TL494_15 Datasheet, PDF (16/33 Pages) Unisonic Technologies – VOLTAGE MODE PWM CONTROL CIRCUIT

TL494
SLVS074G – JANUARY 1983 – REVISED JANUARY 2015
Typical Application (continued)
+
C2
14 +5 V
Oscillator Ramp
Osc
R_T
5
4
R6
7
0.1 V
TL494
Pin 4 Voltage
Oscillator Ramp Voltage
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t_on
PWM Output
Figure 13. Soft-Start Circuit
The soft-start circuit allows the pulse width at the output to increase slowly (see Figure 13) by applying a
negative slope waveform to the dead-time control input (pin 4).
Initially, capacitor C2 forces the dead-time control input to follow the 5-V regulator, which disables the outputs
(100% dead time). As the capacitor charges through R6, the output pulse width slowly increases until the control
loop takes command. With a resistor ratio of 1:10 for R6 and R7, the voltage at pin 4 after start-up is 0.1 × 5 V,
or 0.5 V.
The soft-start time generally is in the range of 25 to 100 clock cycles. If 50 clock cycles at a 20-kHz switching
rate is selected, the soft-start time is:
t = ¹ = ¹ = 50 msper clock cycle
f 20kHz
(12)
The value of the capacitor then is determined by:
C2 = ^{soft - start time} = ^{50 ms ´ 50 cycles} = 2.5 mF
R6
1 kW
(13)
This helps eliminate any false signals that might be created by the control circuit as power is applied.
16
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