CURRENT MODE PWM CONTROLLER
Fig.6 Shutdown Techniques
1K
LM3842A/3A/4A/5A
8
V
REF
1
COMP
330Ω
500Ω
3
I
SENSE
SHUTDOWN
SHUTDOWN
TO CURRENT
SENSE-RESISTOR
Shutdown of the LM3842A 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 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 shoutdown condition at pins 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.
Fig.7 Slope Compensation
V
REF
8
0.1μF
R
T
LM3842A
R
T
/C
T
4
C
T
R1
I
SENSE
R2
I
SENSE
3
R
SENCE
C
A fraction of the oscillator ramp can be resistively summed with the current sense signal to
provide slope compensation for converters requiring duty cycles over 50%.
Note that capacitor, C, forms a filter with R2 to suppress the leading edge switch spikes.
HTC
5