TL494_15 Datasheet, PDF (11/33 Pages) Unisonic Technologies – VOLTAGE MODE PWM CONTROL CIRCUIT

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TL494
SLVS074G – JANUARY 1983 – REVISED JANUARY 2015
Feature Description (continued)
9.3.2 Oscillator
The oscillator provides a positive sawtooth waveform to the dead-time and PWM comparators for comparison to
the various control signals.
The frequency of the oscillator is programmed by selecting timing components R_T and C_T. The oscillator charges
the external timing capacitor, C_T, with a constant current, the value of which is determined by the external timing
resistor, R_T. This produces a linear-ramp voltage waveform. When the voltage across C_T reaches 3 V, the
oscillator circuit discharges it, and the charging cycle is reinitiated. The charging current is determined by the
formula:
3V
I_CHARGE = _RT
(1)
The period of the sawtooth waveform is:
T = ^{3 V ´ CT}
I_CHARGE
(2)
The frequency of the oscillator becomes:
1
f_OSC = _{RT ´ CT}
(3)
However, the oscillator frequency is equal to the output frequency only for single-ended applications. For push-
pull applications, the output frequency is one-half the oscillator frequency.
Single-ended applications:
1
f = _{RT ´ CT}
(4)
Push-pull applications:
1
f=
2R_T ´ C_T
(5)
9.3.3 Dead-time Control
The dead-time control input provides control of the minimum dead time (off time). The output of the comparator
inhibits switching transistors Q1 and Q2 when the voltage at the input is greater than the ramp voltage of the
oscillator. An internal offset of 110 mV ensures a minimum dead time of ∼3% with the dead-time control input
grounded. Applying a voltage to the dead-time control input can impose additional dead time. This provides a
linear control of the dead time from its minimum of 3% to 100% as the input voltage is varied from 0 V to 3.3 V,
respectively. With full-range control, the output can be controlled from external sources without disrupting the
error amplifiers. The dead-time control input is a relatively high-impedance input (I_I < 10 μA) and should be used
where additional control of the output duty cycle is required. However, for proper control, the input must be
terminated. An open circuit is an undefined condition.
9.3.4 Comparator
The comparator is biased from the 5-V reference regulator. This provides isolation from the input supply for
improved stability. The input of the comparator does not exhibit hysteresis, so protection against false triggering
near the threshold must be provided. The comparator has a response time of 400 ns from either of the control-
signal inputs to the output transistors, with only 100 mV of overdrive. This ensures positive control of the output
within one-half cycle for operation within the recommended 300-kHz range.
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