DER-49

-12V/3W, Non-Isolated Buck-Boost Converter

April 20, 2005

Page 5 of 17

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3

Schematic

L1

2.2mH

J4

1

R3

12k 1%

RF2

47/3W

J1

1

J3

1

D1

1N4007

+

C1

10uF/400V

+

C4

10uF/50V

RF1

47/3W

R2

3.3k

NEUTRAL

J2

1

108 - 265VAC

50/60Hz

LINE

+

C2

10uF/400V

C3

100uF/25V

-12V @

250mA

U1

LNK305P

D3

MUR160

C5

0.1uF

L2

1500uH

R4

2.05k 1%

D2

1N4005GP

Figure 2 –Schematic.

4

Circuit Description

The circuit shown in Figure 2 is a non-isolated buck-boost (inverting) topology. The input

voltage range is 108 to 265VAC 50/60Hz and provides a regulated –12V at 250mA. The

buck-boost topology is essentially the non-isolated version of the Flyback Converter, in that

the transformer is replaced with a single low cost inductor (L2).

RF1 is a fusible link resistor. The input AC is half-wave rectified and filtered by D1 and C1.

C1, L1 and C2 form a pi-filter network to reduce common-mode emissions imposed to the

input line, this in conjunction with the built-in frequency jitter of the LinkSwitch-TN (U1)

ensure sufficient conducted EMI margins. U1, D2 and L2 form the buck-boost switching cell,

which converts the rectified bulk positive DC voltage on C2 into a negative voltage on C3

(w/r/t Neutral/GND). D2 samples the output voltage onto C4 as a positive voltage with

respect to the source of U1. The EN pin of U1 is internally set to 1.65V (w/r/t pins 1,2, 7 and

8) this in conjunction with resistors R3 and R4 form a simple voltage divider to precisely set

the output voltage to the desired level. C5 is a bypass capacitor that serves as high

frequency decoupling and energy storage. This capacitor provides power to the IC as well as

controls the auto-restart mechanism in the LinkSwitch-TN. Resistor R2 serves to reduce

peak charging effects on C3 which tend to increase the output voltage, its static power

dissipation is limited to less than 30mW. Without this additional resistor the standby power

consumption would be less than 100mW.