Current-mode PWM controller
Current-mode PWM controller
Current-mode PWM controller
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Philips Semiconductors Linear Products<br />
<strong>Current</strong>-<strong>mode</strong> <strong>PWM</strong> <strong>controller</strong><br />
Product specification<br />
UC3842<br />
OPEN-LOOP LABORATORY TEST FIXTURE<br />
V REF<br />
R T<br />
A<br />
V CC<br />
4.7k<br />
2N2222<br />
100k<br />
1<br />
COMP<br />
UC3842<br />
8<br />
V REF<br />
0.1µF<br />
ERROR AMP<br />
ADJUST<br />
4.7k<br />
5k<br />
I SENSE<br />
ADJUST<br />
2<br />
V FB<br />
3<br />
I SENSE<br />
4<br />
R T /C T<br />
7<br />
V CC<br />
6<br />
OUTPUT<br />
5<br />
GND<br />
0.1µF<br />
1k<br />
1W<br />
OUTPUT<br />
C T<br />
NOTE:<br />
High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be connected close to Pin 5 in a single point<br />
ground. The transistor and 5k potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to Pin 3.<br />
GND<br />
SHUTDOWN TECHNIQUES<br />
4.7k 8<br />
1<br />
COMP<br />
3<br />
4.7k<br />
I SENSE<br />
SHUTDOWN<br />
500<br />
SHUTDOWN<br />
TO CURRENT<br />
SENSE RESISTOR<br />
NOTE:<br />
Shutdown of the UC3842 can be accomplished by two methods; either raise Pin 3 above 1V or pull Pin 1 below a voltage two diode drops above ground. Either method causes the<br />
output of the <strong>PWM</strong> comparator to be high (refer to Block Diagram). The <strong>PWM</strong> latch is reset dominant so that the output will remain low until the next clock cycle after the shutdown<br />
condition at Pins 1 and/or 3 is removed. In the examples shown, an externally-latched shutdown may be accomplished by adding an SCR which will be reset by cycling<br />
V CC below the lower UVLO threshold (10V). At this point all internal bias is removed, allowing the SCR to reset.<br />
August 31, 1994 1105