M51995AP/AFP

REJ03D0835-0201 Rev.2.01 Nov 14, 2007

Page 21 of 40

Current Limiting Section

When the current-limit signal is applied before the crossing instant of “A” pint potential and CF terminal voltage shown

in figure 7, this signal makes the output “off” and the off state will continue until next cycle. Figure 9 shows the timing

relation among them.

The current limiting circuit has two input terminals, one has the detector-sensitivity of

+200 mV to the GND terminal

and the other has

−200 mV. The circuit will be latched if the input signal is over the limit of either terminal.

If the current limiting circuit is set, no waveform is generated at output terminal however this state is reset during the

succeeding dead-time.

So this current limiting circuit is able to have the function in every cycle, and is named “pulse-by-pulse current limit”.

It is rather recommended to use not "CLM

+" but "CLM−" terminal, as the influence from the gate drive current of MOS

FET can be eliminated and wide voltage rating of

+4 V to −4 V is guaranteed for absolute maximum rating.

There happen some noise voltage on RCLM during the switching of power transistor due to the snubber circuit and stray

capacitor of the transformer windings.

To eliminate the abnormal operation by the noise voltage, the low pass filter, which consists of RNF and CNF is used as

shown in figure 10.

It is recommended to use 10 to 100

Ω for R

NF because such range of RNF is not influenced by the flow-out current of

some 200

µA from C

LM terminal and CNF is designed to have the enough value to absorb the noise voltage.

VTHCLM ≈ 200 mV

VTHCLM ≈ −200 mV

(a) +current limit

(b)

−current limit

OSC waveform of

CF terminal

Waveform of

CLM+ terminal

Current limit signal

to set latch

Waveform of

VOUT terminal

OSC waveform of

CF terminal

Waveform of

CLM

− terminal

Current limit signal

to set latch

Waveform of

VOUT terminal

Figure 9 Operating Waveforms of Current Limiting Circuit