UC3842D Просмотр технического описания (PDF) - Fairchild Semiconductor

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UC3842D

SMPS Controller

Fairchild Semiconductor 

UC3842/UC3843/UC3844/UC3845

Figure 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:

tc = 0.55 RT CT

tD=

RT

CTI



n

-0--0-.--0.--0-0--0-6---6-3--3-R---R--T--T--–---–--2--4-.--7-

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

ForRT > 5KΩ,f= ----1---.--8-----

RTCT

Figure 6. Oscillator Dead Time & Frequency

(Deadtime vs CT RT > 5kΩ)

Figure 7. Timing Resistance vs Frequency

Figure 8. Shutdown Techniques

6

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