M51995AP/AFP

REJ03D0835-0300 Rev.3.00 Jun 06, 2008

Page 19 of 40

As shown in figure 5, the internal circuit kills the first output pulse in the output waveform. The output waveform

with that at the stable operating state.

Usually the applied voltage to VF terminal must be proportional the output voltage of the regulator.

So when the over current occurs and the output voltage of the regulator becomes low, the off-duration becomes wide.

There are two methods to get the control voltage, which depends on the output voltage, on primary side. For the fly

back type regulator application, the induced voltage on the third or bias winding is dependent on output voltage. On

the other hand, for the feed forward type regulator application, it can be used that the output voltage depends on the

product of induced voltage and “on-duty”, as the current of choke coil will continue at over load condition, it means

the “continuous current” condition.

Figure 6 shows one of the examples for VF terminal application for the feed forward type regulator.

Voltage waveform of

CF terminal

Voltage waveform of

output terminal at

Max ON duty

≈ 4.4 V

≈ 2.0 V

Figure 4 OSC Waveform with Operation of Intermittent and OSC Control Circuit Operation

Start from 0 V

First

pulse

Operation start

0

0

Voltage waveform of

CF terminal

Voltage waveform of

output terminal at

Max ON duty

No generate

pulse

Figure 5 Relation between OSC and Output Waveform Circuit Operation at Start-up

M51995A

VF

Figure 6 Feedback Loop with Low Pass Filter from Output to VF Terminal