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 2006 Semtech Corp.

www.semtech.com

SC4911

POWER MANAGEMENT

Non-Isolated feedback 48V to 12V application

Figure 4 is an example of low cost, non-isolated feedback,

low power application of the SC4911. T1 is a two winding

transformer. The output voltage is directly fed to the

primary circuitry.

During the start-up, the input source trickle charges the

capacitor C3 through start-up resistor R1. As long as the

voltage across C3 reaches the Vcc turn-on threshold,

the SC4911 turns on abruptly and starts driving the gate

of MOSFET Q1. After the SC4911 starts up the secondary

winding supplies the primary circuitry.

In this 36V to 72V telecom application, LUVLO monitors

the input voltage via resistor R2 and R4 to determine

turn-on and turn-off of the converter. Capacitor C4 helps

filter out the noise from the input line. Voltage feedback

is directly brought from the output to the input error

amplifier of SC4911 through R5 and R7. C6 and R9 are

for frequency compensation. An additional small

capacitor could be used from COMP to GND if needed.

Typical Applications (Cont.)

Vo = 12V

C6

47nF

C4

100pF

D1 1N5819

Q1

Si2320DS

C5

150pF

U1 SC4911

1

2

3

4

5

6

7

8

LUVLO

RT

FB

COMP

ILIM

GND

OUT

VCC

C1

47uF

R6

51

C3

10uF

R7

100K

T1

C7

1000pF

R4

100K

D2 1N5819

R8

1.00

C2

10uF

R1

100K

R9

4.7K

R2

2.7MEG

R3

68

R5

383K

Vin = 48V

R10

100K

Figure 4. Non-isolated 48V to 12V Flyback Converter