TL494

5

MOTOROLA ANALOG IC DEVICE DATA

APPLICATIONS INFORMATION

Description

The TL494 is a fixed–frequency pulse width modulation

control circuit, incorporating the primary building blocks

required for the control of a switching power supply. (See

Figure 1.) An internal–linear sawtooth oscillator is frequency–

programmable by two external components, RT and CT. The

approximate oscillator frequency is determined by:

fosc ≈

1.1

RT • CT

For more information refer to Figure 3.

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 2.)

The control signals are external inputs that can be fed into

the deadtime control, the error amplifier inputs, or the

feedback input. The deadtime control comparator has an

effective 120 mV input offset which limits the minimum output

deadtime to approximately the first 4% of the sawtooth–cycle

time. This would result in a maximum duty cycle on a given

output of 96% with the output control grounded, and 48% with

it connected to the reference line. Additional deadtime may

be imposed on the output by setting the deadtime–control

input to a fixed voltage, ranging between 0 V to 3.3 V.

Functional Table

Input/Output

Controls

Output Function

fout

fosc

=

Grounded

Single–ended PWM @ Q1 and Q2

1.0

@ Vref

Push–pull Operation

0.5

The pulse width modulator comparator provides a means

for the error amplifiers to adjust the output pulse width from

the maximum percent on–time, established by the deadtime

control input, down to zero, as the voltage at the feedback pin

varies from 0.5 V to 3.5 V. Both error amplifiers have a

common mode input range from –0.3 V to (VCC – 2V), and

may be used to sense power–supply output voltage and

current. The error–amplifier outputs are active high and are

ORed together at the noninverting input of the pulse–width

modulator comparator. With this configuration, the amplifier

that demands minimum output on time, dominates control of

the loop.

When capacitor CT is discharged, a positive pulse is

generated on the output of the deadtime comparator, which

clocks the pulse–steering flip–flop and inhibits the output

transistors, Q1 and Q2. With the output–control connected to

the reference line, the pulse–steering flip–flop directs the

modulated pulses to each of the two output transistors

alternately for push–pull operation. The output frequency is

equal to half that of the oscillator. Output drive can also be

taken from Q1 or Q2, when single–ended operation with a

maximum on–time of less than 50% is required. This is

desirable when the output transformer has a ringback

winding with a catch diode used for snubbing. When higher

output–drive currents are required for single–ended

operation, Q1 and Q2 may be connected in parallel, and the

output–mode pin must be tied to ground to disable the

flip–flop. The output frequency will now be equal to that of the

oscillator.

The TL494 has an internal 5.0 V reference capable of

sourcing up to 10 mA of load current for external bias circuits.

The reference has an internal accuracy of

±5.0% with a

typical thermal drift of less than 50 mV over an operating

temperature range of 0

° to 70°C.

Figure 3. Oscillator Frequency versus

Timing Resistance

500 k

100 k

10 k

1.0 k

500

1.0 k 2.0 k 5.0 k

10 k

20 k

50 k

100 k 200 k

500 k 1.0 M

RT, TIMING RESISTANCE (Ω)

VCC = 15 V

0.01

µF

0.1

µF

CT = 0.001 µF