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LT1425 Isolated Flyback Switching Regulator
FEATURES
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DESCRIPTION
The LT ®1425 is a monolithic high power switching regulator specifically designed for the isolated flyback topology. No “third winding” or optoisolator is required; the integrated circuit senses the isolated output voltage directly from the primary side flyback waveform. A high current, high efficiency switch is included on the die along with all oscillator, control and protection circuitry. The LT1425 operates with input supply voltages from 3V to 20V and draws only 7mA quiescent current. It can deliver output power up to 6W with no external power devices. By utilizing current mode switching techniques, it provides excellent AC and DC line regulation. The LT1425 has a number of features not found on other switching regulator ICs. Its unique control circuitry can maintain regulation well into discontinuous mode in most applications. Optional load compensation circuitry allows for improved load regulation. An externally activated shutdown mode reduces total supply current to 15µA for standby operation.
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No Transformer “Third Winding” or Optoisolator Required ± 5% Accurate Output Voltage Without User Trims (See Circuit Below) Resistor Programmable Output Voltage Regulation Maintained Well Into Discontinuous Mode (Light Load) Optional Load Compensation Operating Frequency: 285kHz Easily Synchronized to External Clock Available in 16-Pin Narrow SO Package
U APPLICATIONS
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Isolated Flyback Switching Regulators Ethernet Isolated 5V to – 9V Converters Medical Instruments Isolated Telecom Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
5V to Isolated – 9VOUT
500V ISOLATION BARRIER 5V T1*
9.5 9.4
+
V–
OUTPUT VOLTAGE (V)
VIN
VSW
11 R1 22.6k 3 1%
•
•
LT1425 15 6 4 C3 1000pF SHDN SYNC VC RREF ROCOMP RCCOMP SGND PGND 7 10 RFB
*DALE LPE 4841-330MB R2 3.01k 1%
5 14 13
C4 0.1µF
R3 15k
+
C1 100µF 10V
12
D1 1N5819
C2 47µF 16V
ISOLATED –9V ± 5% AT 20mA TO 200mA
F
9.3 9.2 9.1 9.0 8.9 8.8 8.7 8.6 8.5 0 50 100 150 OUTPUT CURRENT (mA) 200
1425 TA02
1425 TA01
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Load Regulation
1
LT1425
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW GND 1 NC 2 RFB 3 VC 4 RREF 5 SYNC 6 SGND 7 GND 8 16 GND 15 SHDN 14 ROCOMP 13 RCCOMP 12 VIN 11 VSW 10 PGND 9 GND
Supply Voltage ........................................................ 20V Switch Voltage ......................................................... 35V SHDN, SYNC Pin Voltage ........................................... 7V RFB Pin Current....................................................... 2mA Operating Junction Temperature Range Commercial .......................................... 0°C to 100°C Industrial ......................................... – 40°C to 100°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART NUMBER LT1425CS LT1425IS
S PACKAGE 16-LEAD PLASTIC SO
TJMAX = 145°C, θJA = 75°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL IREF IIN gm VCL PARAMETER Reference Current RREF Pin Input Current Feedback Amplifier Transconductance Feedback Amplifier Clamp Voltage Reference Voltage/Current Line Regulation Voltage Gain VIN Sense Error Output Switch BV V(VSW) ILIM Output Switch Breakdown Voltage Output Switch ON Voltage Switch Current Limit Feedback Amplifier
VIN = 5V, TJ = 25°C, VSW open, VC = 1.4V, unless otherwise specified.
MIN 402 396 400 30 TYP 408 500 MAX 414 420 1600 80 0.04 25 UNITS µA µA nA µmho µA V %/V V/V mV V 0.85 1.9 1.9 V A A A V V A/V 300 320 260 kHz kHz ns ns ns 95 %
CONDITIONS Measured at RFB Pin with RREF = 3.000k
q
∆IC = ± 10µA (Note 2)
q q
1000 50 1.9 0.01 500 10
ISOURCE, ISINK Feedback Amplifier Source or Sink Current 5V ≤ VIN ≤ 18V (Note 3)
q
q
IC = 5mA ISW = 1A Duty Cycle = 50%, 0°C ≤ TJ ≤ 100°C Duty Cycle = 50%, – 40°C ≤ TJ ≤ 100°C Duty Cycle = 80% Duty Cycle = Minimum
q q q q
35 1.35 1.25
50 0.55 1.60 1.60 1.30 1.2 2
Current Amplifier Control Pin Threshold Control Voltage to Switch Transconductance Timing f tON tED tEN Switching Frequency
q q
0.95 0.85
1.3 1.4
260 240 170
285 210 150 180
Minimum Switch ON Time Flyback Enable Delay Time Minimum Flyback Enable Time Maximum Switch Duty Cycle
q
85
90
2
U
W
U
U
W W
W
LT1425
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER ∆VRCCOMP/∆ISW SYNC Function Minimum SYNC Amplitude Synchronization Range SYNC Pin Input Resistance Power Supply VIN(MIN) ICC Minimum Input Voltage Supply Current Shutdown Mode Supply Current Shutdown Mode Threshold Load Compensation
VIN = 5V, TJ = 25°C, VSW Open, VC = 1.4V, unless otherwise specified.
MIN TYP 0.45
q
CONDITIONS
MAX
UNITS Ω
1.5 320 40
2.2 450
V kHz kΩ
q q q q
2.8 7.0 15 0.4 0.9
3.1 9.5 40 1.3
V mA µA V
The q denotes the specifications which apply over the full operating temperature range. Note 1: Absolute Maximum Ratings are those val.