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AN1317 - APPLICATION NOTE

switching frequency will be comprised between 20 kHz and 30 kHz, just above audible values.

During the start up phase, the VIPer20 is in standby mode and its on chip high voltage current source

Then, this current source is turned off and the device starts switching. After a transition phase during

finally, from the positive output through the diode D2 when the output voltage becomes higher than the

Figure 1: 2W typical single output not isolated power supply with Buck topology

In normal operation, the output voltage regulation is achieved by the VIPer20 error amplifier which

diode D2 is here partially compensated by the forward voltage across the diode D3. So, the output

voltage and the on chip voltage reference values are equal, except for diode forward voltage differences

due to different diodes current: It is generally higher in the free wheeling diode D3, resulting in a slightly

lower output voltage.

A typical characteristic of the Buck is that the inductor charge and discharge paths are exclusively done

through the output load. It is a slight advantage in normal operation because the energy is transferred to

the load during both turn on and turn off cycles, but in very low or no load conditions, it has two

drawbacks:

• The charge of the start up tank capacitor C2 is impossible, especially when the input voltage is slowly

13V

OSC

COMP

SOURCE

DRAIN

VDD

-

+

VIPer20

R2

3.9k

C4

100nF

R1

10k

C3

10nF

D1

1N4007

C2

22uF

400V

L1

470uH

D3

BYT01-400V

C5

33uF

16V

D2

BYT01-400V

C1

10uF

16V

DZ1

BZX55C15V

AC IN

AC IN

+13V

GND OUT