TL494 pulse width modulation control circuit

Main features Integrated all pulse width modulation circuits . Built-in linear sawtooth oscillator, only two external oscillation components (one resistor and one capacitor ). Built-in error amplifier. Internal 5V reference voltage source. Adjustable dead time. Built-in power transistor provides 500mA drive capability. Push or pull output.

Brief description of working principle TL494 is a fixed frequency pulse width modulation circuit with a built-in linear sawtooth oscillator. The oscillation frequency can be adjusted by an external resistor and a capacitor. The oscillation frequency is as follows: The width of the output pulse is achieved by comparing the positive polarity sawtooth voltage on the capacitor CT with the other two control signals. The power output tubes Q1 and Q2 are controlled by the NOR gate. The bistable trigger will be selected only when the clock signal is low, that is, it will be selected only when the sawtooth voltage is greater than the control signal. When the control signal increases, the width of the output pulse will decrease. See Figure 2.

The control signal is input from the external input of the integrated circuit , one way to the dead time comparator and one way to the input of the error amplifier. The dead time comparator has an input offset voltage of 120mV, which limits the minimum output dead time to approximately 4% of the sawtooth wave period. When the output terminal is grounded, the maximum output duty cycle is 96%, and when the output terminal is connected to the reference level, the duty cycle is 48%. When the dead time control input terminal is connected to a fixed voltage (range between 0-3.3V), additional dead time can be generated on the output pulse.

The pulse width modulation comparator provides a means for the error amplifier to adjust the output pulse width: when the feedback voltage changes from 0.5V to 3.5, the output pulse width drops from the maximum conduction percentage time determined by the dead zone to zero. The two error amplifiers have a common-mode input range from -0.3V to (Vcc-2.0), which can be detected from the output voltage and current of the power supply. The output of the error amplifier is usually at a high level, which is "OR" with the inverting input of the pulse width modulator. It is this circuit structure that the amplifier only needs a minimum output to dominate the control loop.

When the comparator CT discharges , a positive pulse appears at the output of the dead zone comparator, and the pulse-constrained bistable trigger is timed and the output tubes Q1 and Q2 are stopped at the same time. If the output control terminal is connected to the reference voltage source, the modulated pulse is output alternately to the two output transistors, and the output frequency is equal to half of the pulse oscillator. If it works in a single-ended state and the maximum duty cycle is less than 50%, the output drive signal is obtained from transistor Q1 or Q2 respectively. A feedback winding and a diode of the output transformer provide feedback voltage. In the single-ended working mode, when a higher drive current output is required, Q1 and Q2 can also be used in parallel. At this time, the output mode control pin needs to be grounded to turn off the bistable trigger. In this state, the output pulse frequency will be equal to the frequency of the oscillator. TL494 has a built-in 5.0V reference voltage source. When using an external bias circuit, it can provide a load current of up to 10mA. Under the typical 0-70℃ temperature range and 50mV temperature drift conditions, the reference voltage source can provide ±5% accuracy.

TL494 limit parameters
name	Code	Limit value	unit
Operating voltage	Vcc	42	V
Collector output voltage	V c1 ,V c2	42	V
Collector output current	I c1 ,I c2	500	mA
Amplifier Input Voltage Range	V I R	-0.3V—+42	V
Power consumption	P D	1000	mW
Thermal resistance	R θJA	80	℃/W
Operating Junction Temperature	T J	125	℃
Working environment temperature TL494B TL494C TL494I NCV494B	T A	-40—+125 0—+70 -40—+85 -40—+125	℃
Rated ambient temperature	T A	40	℃

TL494 pulse width modulation control circuit