M51996AP/AFP

REJ03D0836-0201 Rev.2.01 Nov 14, 2007

Page 19 of 35

Voltage Detector Circuit (DET) Section

The DET terminal can be used to control the output voltage which is determined by the winding ratio of fly back

transformer in fly-back system or in case of common ground circuit of primary and secondary in feed forward system.

The circuit diagram is quite similar to that of shunt regulator type 431 as shown in figure 11. As well known from

figure 11 and figure 12, the output of OP AMP has the current-sink ability, when the DET terminal voltage is higher

than 2.5 V.

But it becomes high impedance state when lower than 2.5 V DET terminal and F/B terminal have inverting phase

characteristics each other, so it is recommended to connect the resistor and capacitor in series between them for phase

compensation. It is very important one can not connect by resistor directly as there is the voltage difference between

them and the capacitor has the DC stopper function.

DET

5.4 k

10.8 k

10.8 k

1.2 k

F/B

6 S

3 k

500

Ω

1 S

10 S

Figure 11 Voltage Detector Circuit Section (DET)

F/B

DET

2.5 V

500

Ω

6 S

3 k

1 S

+

−

OP

AMP

Figure 12 Schematic Diagram of Voltage Detector Circuit Section (DET)