LM3845AD 参数 Datasheet PDF下载

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