Features

Complete PWM Power Control Circuit

200mA sink or source current uncommitted output

Output control selects single-ended or push-pull operation

Internal circuitry inhibits double pulses on either output

Variable dead time provides control over the entire range

Internal Regulator Provides Stable 5-V Reference Supply With 5% Tolerance

Circuit structure allows easy synchronization

2 apps

Desktop PC

Micro-wave oven

Power supply: AC/DC, isolated, with power factor correction, >90 watts

Server PSU

Solar Micro Inverter

Washing machines: low-end and high-end

electric bicycle

Power: AC/DC, Isolated,

No power factor correction, <90 watts

Power Supply: Telecom/Server AC/DC Power Supply: Dual Controller: Analog

Smoke Detectors

Solar inverters

Pin Diagram

illustrate

The TL494 device integrates all the functions required to build a pulse width modulation (PWM) control circuit on a single chip. This device is primarily designed for power control, it provides the flexibility to customize the power control circuit 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% accuracy regulator, and output control circuitry.

The common-mode voltage range of the error amplifier is -0.3V to VCC-2V. 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 referenced to the output and providing a sawtooth input to CT, or it can drive common circuits in synchronous multi-rail supplies.

Uncommitted output transistors provide common emitter or emitter follower output capability. The TL494 device offers push-pull or single-ended output operation, selectable through the output control function. The structure of the device prohibits two pulse outputs during a push-pull operation.

The TL494C device is characterized for operation from 0°C to 70°C. The TL494I device is characterized for operation in temperatures ranging from -40°C to 85°C.

Parameter measurement information

Overview

The design of the TL494 not only contains the main blocks required to control a switching power supply, but it also solves many fundamental problems, reducing the number of additional circuits required in the overall design. TL494 is a fixed frequency pulse width modulation (PWM) control circuit. The modulation of the output pulse is accomplished by comparing the sawtooth generated by the internal oscillator on the timing capacitor (CT) with either of the two control signals. When the sawtooth voltage is greater than the voltage control signal, the output stage is enabled. As the control signal increases, the time for which the sawtooth input is larger decreases; therefore, the output pulse duration decreases.

Functional block diagram

Feature description

5-V Reference Regulator

The 5-V reference regulator output inside the TL494 is the reference pin. In addition to providing a stable reference, it acts as a pre-regulator and establishes a stable power supply from which to power the output control logic, pulse-controlled flip-flops, oscillators, dead-time control comparators, and PWM comparators. The regulator employs a bandgap circuit as its primary reference to maintain thermal stability of less than 100 mV variation over the operating free air temperature range of 0°C to 70°C. Short-circuit protection is provided to protect the internal reference and preregulator; 10 mA load current is available for additional bias circuits. The reference is internally programmed with an initial accuracy of ±5% and is stable to less than a 25 mV change over an input voltage range of 7 V to 40 V. For input voltages less than 7 V, the regulator saturates within 1 V of the input voltage and tracks it.

Characterization (continued)

oscillator

The oscillator provides a positive sawtooth wave to the dead-time and PWM comparators for comparison with various control signals.

The frequency of the oscillator is programmed by selecting timing elements 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 on the current transformer reaches 3V, the oscillator circuit discharges it and the charging cycle starts over. The charging current is determined by the following formula:

Dead time control

The dead time control input provides control of the minimum dead time (off time). When the voltage at the input is greater than the oscillator's ramp voltage, the output of the comparator inhibits switching transistors Q1 and Q2. An internal offset of 110 mV ensures a minimum deadtime of 3% with the deadband control input grounded. Applying a voltage to the dead time control input can apply additional dead time. This provides linear control of dead time from a minimum of 3% to 100% when the input voltage is varied from 0 V to 3.3 V, respectively. With full range control, the output can be controlled from an external source without disturbing the error amplifier. The dead-band control input is a relatively high impedance input (II < 10µA) 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.

