LM3842A データシートの表示（PDF） - HTC Korea

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LM3842A

CURRENT MODE PWM CONTROLLER

HTC Korea 

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(

0.0063 RT - 2.7)

0.0063 RT - 4

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

For

RT>5KΩ,

f

~~

1.8

RT CT

Fig.6 Shutdown Techniques

1K

8 VREF

1 COMP

SHUTDOWN

330Ω

3 ISENSE

500Ω

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 VCC below the lower UVLO threshold. At this point the

reference turns off, allowing the SCR to reset.

Fig.7 Slope Compensation

VREF 8

LM3842A

RT/CT 4

ISENSE 3

0.1μF

RT

CT

R1

R2

C

ISENSE

RSENCE

−5−

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