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SLUS223C – APRIL 1997 – REVISED JUNE 2007
OPEN-LOOP LABORATORY FIXTURE
High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypas
capacitors should be conected close to pin 5 in a single point ground. The transistor and 5k potentiometer are
used to sample the oscillator waveform and apply an adjustable ramp to pin 3.
V
REF
R1
2N2222
4.7 kW
1 kW
ERROR AMP
ADJUST
4.7 kW
100 kW
1
2
5 kW
I
SENSE
ADJUST
3
4
COMP
V
FB
I
SENSE
R
T
/ C
T
UC1842
V
REF
8
0.1
mF
0.1
mF
1 kW 1 W
OUTPUT
A
V
CC
V
CC
7
OUTPUT 6
GROUND 5
GROUND
C
T
SHUTDOWN TECHNIQUES
Shutdown of the UC1842 can be accomplished by two methods; either raise pin 3 above 1 V or pull pin 1 below
a voltage two diode drops above ground. Either method causses the output of the PWM comparator to be high
(refer to block diagram). The PWM latch is reset dominant so that the output will remain low until the next clock
cycle after the shutdown condition at pin 1 and/or 3 is removed. In one example, an externally latched shutdown
may be accomplished by adding an SCR which will be reset by cycling V
CC
below the lower UVLO threshold. At
this pint the reference turns off, allowing the SCR to reset.
1 kW
8
V
REF
1
COMP
SHUTDOWN
330
W
500
W
SHUTDOWN
To Current
SENSE RESISTOR
3
I
SENSE
7
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