M51996AP/AFP

REJ03D0836-0201 Rev.2.01 Nov 14, 2007

Page 20 of 35

OVP Circuit (Over Voltage Protection Circuit) Section

OVP circuit is basically positive feedback circuit constructed by Q2, Q3 as shown in figure 13.

Q2, Q3 turn on and the circuit operation of IC stops, when the input signal is applied to OVP terminal. (threshold

voltage

≈ 750 mV)

The current value of I2 is about 150

µA when the OVP does not operates but it decreases to about 2 µA when OVP

operates.

It is necessary to input the sufficient larger current (800

µA to 8 mA) than I2 for triggering the OVP operation.

It is necessary that OVP state holds by circuit current from R1 in the application example, so this IC has the

On the other hand, the circuit current is large in the higher supply voltage, so the supply voltage of this IC doesn’t

become so high by the voltage drop across R1.

This characteristic is shown in figure 14.

The OVP terminal input current in the voltage lower than the OVP threshold voltage is based on I2 and the input current

in the voltage higher than the OVP threshold voltage is the sum of the current flowing to the base of Q3 and the current

flowing from the collector of Q2 to the base.

So if the capacitor is connected between the OVP terminal and GND, even though Q2 turns on in a moment by the

by charging this capacitor.

For resetting OVP state, it is necessary to make the OVP terminal voltage lower than the OVP L threshold voltage or

As the OVP reset voltage is settled on the rather high voltage of 9.0 V, SMPS can be reset in rather short time from the

switch-off of the AC power source if the smoothing capacitor is not so large value.

OVP

GND

8 k

12 k

Q1

Q2

2.5 k

400

7.8 V

100

µA

Q3

Figure 13 Detail Diagram of OVP Circuit