Phone

    00852-6915 1330

TL494 PWM Controller Working Principle [FAQ]

  • Contents

I Description

TL494, is a switching power supply pulse-width modulation (PWM) control chip.

Designed in the early 1980s, the TL494 gained immediate and widespread market acceptance, especially in ATX half-bridge power supplies for PC computers.

TL494 PWM Controller

TL494 has become an industry-standard chip, produced by many integrated circuit manufacturers. Widely used in single-ended forward dual-tube, half-bridge, and full-bridge switching power supplies. TL494 has two packaging forms, SO-16 and PDIP-16, to meet the requirements of different occasions.

Catalog

I Description

II TL494 Features

III TL494 Internal Structure

3.1 5V Reference Source

3.2 Sawtooth Oscillator

3.3 Operational Amplifier

3.4 Comparator

3.5 Pulse Trigger

3.6 Quiet Time Comparator

IV TL494 Working Principle

V Conclusion

FAQ

Ordering & Quantity

II TL494 Features

  • Complete PWM Power-Control Circuitry
  • Uncommitted Outputs for 200-mA Sink or Source Current
  •  Output Control Selects Single-Ended or Push-Pull Operation
  • Internal Circuitry Prohibits Double Pulse at Either Output
  • Variable Dead Time Provides Control Over Total Range
  • Internal Regulator Provides a Stable 5-V Reference Supply With 5% Tolerance
  • Circuit Architecture Allows Easy Synchronization

III TL494 Internal Structure

tl494 structureFigure 1. TL494 Internal Structure

3.1 5V Reference Source

TL494 has a built-in reference source based on the band gap principle. The stable output voltage of the reference source is 5V. The condition is that the VCC voltage is above 7V. The error is within 100mV. The output pin of the reference source is the 14th pin REF.

3.2 Sawtooth Oscillator

TL494 has a built-in linear sawtooth wave oscillator, which generates a 0.3~3V sawtooth wave. The oscillation frequency can be adjusted by an external resistor Rt and a capacitor Ct. Its oscillation frequency is:

f=1/Rt*Ct

Among them:

  • The unit of Rt is ohm;
  • The unit of Ct is farad.

The sawtooth wave can be measured at the Ct pin.

3.3 Operational Amplifier

TL494 integrates two operational amplifiers powered by a single power supply. The transfer function of the operational amplifier is ft(ni, inv)=A(ni-inv), but it cannot exceed the output swing. In general power circuits, the op-amp is connected to operate in a closed-loop. Open-loop is used in a few special cases, and the signal is input from the outside. The output terminals of the two operational amplifiers are respectively connected to a diode, which is connected to the COMP pin and the subsequent circuit (comparator). This ensures that the higher output of the two op-amps enters the subsequent circuit.

3.4 Comparator

The signal (COMP pin) output by the operational amplifier 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 is higher than the signal of the COMP pin, the comparator outputs 0, otherwise, it outputs 1.

3.5 Pulse Trigger

The pulse flip-flop turns on at the falling edge of the sawtooth wave and the comparator outputs 1. This makes one of the two outputs (in turn) on-chip transistors are turned on and cut off when the comparator output drops to zero.

3.6 Quiet Time Comparator

The dead zone time is set by Dead Time Control pin 4. It uses a comparator to interfere with the pulse trigger and limit the maximum duty cycle. The upper limit of the duty cycle of each end can be set up to 45%, and the upper limit of the duty cycle is about 42% when the operating frequency is higher than 150KHz. (When the DTC pin level is set to 0).

IV TL494 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 sawtooth voltage on the capacitor CT with the other two control signals. The power output tubes Q1 and Q2 are controlled by a NOR gate. It will be strobed when the clock signal of the flip-flop is low. That is, it will be gated only when the sawtooth voltage is greater than the control signal. When the control signal increases, the output pulse width will decrease. We can take a look at the picture below.

tl494 pulse control waveform

Figure 2. TL494 Pulse Control Waveform

​The control signal is input from the outside of the integrated circuit. One is sent to the dead time comparator, and one is sent to the input of the error amplifier. The dead-time comparator has an input compensation voltage of 120mV, which limits the minimum output dead-time to approximately 4% of the sawtooth period. When the output terminal is grounded, the maximum output duty cycle is 96%. When the output terminal is connected to the reference level, the duty cycle is 48%. When the dead time control input 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 to zero from the maximum on-percentage time determined by the dead zone. The two error amplifiers have a common mode input range from -0.3V to (Vcc-2.0), which may be detected from the output voltage and current of the power supply. The output of the error amplifier is always at a high level. It performs "OR" operation with the inverting input terminal of the pulse width modulator. It is this circuit structure that the amplifier can dominate the control loop with minimal output.