Comparator

The comparator is offset from the 5V reference regulator. This provides isolation from the input power supply for improved stability. The comparator's input has no hysteresis, so protection against false triggering must be provided near the threshold. The comparator has a response time of 400 ns from either control signal input to the output transistor, with only 100 mV of overdrive. This ensures positive control of the output for half a cycle within the recommended 300 kHz range.

Feature description

Pulse Width Modulation (PWM)

The comparator also provides modulation control of the output pulse width. To do this, the ramped voltage across the timing capacitor CT is compared with the control signal at the output of the error amplifier. The timing capacitor input contains series diodes omitted from the control signal input. This requires the control signal (error amplifier output) to be 0.7 V greater than the voltage on CT to suppress the output logic and ensure maximum duty cycle operation without the control voltage dropping to actual ground potential. When the voltage output by the error amplifier varies from 0.5v to 3.5v, respectively, the output pulse width varies from 97% of the period to 0.

Error amplifier

Both high-gain error amplifiers receive their bias from the VI supply rail. This allows a common-mode input voltage range from -0.3V to 2V less than VI. Both amplifiers are characterized as single-ended, single-supply amplifiers because each output is active high. This enables each amplifier to be pulled up independently to reduce output pulse width requirements. When both outputs are ORed simultaneously at the inverting input node of the PWM comparator, the amplifier requiring the smallest pulse output dominates. When both amplifiers are biased, the amplifier output is biased low by the current sink to provide maximum pulse width.

output control input

The output control input determines whether the output transistors operate in parallel or push-pull. This input is the power supply for the pulse-controlled flip-flop. The output control input is asynchronous and controls the output directly, independent of oscillators or pulse-controlled flip-flops. Input conditions are fixed conditions defined by the application. For parallel operation, the output control input must be grounded. This disables the pulse-controlled flip-flop and disables its output. In this mode, the pulses seen at the output of the dead-time control/PWM comparator are transmitted in parallel by the two output transistors. For push-pull operation, the output control input must be connected to an internal 5 V reference regulator. In this case, each output transistor is alternately enabled by a pulse-controlled flip-flop.

output transistor

There are two output transistors on the TL494. Both transistors are configured as open collector/open emitter, each capable of sinking or sourcing up to 200mA. The saturation voltage of the transistor is less than 1.3v in common emitter configuration and less than 2.5v in emitter follower configuration. Protect the outputs from excessive power dissipation to prevent damage, but do not employ sufficient current limit to allow them to operate as current source outputs.

Device functional mode

When the output control pin is connected to ground, the TL494 operates in single-ended or parallel mode. When the output control pin is connected to VREF, the TL494 operates in normal push-pull operation.

Application Information

The following design example uses the TL494 to create a 5-V/10-a power supply.

typical application

Switch and Control Section

Design requirements

VI=32V

VO=5V

IO=10A

fOSC=20 kHz switching frequency

VR=20 mV peak-to-peak voltage (VRIPPLE)

IL=1.5-A inductor current change

Detailed design procedure

input power

The 32-V DC power supply for this power supply uses a 120-V input, 24-V output transformer rated at 75 VA. The 24-V secondary winding powers the full-wave bridge rectifier, followed by the current limiting resistor (0.3Ω) and two filter capacitors

Error amplifier

The error amplifier compares a sample of the 5-V output to a reference voltage and adjusts the pulse width modulation to maintain a constant output current.

Error Amplifier Section

The 5V reference voltage inside the TL494 is divided by R3 and R4 by 2.5V. The output voltage error signal is also divided into 2.5v by R8 and R9. If the output must be adjusted precisely to 5.0 V, a 10-kΩ potentiometer can be used in place of R8 for adjustment.

In order to improve the stability of the error amplifier circuit, the output of the error amplifier circuit is fed back to the inverter input through RT, and the gain is reduced to 101.

