CURRENT MODE PWM CONTROLLER

LM3842A/3A/4A/5A

duty cycle. Charge and discharge times are determined by the formulas:

tc

0.55 RT CT

td

RT CT ∫n(

)

Frequency, then, is: f = (tc + td) -1

For RT>5KΩ, f

Fig.6 Shutdown Techniques

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 VCC below the lower UVLO threshold. At this point the

reference turns off, allowing the SCR to reset.

Fig.7 Slope Compensation

HTC

−

5 −

~

~

~

~

1.8

~

~

1

SHUTDOWN

COMP

8

3

1K

330Ω

500Ω

SHUTDOWN

TO CURRENT

SENSE-RESISTOR

8

4

3

R2

0.1μF

R1

C

LM3842A