When the comparator CT discharges, a positive pulse appears at the output of the dead zone comparator, and the flip-flop constrained by the pulse is timed. At the same time stop the work of the output tubes Q1 and Q2. If the output control terminal is connected to the reference voltage source, the modulated pulse is alternately output 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 the transistor Q1 or Q2 respectively. A feedback winding and diode of the output transformer provide feedback voltage. In single-ended operating 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 flip-flop. In this state, the output pulse frequency will be equal to the oscillator frequency.

V Conclusion

This blog summarizes the characteristics, internal structure and working principle of TL494. Although the architecture of TL494 has been proven to be extremely excellent in history, it is facing elimination in the high-end market due to its old technology, low frequency, and lack of new energy-saving features. However, it is worth mentioning that TL494 is still widely adopted in the low-end and mid-end markets.


FAQ

  • What is TL494?

TL494 is a PWM controller IC used for power electronics circuits. It comprises of on-chip two error amplifiers an oscillator with adjustable frequency feature, an output flip-flop having pulse steering control, and an output control circuit with feedback.

  • What is the detailed description of TL494?

The TL494 device incorporates all the functions required in the construction of a pulse-width-modulation (PWM) control circuit on a single chip. Designed primarily for power-supply control, this device offers the flexibility to tailor the power-supply control circuitry to a specific application. The TL494 device contains two error amplifiers, an on-chip adjustable oscillator, a dead-time control (DTC) comparator, a pulse-steering control flip-flop, a 5-V, 5%-precision regulator, and output-control circuits. The error amplifiers exhibit a common-mode voltage range from –0.3 V to VCC – 2 V. The dead-time control comparator has a fixed offset that provides approximately 5% dead time. The on-chip oscillator can be bypassed by terminating RT to the reference output and providing a sawtooth input to CT, or it can drive the common circuits in synchronous multiple-rail power supplies. The uncommitted output transistors provide either common-emitter or emitter-follower output capability. 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.

  • What are TL494 product features?
    • Complete PWM Power-Control Circuitry
    • Uncommitted Outputs for 200-mA Sink or Source Current
    • Output Control Selects Single-Ended or Push-Pull Operation
    • Internal Circuitry Prohibits Double Pulse at Either Output
    • Variable Dead Time Provides Control Over Total Range
  • What is PWM IC?

The TL494 fixed frequency PWM Controller can be used for DC to DC conversion regardless of buck or boost topology. ... This IC feature an output control circuit, a flip flop, a dead time comparator, two different error amplifiers, a 5V reference voltage, an oscillator, and a PWM comparator.

  • How does PWM IC work?

As its name suggests, pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.

  • Which IC is better for a buck converter, TL494 or UC3843?

They mainly differ in type of control…
TL494 => voltage mode control (One loop control) ….
while UC3843 uses current mode control (Nested loop control, with a inner/fast current loop and another outer/slower voltage loop)…
Typically voltage mode are used in multiple output converters with good cross-regulation. Current mode when you want to parallel multiple converters to make a single converter with higher current rating…
TL494 is a very popular IC. If you have simple requirements… TL494 is recommended…

  • How do I properly set the feedback pin on a TL494 SMPS IC?

The feedback pin is the output of both error amplifiers, used in comparing and adjusting the output pulse width to the DC control voltage.
On various circuits I have looked up, the op-amp connected to pins 2 & 3 are used to set the gain of the feedback loop, using 2 resistors with one resistor connecting to 2.5V potential divider on 5V reference voltage. With the other connecting to the output (via suitable isolation)
The gain appears to be set at 101, using a 51k feedback with 510 ohms to the 2.5V reference. It is used to control the gain of the feedback voltage. No literature I have yet found, gives an indication on how this gain be set, except a graph showing an open loop gain of 1000, presumably the gain is set for the best stability, although there will also be a time constant.

  • Why is there no frequency compensation required in TI's TL494 example buck regulator design (operational amplifier, buck phase, shift phase, margin, TL494, electronics)?

It's a fixed frequency PWM controller with internal dead time timer. Frequency compensation is not required. Take a look at the datasheet.

  • How to use TL494?

 

Kynix

Kynix was founded in 2008, specializing in the electronic components distribution business. We adhere to honesty and ethics as our business philosophy and have gradually established an excellent reputation and credibility in our international business. With the accurate quotation, excellent credit, reasonable price, reliable quality, fast delivery, and authentic service, we have won the praise of the majority of customers.

Join our mailing list!

Be the first to know about new products, special offers, and more.

Leave a Reply

We'd love to hear from you! Feel free to share your thoughts and comments below. Rest assured, your email address will remain private.

Name *
Email *
Captcha *
Rating:

Kynix

  • How to purchase

  • Order
  • Search & Inquiry
  • Shipping & Tracking
  • Payment Methods
  • Contact Us

  • Tel: 00852-6915 1330
  • Email: info@kynix.com
  • Follow Us

authentication

Kynix

© 2008-2026 kynix.com all rights reserved.