Daewoo DTH-14Q1FS, Service Manual

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6) OCP (overcurrent protection) function
Overcurrent protection is performed pulse by pulse by directly detecting collector current of the power transistor.
Configuration of the OCP circuit is shown in . Detecting voltage is set to -1V below a reference point of GND
(ground). In additlon, since the detecting voltage is set by a comparator, very stable characteristics against
temperature is achieved and drift of the detecting voltage against temperature change is almost 0V.
7) Latch circuit
It is a circuit which sustains an output from the oscillator low and stops operation of the power supply when
overvoltage protection (OVP) circuit and thermal shutdown (TSD) circuit are in operation. As the sustaining
current of the latch circult is 500
¥
A maximum when V
IN
terminal voltage is 4V. the power supply circuit sustains
the off state as long as current of 500
¥
A minimum flows to V
IN
terminal from a start-up resistor. In order to
prevent a malfunction to be caused by a noise and so on, delay time is provided by C1 incorporated in the IC
and, therefore, the latch circuit operates when the OVP of TSD circuit is in operation, of an external signal input
is provided. for about 10
¥
sec or longer.In addition, even after the latch circuit start operating, the constant
voltage regulator (Reg) circuit is in operation and the circuit current is at high level. As a result. V
IN
terminal
voltage rapidly decreases. When V
IN
terminal voltage becomes lower than the shutdown voltage, V
IN
(OFF),
(4.9V typical), it starts in -creasing as the circuit current is below 500
¥
A. When it reaches the ON-state voltage.
V
IN
(ON), (8V typical), V
IN
terminal voltage starts decreasing because the circult current increases again.
Drive
OSC
+
– -1V
5.OCP
2.GND
R811
Fig 11 Overcurrent protection Circuit