M51995AP/AFP

REJ03D0835-0300 Rev.3.00 Jun 06, 2008

Page 20 of 40

PWM Comparator and PWM Latch Section

Figure 7 shows the PWM comparator and latch section. The on-duration of output waveform coincides with the rising

duration of CF terminal waveform, when the infinitive resistor is connected between F/B terminal and GND.

When the F/B terminal has finite impedance and current flows out from F/B terminal, “A” point potential shown in

figure 7 depends on this current. So the “A” point potential is close to GND level when the flow-out current becomes

large.

“A” point potential is compared with the CF terminal oscillator waveform and PWM comparator, and the latch circuit is

set when the potential of oscillator waveform is higher than “A” point potential.

On the other hand, this latch circuit is reset by high level signal during the dead-time of oscillation (falling duration of

oscillation waveform). So the “B” point potential or output waveform of latch circuit is the one shown in figure 8.

The final output waveform or “C” point potential is got by combining the “B” point signal and dead-time signal

logically. (please refer to figure 8)

+

−

≈ 7.1 V

5.8 V

Point A

PWM

COMP.

CF

M51995A

F/B

6 S

1 S

Latch

Point B

To

output

Point C

From

OSC

200

µA

Figure 7 PWM Comparator and Latch Circuit

Waveform of

OSC & point A

Point B

Point C

OSC waveform

Point A

Figure 8 Waveforms of PWM Comparator Input point A, Latch Circuit Points B and C