Get to Know the TL494 Pulse-Width-Modulation Control Circuits
28 November 2023
Ⅱ. What are the features of TL494?
Ⅲ. TL494 symbol, footprint and pin configuration
Ⅳ. Internal structure and working parts of TL494
Ⅶ. What are the applications of TL494?
In this article, we will provide a comprehensive introduction to the TL494 PWM generator, a fixed-frequency pulse width modulation control integrated circuit manufactured by Texas Instruments. Renowned for its diverse features, this integrated circuit finds widespread use across various applications.
TL494 is a fixed frequency pulse width modulation circuit produced by Texas Instruments in the United States. It contains all the functions required for switching power supply control and is widely used in single-ended forward dual-tube, half-bridge, and full-bridge switching power supplies. It is the core control device of switching power supplies. TL494 is packaged in SO-16 and PDIP-16 to meet the requirements of different occasions. In addition, it adopts dual power supply mode, the operating frequency is up to 500kHz, and the output voltage and duty cycle can be flexibly adjusted. The TL494 features a precision 5 V regulator, a dead-time controller stage (DTC), a built-in variable oscillator, a flip-flop control for pulse steering, two error amplifiers and output buffer circuitry.
Replacements and equivalents:
• LM3524
• SG3524
• UC3842
• UC3843
• It has a built-in linear sawtooth oscillator, and the external oscillation components only include a resistor and a capacitor.
• TL494 also has over-current protection function, which can effectively protect the power system from accidental over-current.
• The PWM chip features two fully integrated pulse width modulation control circuits.
• It allows users to adjust the operating frequency of the PWM signal through external components to meet the requirements of different applications.
• The adjustable dead time function in the TL494 minimizes cross-conduction of power switching devices during switching, leading to enhanced system efficiency.
The following figures show the symbol, footprint and pin configuration of TL494 respectively.
TL494 has a total of 16 pins, their names and functions are as follows.
Pin 1: It is the non-inverting input terminal of the error amplifier I, with a withstand voltage of 41V.
Pin 2: It is the inverting input terminal of the error amplifier I, with a withstand voltage of 41V.
Pin 3: It is the feedback terminal, used for feedback compensation of the error amplifier output signal, the maximum voltage is 4.5V. It is often used to provide an input signal that forms the PG signal.
Pin 4: It is the dead time control terminal. By applying a voltage of 0 to 3.5V to this terminal, we can change the duty cycle between 49% and 0, thereby controlling the output of the output terminal.
Pin 5: It is the timing capacitor end of the oscillator.
Pin 6: It is the timing resistor end of the oscillator.
Pin 7: It is the ground terminal.
Pin 8: It is the collector of the first pulse width modulation square wave output transistor (withstand voltage value is 41V, maximum current is 250mA).
Pin 9: It is the emitter of the first pulse width modulation square wave output transistor (withstand voltage value is 41V, maximum current is 250mA).
Pin 10: It is the emitter of the second pulse width modulation square wave output transistor (withstand voltage value is 41V, maximum current is 250mA).
Pin 11: It is the collector of the second pulse width modulation square wave output transistor (withstand voltage value is 41V, maximum current is 250mA).
Pin 12: This terminal serves as the power input point, with a maximum allowable voltage of 41V. The circuit will not initiate if the voltage falls below 7V.
Pin 13: It is the output mode control terminal. When pins 13 and 14 are connected, the two tubes output in push-pull mode. When pin 13 is connected to ground, the two tubes output in parallel. In the parallel output state, the emitters of the two tubes can be connected to the emitter, and the collector to the collector can also be connected. The output current after parallel connection can reach 400mA.
Pin 14: It is used to provide a reference voltage value for each comparison circuit, with a maximum current of 10mA.
Pin 15: It is the inverting input terminal of error amplifier II, with a withstand voltage of 41V.
Pin 16: It is the non-inverting input terminal of error amplifier II, with a withstand voltage of 41V.
The TL494 circuit is shown in the figure. Its main components include operational amplifier, dead zone time comparator, pulse flip-flop, 5V reference regulator, comparator, error amplifier and sawtooth wave oscillator.
1. Operational amplifier
The TL494 integrates two single-supply operational amplifiers. The transfer function of the operational amplifier is ft(ni,inv)=A(ni-inv), and its output swing is limited by the circuit design. In general power supply circuits, operational amplifiers are usually connected to operate in a closed loop. In a few special cases, an open loop is used to input signals from the outside. The output terminals of the two operational amplifiers are each connected to a diode, which is connected to the COMP pin and the subsequent circuit (comparator). This ensures that the higher output level of the two operational amplifiers can enter the subsequent stage circuit, while protecting the latter stage circuit from high voltage impact.
2. Dead zone time comparator
The dead zone time is set by Dead Time Control pin 4. It interferes with the pulse flip-flop via a comparator to limit the maximum duty cycle. The upper limit of the duty cycle of each end that can be set is up to 45%. When the operating frequency is higher than 150KHz, the upper limit of the duty cycle is approximately 42% (when the DTC pin level is set to 0).
