UC2845 datasheet(7/11 Pages) STMICROELECTRONICS

Il numero della parte	UC2845

Spiegazioni elettronici	CURRENTMODE PWM CONTROLLER

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Figure 9 : Open Loop Test Circuit.

High peak currents associatedwith capacitive loads

necessitate careful grounding techniques. Timing

and bypass capacitors should be connected close

to pin 5 in a single point ground. The transistor and

Ω potentiometerareusedtosampletheoscillator

waveform and apply an adjustable ramp to pin 3.

Figure 10 : Shutdown Techniques.

Shutdown of the UC2842 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 cause the output of the PWM com-

parator 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 shut-

down conditionat pins 1 and/or 3 is removed. In one

example, an externally latched shutdown may be

accomplishedby addingan SCR which will be reset

by cycling Vi belowthe lower UVLOthreshold.At this

point the referenceturns off, allowing the SCR to re-

set.

UC2842/3/4/5-UC3842/3/4/5

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