Si9117

Vishay Siliconix

www.vishay.com

10

Document Number: 70027

S-40750—Rev. E, 19-Apr-04

Feedback

Comparator

UV-low

4.6 V

20 mA

Error Amp

FIGURE 4. Soft-Start

Oscillator

50% duty cycle, so that core saturation can be avoided. When

disconnects the oscillator output from the SYNC terminal using

the current mode comparator circuit.

equations:

8

1

2 x

1

(1)

(2)

(3)

Actual values taken from a prototype board have been

plotted (Figure 6), and are a close match (except for 47 pF,

where stray parasitics have a more significant effect).

In certain circumstances, such as current limiting, it may be

desirable to change the frequency of the converter for a period

of time to overcome current tails (see Figure 14 for further

explanation). With the Si9117, this is easily done by adding or

nal emitter follower from the reference.

The frequency can be changed easily by supplying some of

internal current source, and slowing the frequency down.

(See Figure 7.)

DQ

Q

CLK

SYNC

FIGURE 5. Oscillator