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Reg
Cfrcu1
PIN +2Di
R7
OSD
+
COMP 1
–
R6
+
R1
COMP 2
–
R7
R8
R9
C1
C2
R2
Fig 3 Oscillator Circuit Configulation
As the circuit in Fig. 3 shows, the ON-time is controlled by changing a current charged by C1, which is as the
result of that the detection winding (L1),which detects a change of voltage in a secondary side, connected to the
sensing terminal (Pin No. 7) has the current in accordance with an output signal from an output voltage
detection circuit (an error amplifier) built in. As an AC input voltage to the power supply gets the higher and a
load current the smaller, the current flowing to the SENS terminal gets the larger, and the ON-time gets the
shorter.
3) Function of INH terminal (Pin #6), control of OFF-time
Signal to the INH terminal is used as inputs to COMP.1 and COMP.2 inside of the control IC. A threshold voltage
of COMP.1 V
is set at 0.75V (Ta=25°C) and an input signal to a drive circuit becomes almost 0V (the power
TH1
transistor is in OFF mode) when a voltage at the INH terminal reaches the V
voltage does not get lower than V
voltage of COMP.2 V
TH2
COMP.2 reverses and, as a result, C2 starts firing and a voltage across C2 drops to almost 0V in a moment. As
the result of this immediate discharge of C2, the OFF-time of the oscillator which has been determined by the
product of C2 and R2 (⁄ 55 ¥ sec) can be quicker up to approx. 2¥sec. As long as the INH terminal voltage
does not get lower than V
power transistor being on. The relation between the INH terminal voltage and the function of the oscillator
described above is shown in Fig. 6 and Fig. 7
Osc.
Output
PTR
INH
Ter. Voltage
Fig .6 Operation Waveforms of INH Terminal(V
L1
2.3V
C2 ACROSS
Voltage
1V
Drive
INH
0.75V
Tre
C1 ACROSS
VTH1
Voltage
0V
VTH2
Osc.
Output
PTR
0V
Fig 4 Operation Waveforms of Oscillator without F/B nor
INH Signal
. the power transistor sustains OFF mode. On the other hand, a threshold
TH1
, is set at 1.5V (Ta=25°C).When the INH terminnal voltage reaches V
, AVoltage across C2 stays at almost 0V and a output from the oscillator keeps the
TH2
ON
OFF
V
TH1
TH1
ON
OFF
Osc.
Output
PTR
2.3V
C2 ACROSS
Voltage
0V
INH
Ter. Voltage.
)
Fig .7 Operation Waveforms of INH Terminal(V
20
2.3V
C2 ACROSS
Voltage
1V
0.75V
C1 ACROSS
Voltage
0V
Osc.
Output
PTR
0V
Fig 5 Operation Waveforms of Oscillator with F/B Signal
. As long as the INH terminal
TH1
TH2
ON
OFF
V
TH2
)
TH2
, an output from
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