The TL494 is HeroMicro a classic fixed-frequency pulse-width modulation (PWM) controller IC. Renowned for its high flexibility and integration, it has been widely used in switch-mode power supplies, motor control, inverters, and various other power conversion systems since its introduction. It integrates an oscillator, two error amplifiers, a dead-time control comparator, a PWM flip-flop, and totem-pole output stages, making it easy to implement both single-ended and push-pull topologies.
1. Key Features Overview
Based on the provided document (TL494.pdf), the key specifications and functions are:
Control Mode: Voltage-mode PWM control (duty cycle regulated via external feedback).
Dual Error Amplifiers: Enable complex feedback loops, such as simultaneous voltage and current monitoring, or multi-output regulation.
Programmable Oscillator: Operating frequency (typically 1kHz - 300kHz) is set by external resistor (RT) and capacitor (CT).
Dead Time Control: The DEAD TIME CONTROL pin (Pin 4) allows linear adjustment of the maximum duty cycle (from 0% up to a theoretical maximum of ~45%), effectively preventing shoot-through in push-pull applications.
Output Stage:
Totem-Pole Outputs: Capable of sourcing/sinking up to 200mA, sufficient to directly drive small power transistors or act as a pre-driver.
Single-Ended or Push-Pull Operation: Configurable via the OUTPUT CONTROL pin (Pin 13).
On-Chip Reference: Provides a precise +5V ±5% reference voltage (VREF, Pin 14) for internal circuits and external components (e.g., RT, feedback network).
Protection Features: Soft-start and over-current protection can be implemented through the DEAD TIME CONTROL pin (output shuts down when this pin voltage exceeds 3.3V).
2. Key Pin Functions
| Pin | Name | Description |
|---|---|---|
| 1, 2 | COMP1, INV1 | Output and inverting input of Error Amplifier 1 |
| 3 | FEEDBACK | Non-inverting input of the error amplifier (typically for feedback signal) |
| 4 | DEAD TIME CONTROL | Dead time control / soft-start / shutdown input. Higher voltage = larger dead time; output disabled if >3.3V. |
| 5, 6 | CT, RT | Timing capacitor and resistor connections for the oscillator. |
| 7 | GND | Ground. |
| 8, 11 | C1, C2 | Collector outputs 1 and 2. |
| 9, 10 | E1, E2 | Emitter outputs 1 and 2. |
| 12 | VCC | Supply voltage (typically 7V - 40V). |
| 13 | OUTPUT CONTROL | Output configuration control. High (VCC) for push-pull, Low (GND) for single-ended. |
| 14 | VREF | +5V internal reference voltage output. |
| 15, 16 | INV2, COMP2 | Inverting input and output of Error Amplifier 2 |
3. Critical Design Guidelines
(1) Setting the Operating Frequency
The document example (p.13) shows how to set the oscillator to 20kHz:
Choose CT = 0.001 μF (1 nF)
Calculate RT = 1 / (f × CT) = 1 / (20,000 × 0.001×10⁻⁶) ≈ 50 kΩ
Formula: f ≈ 1 / (RT × CT) (consult the official datasheet for the exact constant).
(2) Configuring Output Mode
Single-Ended Output: Connect OUTPUT CONTROL (Pin 13) to GND. C1/E1 and C2/E2 output identical PWM signals, which can be paralleled for higher drive current.
Push-Pull Output: Connect OUTPUT CONTROL (Pin 13) to VCC. C1/E1 and C2/E2 output complementary (180° out of phase) PWM signals, suitable for push-pull or half-bridge topologies.
(3) Feedback and Regulation
The document example (p.13) demonstrates a 5V regulated output circuit:
The internal VREF (5V) is divided by R3 and R4 (both 5.1kΩ) to create a 2.5V reference.
The output voltage VO is divided by R8 and R9 (both 5.1kΩ) to also yield 2.5V, which is fed into the non-inverting input (Pin 3).
By comparing these two 2.5V signals, the error amplifier adjusts the PWM duty cycle to maintain VO = 5V.
Dual Error Amplifier Application: One amplifier can be used for voltage feedback, while the other can be configured for current limiting (as mentioned in the "Current Limit Amplifier" section on p.14).
(4) Dead Time and Protection
Dead Time Adjustment: Apply a DC voltage between 0-3.3V to DEAD TIME CONTROL (Pin 4) to linearly adjust the max duty cycle from ~45% down to near 0%.
Soft-Start: Connect a capacitor from Pin 4 to GND. At power-up, the capacitor voltage ramps up from 0V, causing the output duty cycle to start from zero and gradually increase, preventing inrush current.
Over-Current/Over-Voltage Protection: An external comparator can monitor for faults. Upon detection, it pulls Pin 4 above 3.3V, immediately disabling all outputs.
(5) Driving Power Switches
The TL494's output stage (C1/E1, C2/E2) can directly drive small transistors.
For high-power applications (e.g., the 10A output shown on p.16), external Darlington pairs or MOSFET drivers are required.
The document calculates that a base drive current of 144mA is needed for a peak current of 10.8A.
Based on this, a 220Ω base current-limiting resistor (R10) was selected.
(6) Output Filtering
To keep output ripple voltage within a target (e.g., <100mV as in the document), the output filter capacitor must be chosen carefully.
Given a ripple current of 1.5A and a target ripple voltage of 0.1V, the document calculates a minimum capacitance C3 ≥ 94μF.
It also emphasizes the importance of the capacitor's Equivalent Series Resistance (ESR), calculating a maximum allowable ESR of 0.067Ω.
4. Summary
The TL494 is a powerful and enduring PWM controller. Its dual error amplifiers, flexible output configuration, programmable dead time, and on-chip reference make it adaptable to a wide range of power supply designs, from simple to complex. By carefully studying the documentation and following the guidelines above, engineers can efficiently leverage the TL494 to build stable, reliable, and high-performance switch-mode power supplies.
