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 2006 Semtech Corp.

www.semtech.com

SC4911

POWER MANAGEMENT

Typical Applications (Cont.)

Isolated feedback 48V to 12V application

Figure 3 gives an example of isolated feedback and low

power flyback application of the SC4911. T1 is a three

winding transformer. The winding 1-2 is a bias winding

which supplies the primary circuitry after the SC4911

starts up. During the startup, the input source trickle

charges the capacitor C1 through the start-up resistor

R1. As long as voltage across C1 reaches the Vcc turn-

on threshold, the SC4911 turns on abruptly and start

driving the gate of MOSFET Q1. As Q1 starts switching,

regenerative feedback from the bias winding will provide

current via D1 to capacitor C1.

In this configuration, the input voltage is in the range of

36V to 72V for telecom applications, and the LUVLO pin

monitors the input voltage via resistor R2 and R5 to

determine turn-on and turn-off of the converter. Capacitor

C5 helps filter out the noise from the input line.

Timing resistor R13 sets switching frequency. R3 and

C6 are used to suppress the voltage spikes across the

switch due to leakage inductance of T1. R11 is current

sense resistor to set current limit of the convertor. R10

and C9 filter spikes caused by parasitic capacitance

during the turning on of Q1. The U3 SC431 is shunt

regulator integrating an accurate 2.5V bandgap

reference, op amp and driver into a single device. It

compares sensed output voltage divided by R7 and R9

to its internal reference and drives the amplified error

signal to opto-coupler U1. C7 keeps ripple and noise

from over driving the device. U1 delivers the isolated

feedback signal to the SC4911. C8 and R12 are for

compensation. An additional small capacitor could be

used from COMP to GND if needed.

D2 1N5819

C2

100uF

D1 1N5819

C5

100pF

Q1

Si2320DS

R12

4.7K

R7

10K

C7

220pF

R2

2.7MEG

R10

51

R5

100K

R9

10K

C1

10uF

NEC2501

U1

C9

1000pF

Vin = 48V

C8

47nF

R4

330

C4

0.1uF

R11

1.00

R13

100K

R3

68

R1

100K

Vo = 5V

C3

10uF

U3

SC431

C6

150pF

R6

1K

R8

1K

U2 SC4911

1

2

3

4

5

6

7

8

LUVLO

RT

FB

COMP

ILIM

GND

OUT

VCC

T1

3

5

8

10

2

1

Figure 3. Isolated 48V to 12V Flyback Converter