LM3845AD datasheet(4/7 Pages) HTC

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CURRENT MODE PWM CONTROLLER

LM3842A/3A/4A/5A

Fig.4 Current Sense Circuit

Peak current (IS) is determined by the formula:

IS(MAX)

A small RC filter may be required to suppress switch transients.

Fig.5 Oscillator Waveforms and Maximum Duty Cycle

Oscillator timing capacitor, CT, is charged by VREF through RT, and discharged by an internal

current source.

During the discharge time, the internal clock signal blanks the output to the

low state. Selection of RT and CT therefore determines both oscillator frequency and maximum

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

0.55 RT CT

RT CT ∫n(

)

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

-1

For RT>5KΩ, f

HTC

ERROR

AMP

CURRENT

SENSE

COMPARATOR

COMP

CURRENT

SENSE

GND

1.0V

0.0063 RT - 2.7

0.0063 RT - 4

1.8

RT CT

VREF

RT/CT

GND

LARGE RT

SMALL CT

INTERNAL CLOCK

LARGE RT

SMALL CT

INTERNAL CLOCK

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