The frequency of the oscillator is programmed by selecting timing components RT and CT. The oscillator charges

the external timing capacitor, CT, with a constant current, the value of which is determined by the external

timing resistor, RT. This produces a linear-ramp voltage waveform. When the voltage across CT reaches 3 V, the

oscillator circuit discharges it, and the charging cycle is reinitiated. The charging current is determined by the

formula:

CHARGE

T

3 V

I

R

=

(1)

The period of the sawtooth waveform is:

T

CHARGE

3 V

C

T

I

´

=

(2)

The frequency of the oscillator becomes:

OSC

T

T

1

f

R

C

=

´

(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:

T

T

1

f

R

C

=

´

(4)

Push-pull applications:

T

T

1

f

2R

C

=

´

(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 (II < 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.

9.3.5 Pulse-Width Modulation (PWM)

The comparator also provides modulation control of the output pulse width. For this, the ramp voltage across

timing capacitor CT is compared to the control signal present at the output of the error amplifiers. The timing

capacitor input incorporates a series diode that is omitted from the control signal input. This requires the control

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TL494

SLVS074I – JANUARY 1983 – REVISED JULY 2022

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