Current limiting amplifier

The power supply is designed for a load current of 10 a and an IL swing of 1.5 a, so the short circuit current should be:

Current limiting circuit

Resistors R1 and R2 set a reference voltage of about 1v on the inverting input of the current limiting amplifier. Resistor R13 in series with the load applies 1v to the non-vertical side of the current limiting amplifier when the load current reaches 10a. The output pulse width is reduced accordingly

Soft start and stop times

In order to reduce the stress of the switching transistor during startup, it is necessary to reduce the startup surge when the output filter capacitor is charged. Availability of dead-band control makes soft-start circuit implementation relatively simple

Soft start circuit

The soft-start circuit slowly increases the pulse width at the output by applying a negative slope waveform to the dead-band control input (pin 4).

Initially, capacitor C2 forces the dead-band control input to follow the 5-V regulator, thereby disabling the output (100% dead-time). As the capacitor is charged through R6, the output pulse width slowly increases until the control loop accepts the command. When the resistance ratio of R6 and R7 is 1:10, the voltage of pin 4 after starting is 0.1×5V or 0.5V.

Power Recommendations

The TL494 is designed to operate over an input voltage supply range between 7 V and 40 V. This input supply should be well regulated. If the input power supply is more than a few inches away from the device, additional bulk capacitors are required in addition to ceramic bypass capacitors. A tantalum capacitor with a value of 47µF is a typical choice, but this may vary depending on the output power.

Layout Guidelines

Always try to use ferrite type closed core low EMI inductors. Some examples would be toroidal and packaged E-core inductors. Open cores can be used if they have low EMI characteristics and are located a little further away from low power tracks and components. If using an open core, also keep the poles perpendicular to the PCB. Rod cores usually make the most unwanted noise.

feedback record

Try to run the feedback tracking as far away from the inductive and noisy power tracking as possible. You also want the feedback tracking to be as direct as possible and a little thick. There are sometimes tradeoffs between these two situations, but keeping it away from inductor EMI and other noise sources is the most critical. Run the feedback trace on the side of the PCB opposite the inductor, with the ground plane separating the two.

Input/Output Capacitors

When using low value ceramic input filter capacitors, place them as close as possible to the VCC pin of the IC. This will remove as much tracking inductance effects as possible and give a cleaner voltage supply to the inter-IC tracks. Some designs require a feedforward capacitor connected from the output to the feedback pin, usually for stability reasons. In this case, it should also be placed as close to the IC as possible. Using surface mount capacitors also reduces lead length and reduces the chance of noise coupling into the effective antenna created by through-hole components.

Compensation components

External stabilization compensation components should also be placed close to the IC. Surface mount components are also recommended here for the same reasons as filter capacitors. These also should not be located very close to the sensor.

Traces and Ground Planes

Keep all power (high current) traces as short, direct, and thick as possible. It is good practice to have an absolute minimum trace of 15 mils (0.381 mm) per amp on a standard PCB board. The inductor, output capacitor, and output diode should be placed as close together as possible. This helps reduce EMI radiated by power tracking due to the high switching currents through them. This will also reduce lead inductance and resistance, thereby reducing noise spikes, ringing, and resistive losses that create voltage errors. The grounds of integrated circuits, input capacitors, output capacitors, and output diodes (if applicable) should be connected directly to the ground plane. It's also a good idea to have a ground plane on both sides of the PCB. This will reduce noise by reducing ground loop errors and absorbing more EMI radiated by the inductor. For multi-layer boards with more than two layers, a ground plane can be used to separate the power plane (where the power traces and components are) and the signal plane (where the feedback and compensation are and where the components are) to improve performance. On multilayer boards, vias are required to connect traces and different planes. If the trace needs to conduct a lot of current from one plane to another, it is good practice to use a standard via for every 200mA of current. Arrange these elements so that the switching current loops curl in the same direction. Because of the way switching regulators work, there are two power states. One state is when the switch is on and the other state is when the switch is off. In each state, there is a current loop consisting of the power components that are currently conducting. Place the power components so that the current loop conducts in the same direction in each of the two states. This prevents magnetic field reversal induced by the track between half cycles and reduces radiated EMI.

Op amp board layout in non-vertical configuration