3. Pulse trigger
The pulse flip-flop is turned on when the falling edge of the sawtooth waveform and the comparator output is 1, causing one of the two output terminals (in turn) to turn on the on-chip transistor. When the comparator output drops to 0, the output is turned off.
4. 5V reference regulator
The TL494's reference regulator is built-in. It works based on the band gap principle, and TL494 is able to provide a stable 5V output voltage. But there is a condition we need to pay attention to, that is, the VCC voltage needs to be above 7V, and the error must be within 100mV. According to the pin configuration table, the reference source will use pin 14 REF as its output pin.
5. Comparator
The signal output by the operational amplifier (COMP pin) enters the positive input terminal of the comparator inside the chip and is compared with the sawtooth wave entering the negative input terminal. When the sawtooth wave exceeds the signal at the COMP pin, the comparator outputs a low level (0), otherwise it outputs a high level (1).
6. Error amplifier
By adjusting the two error amplifiers through the power rails of the IC, we can achieve a high gain for the error amplifier, allowing for a common-mode input range that is 0.3v to 2v lower than V1. These error amplifier setups typically function akin to single-supply amplifiers, resulting in outputs with active-high functionality. Consequently, the amplifiers can be selectively activated to fulfill PWM requirements and maintain a constant current.
7. Sawtooth oscillator
TL494 has a built-in linear sawtooth wave oscillator, which generates a sawtooth waveform of 0.3 to 3V. The oscillation frequency can be adjusted through the external resistor Rt and capacitor Ct. The specific oscillation frequency is: f = 1/(Rt×Ct). The units of Ct and Rt are Farads and Ohms respectively. The sawtooth wave can be measured on the Ct pin.
TL494 is a fixed frequency pulse width modulation circuit with a built-in linear sawtooth oscillator whose oscillation frequency can be adjusted through an external resistor and capacitor.
The pulse of the output capacitor is achieved by comparing the positive polarity sawtooth voltage on the capacitor with two other control signals. Power output tubes Q1 and Q2 are controlled by NOR gates. The output is only passed when the bistable voltage regulator's clock signal is low. This means that the output will only be gated if the sawtooth voltage is greater than the control signal. When the control signal increases, the width of the output pulse decreases accordingly.
The integrated circuit receives an external control signal, with a portion directed to the time dead time comparator and the remainder directed to the input of the error amplifier. The dead-time comparator incorporates a 120mV input compensation voltage, restricting the minimum output dead-time to around 4% of the sawtooth wave period. Ground connection at the output terminal yields a maximum output duty cycle of 96%, while connection to the reference level results in a 48% duty cycle. Attaching the dead time control input to a fixed voltage allows for the addition of a specific dead time to the output pulses.
The pulse width modulator provides a means for the error amplifier to adjust the output pulse width. When the feedback voltage changes from 0.5V to 3.5V, the output pulse width gradually decreases to zero from the maximum on-time limited by the dead zone. The two error amplifiers have a common-mode input range from -0.3V to (vcc-2.0), which may be affected by the output voltage and current of the power supply. The output of the error amplifier is normally held high and ORed with the inverting input of the pulse width modulator. This circuit structure allows the amplifier to require only a smaller output to control the circuit.
• Inverter design: It has the capability to be employed in creating an inverter for the transformation of DC power into AC power.
• Communication equipment design: In network equipment such as routers and switches, TL494 can be used to implement functions such as voltage adjustment and power management.
• Single-loop controller: TL494 can form a single-loop controller together with corresponding input and output circuits to control the output of the power circuit.
• Lighting control: The TL494 is versatile and can be applied in the creation of lighting systems, including LED drivers and lighting controllers.
• Power circuit: TL494 is often used in various switching power supplies such as single-ended forward double-tube, half-bridge, and full-bridge switching power supplies. It is a fixed frequency pulse width modulation circuit that contains all the functions required for switching power supply control.
• Industrial control design: In the realm of industrial control design, one can employ it to execute tasks like motor control and temperature regulation. For instance, by using a pulse width modulation (PWM) controller composed of TL494, we can control the speed of a motor or control the temperature of equipment such as coolers and heaters.
Frequently Asked Questions
1. What is the function of TL494?
The TL494 device provides for push-pull or single-ended output operation, which can be selected through the output-control function. The architecture of this device prohibits the possibility of either output being pulsed twice during push-pull operation.
2. What are the basics of TL494?
TL 494 is a chip that deals with all of the functions required for Pulse Width Modulation (PWM) control circuits. It consists of two error amplifiers, oscillator, flipflop, 5V reference voltage, dead time comparator, PWM comparator etc. The range of the operating frequency for this device is from 1kHz to 300kHz.
3. How to calculate TL494 frequency?
The first thread recommends using 1.2/(Rt*Ct) to calculate the frequency. The second thread makes the same recommendation and says they will look into changing the datasheet. The third thread created a PCB with 12.2k and 0.1uF, which would be 819 Hz using 1/(Rt*Ct) and 984 Hz using 1.2/(Rt*Ct).
4. How does TL494 works?
The TL494 is a fixed-frequency pulse-width-modulation (PWM) control circuit. Modulation of output pulses is accomplished by comparing the sawtooth waveform created by the internal oscillator on the timing capacitor (CT) to either of two control signals.
