Data Sheet G14309EJ2V0DS00

28

µµµµPC1909

4.8 Overvoltage Protection Circuit Setting

The overvoltage protection circuit based on the overvoltage latch in the

µPC1909 is configured as shown in Figure 4-

12 below. The threshold voltage connected to the overvoltage detection comparator (VTH(OV)) is 2.0 to 2.8 V (2.4 V

TYP.). If the input at the OV pin exceeds VTH(OV), the flip-flop in the IC latches OUT1 and OUT2 to low level. Once the

overvoltage latch circuit is latched, it is not released until the power supply voltage of the IC (VCC) falls below the OVL

release voltage (2 V TYP.) (because ICC is higher than ICC(SB) when the circuit is latched, the operation status will not be

restored in the steady input state).

Figure 4-12.

µµµµPC1909 Overvoltage Protection Circuit

VTH(OV)

SQ

R

VREF

VCC

GND

OV

1

3

9

14

IB(OV)

From converter output

Photo-

coupler

To output circuit

of OUT1, OUT2

Overvoltage detection comparator

Flip-flop

Reference

voltage

Zener diode

Discharge current flows through the OV pin. This is the input bias current of the overvoltage latch block in the electrical

specifications (IB(OV) = 4

µA MAX.). To prevent the detection level fluctuating due to IB(OV), and to allow for the effect of

the leakage current between C and E in the photocoupler, be sure to set the overvoltage detection resistor connected to

the OV pin to no more than 100 k

Ω.

Because the OVL (overvoltage latch) detection delay time is about 750 ns, a capacitor with good frequency

characteristics should be connected between the OV pin and GND to prevent malfunction due to noise. However, if the

overvoltage latch circuit does malfunction due to electrostatic discharge or other such external noise, an effective

countermeasure is to connect a capacitor with good frequency characteristics such as a film capacitor between VCC and

GND.

Caution If the power is reapplied immediately after being stopped (VREF drop) while there is charge remaining

in the filter’s capacitor (such as when VCC (auxiliary coil voltage) is being monitored from VREF without

configuring an overvoltage protection circuit such as is shown in Figure 4-12), the overvoltage latch

threshold value will be boosted in proportion with the boosting of VREF, possibly causing the over-

voltage latch to latch too easily.