KA3842 데이터 시트보기 (PDF) - Fairchild Semiconductor

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KA3842

SMPS Controller

Fairchild Semiconductor 

KA3842B/KA3843B/KA3844B/KA3845B

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 dis-

charge 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= RTCTIn-0--0-.--0.--0-0--0-6---63---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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