KA3842B Datasheet, PDF (6/12 Pages) Fairchild Semiconductor – SMPS Controller

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
t_D= R_TC_TI_{n}-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-----
R_TC_T
Figure 6. Oscillator Dead Time & Frequency
(Deadtime vs CT RT > 5kΩ)
Figure 7. Timing Resistance vs Frequency
Figure 8. Shutdown Techniques
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