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AN4003

PC POWER SUPPLY DESIGN WITH KA3511

Fairchild Semiconductor 

6. BLOCK DESCRIPTION & APPLICATION INFORMATIONS

6.1 OSCILLATOR BLOCK

Vref

VCC

12

1

12

RT

CT

12

Figure 1. Oscillator RT, CT

The KA3511 is a fixed-frequency pulse width modulation control circuit. An internal-linear sawtooth

oscillator is frequency-programmable by two external components, RT and CT. The oscillator fre-

quency is determined by

fosc = R-----T--1---×.--1--C-----T-

300K

100K

VCC=15V

0.001µF

10K

CT=0.01 µF

1K

0.1µF

100

30

1K 2K

1.0µF

5K 10K 20K

50K 100K 200K

RT. TIMING RESISTANCE( Ω)

500K 1M

Figure 2. Oscillator Frequency vs. Timing Resistance

6.2 PWM CONTROL BLOCK

Output pulse width modulation is accomplished by comparison of the positive sawtooth waveform

across capacitor CT to either of two control signals. The NOR gates, which drive output transistors

Q1 and Q2, are enabled only when the flip-flop clock-input line is in its low state. This happens only

during that portion of time when the sawtooth voltage is greater than the control signals. Therefore,

an increase in control-signal amplitude causes a corresponding linear decrease of output pulse

width. (Refer to the timing diagram shown in Figure 4)

Rev C, November 1999

8

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