifically designed for the isolated flyback topology. It drives the gate of an external MOSFET and is generally powered from a DC supply voltage. Output voltage feedback information may be supplied by a variety of methods including a third transformer winding, the primary winding or even direct DC feedback (see Applications Information). Its gate drive capability, coupled with a suitable external MOSFET and other power path components, can deliver load power up to tens of Watts. The LT1737 has a number of features not found on other isolated flyback controller ICs. By utilizing current mode switching techniques, it provides excellent AC and DC line regulation. Its unique control circuitry can maintain regulation well into discontinuous mode in most applications. Optional load compensation circuitry allows for improved load regulation. An optional undervoltage lockout pin halts operation when the application input voltage is too low. An optional external capacitor implements a softstart function.
, LTC and LT are registered trademarks of Linear Technology Corporation.
U APPLICATIO S
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Isolated Flyback Switching Regulators Medical Instruments Instrumentation Power Supplies
TYPICAL APPLICATIO
R2 35.7k 1% D2 BAS16
12V-18V to Isolated 15V Converter
VIN C1 22µF
+
Q1 2N3906
T1 COILTRONICS CTX150-4
240k 33k C3 0.1µF
150µH
• •
150µH
UVLO FB VC R3 3.01k 1% 1nF OSCAP 47pF LT1737
VCC GATE ISENSE
M1 IRFL014 R1 0.27Ω
tON ENDLY MINENAB ROCMP 75k 150k 100k 4.7k
RCMPC
SGND
PGND
0.1µF
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D1 MBRS1100
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C2 33µF R4 7.5k VOUT = 15V IOUT = 300mA NOTE: SEE APPLICATIONS INFORMATION FOR ADDITIONAL COMPONENT SPECIFICATIONS
1737 TA01
1737f
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LT1737
ABSOLUTE
(Note 1)
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RATI GS
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TOP VIEW PGND 1 ISENSE 2 SFST 3 ROCMP 4 RCMPC 5 OSCAP 6 VC 7 FB 8 16 GATE 15 VCC 14 tON 13 ENDLY 12 MINENAB 11 SGND 10 UVLO 9 3VOUT
VCC Supply Voltage ................................................. 22V UVLO Pin Voltage .................................................... VCC ISENSE Pin Voltage .................................................... 2V FB Pin Current ..................................................... ± 2mA Operating Junction Temperature Range LT1737C ............................................... 0°C to 100°C LT1737I ............................................ –40°C to 125°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................ 300°C
ELECTRICAL CHARACTERISTICS
SYMBOL Power Supply VCC(MIN) ICC Minimum Input Voltage Supply Current Shutdown Current Feedback Voltage PARAMETER
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = 14V, GATE open, VC = 1.4V unless otherwise noted.
CONDITIONS
q
Feedback Amplifier VFB IFB gm ISRC, ISNK VCL
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Feedback Pin Input Current Feedback Amplifier Transconductance Feedback Amplifier Source or Sink Current Feedback Amplifier Clamp Voltage Reference Voltage/Current Line Regulation Voltage Gain Soft-Start Charging Current Soft-Start Discharge Current 4.75V ≤ VIN ≤ 18V VC = 1V to 2V VSFST = 0V VSFST = 1.5V, VUVLO = 0V IGATE = 100mA IGATE = 500mA IGATE = 100mA IGATE = 500mA VGATE = 2V CL = 1000pF CL = 1000pF
q q q q q q
Gate Output VGATE Output High Level Output Low Level IGATE tr tf Output Sink Current in Shutdown, VUVLO = 0V Rise Time Fall Time 11.5 11.0 12.1 11.8 0.3 0.6 1.2 2.5 30 30 0.45 1.0 V V V V mA ns ns
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ORDER PART NUMBER LT1737CGN LT1737CS LT1737IGN LT1737IS GN PART MARKING 1737 1737I
GN PACKAGE S PACKAGE 16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 110°C/W (GN) TJMAX = 125°C, θJA = 110°C/W (SO)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
MIN
TYP 4.1 10 50
MAX 4.5 15 150 1.260 1.270 1800 80 0.05 50
UNITS V mA µA V V nA µmho µA V %/V V/V µA mA
VC = Open VUVLO = 0V, VC = Open
q q
1.230 1.220 400 30
1.245 500
∆lC = ±10µA
q q
1000 50 2.5 0.01 2000
25 0.8
40 1.5
1737f
LT1737
ELECTRICAL CHARACTERISTICS
SYMBOL VC VISENSE PARAMETER Control Pin Threshold Switch Current Limit Current Amplifier Duty Cycle = Min
q
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = 14V, GATE op.