UC1842/3/4/5
UC2842/3/4/5
UC3842/3/4/5
OUTPUT SATURATION CHARACTERISTICS
ERROR AMPLIFIER OPEN-LOOP
FREQUENCY RESPONSE
OPEN-LOOP LABORATORY FIXTURE
High peak currents associated with capacitive loads necessi-
tate careful grounding techniques. Timing and bypass capaci-
tors should be connected close to pin 5 in a single point
ground. The transistor and 5k potentiometer are used to sam-
ple the oscillator waveform and apply an adjustable ramp to
pin 3.
SHUT DOWN TECHNIQUES
Shutdown of the UC1842 can be accomplished by two meth-
ods; either raise pin 3 above 1V or pull pin 1 below a voltage
two diode drops above ground. Either method causes the out-
put 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 point the reference turns off, allowing the
SCR to reset.
6
TI [ TEXAS INSTRUMENTS ]
