M51995AFP datasheet(25/43 Pages) RENESAS

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M51995AP/AFP

REJ03D0835-0300 Rev.3.00 Jun 06, 2008

Page 23 of 40

Intermittent Action Circuit Section

Intermittent action circuit will start to operate when the output signal from the intermittent action and oscillation control

circuit are “high” and also VF terminal voltage is lower than VTHTIME of about 3 V.

Figure 12 shows the block diagram of intermittent action circuit. Transistor Q is on state when VF terminal voltage is

higher than VTHTIME of about 3 V, so the CT terminal voltage is near to GND potential.

When VF terminal voltage is lower than VTHTIME, Q becomes “off” and the CT has the possibility to be charged up.

Under this condition, if the intermittent action and oscillation control signal become “high” the switch SWA will close

only in this “high” duration and CT is charged up by the current of 120 µA through SWA (SWB is open) and CT terminal

potential will rise. The output pulse can be generated only this duration. When the CT terminal voltage reaches to 8 V,

the control logic circuit makes the SWA “off” and SWB “on”, in order to flow in the ITIMEOFF of 15 µA to CT terminal.

The IC operation will be ceased in the falling duration.

On the other hand, when CT terminal voltage decreases to lower than 2 V, the IC operation will be reset to original state,

as the control logic circuit makes the SWA “on” and SWB “off”.

Therefore the parts in power circuit including secondary rectifier diodes are protected from the overheat by the over

current.

ITIMEOFF

(

≈ 15 µA)

ITIMEON

(

≈ 120 µA)

Control

logic

VTHTIME (≈ 3 V)

−

Figure 12 Block Diagram of Intermittent Action Circuit

No operating

duration

8 V

2 V

Figure 13 Waveform of CT Terminal

Figure 14 shows the ICC versus VCC in this timer-off duration.

In this duration the power is not supplied to IC from the third winding of transformer but through from the resistor R1

connected to VCC line.

If the R1 shown in Application Example is selected adequate value, VCC terminal voltage will be kept at not so high or

low but adequate value, as the ICC versus VCC characteristics has such the one shown in figure 14.

To ground the CT terminal is recommended, when the intermittent mode is not used.

In this case the oscillated frequency will become low but the IC will neither stop the oscillation nor change to the

intermittent action mode, when the current limit function becomes to operate and the VF terminal voltage becomes low.

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