SLVS074J January   1983  – September 2026 TL494

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1  Absolute Maximum Ratings
    2. 5.2  ESD Ratings
    3. 5.3  Recommended Operating Conditions
    4. 5.4  Thermal Information
    5. 5.5  Electrical Characteristics, Reference Section
    6. 5.6  Electrical Characteristics, Oscillator Section
    7. 5.7  Electrical Characteristics, Error-Amplifier Section
    8. 5.8  Electrical Characteristics, Output Section
    9. 5.9  Electrical Characteristics, Dead-Time Control Section
    10. 5.10 Electrical Characteristics, PWM Comparator Section
    11. 5.11 Electrical Characteristics, Total Device
    12. 5.12 Switching Characteristics
    13. 5.13 Typical Characteristics
  7. 6 Parameter Measurement Information
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 5V Reference Regulator
      2. 7.3.2 Oscillator
      3. 7.3.3 Dead-time Control
      4. 7.3.4 Comparator
      5. 7.3.5 Pulse-Width Modulation (PWM)
      6. 7.3.6 Error Amplifiers
      7. 7.3.7 Output-Control Input
      8. 7.3.8 Output Transistors
    4. 7.4 Device Functional Modes
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Input Power Source
        2. 8.2.2.2 Control Circuits
          1. 8.2.2.2.1 Oscillator
          2. 8.2.2.2.2 Error Amplifier
          3. 8.2.2.2.3 Current-Limiting Amplifier
          4. 8.2.2.2.4 Soft Start and Dead Time
        3. 8.2.2.3 Inductor Calculations
        4. 8.2.2.4 Output Capacitance Calculations
        5. 8.2.2.5 Transistor Power-Switch Calculations
      3. 8.2.3 Application Curves for Output Characteristics
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Feedback Traces
        2. 8.4.1.2 Input/Output Capacitors
        3. 8.4.1.3 Compensation Components
        4. 8.4.1.4 Traces and Ground Planes
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Receiving Notification of Documentation Updates
    2. 9.2 Support Resources
    3. 9.3 Documentation Support
      1. 9.3.1 Related Documentation
    4. 9.4 Trademarks
    5. 9.5 Electrostatic Discharge Caution
    6. 9.6 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information
8.2.2.2.4 Soft Start and Dead Time

To reduce stress on the switching transistors at the start-up time, reduce the start-up surge that occurs as the output filter capacitor charges. The availability of the dead-time control makes implementation of a soft-start circuit relatively simple (see Figure 8-5).

TL494 Soft-Start CircuitFigure 8-5 Soft-Start Circuit

The soft-start circuit allows the pulse duration at the output to increase slowly (see Figure 8-5) by applying a negative slope waveform to the dead-time control input (pin 4).

Initially, capacitor C2 forces the dead-time control input to follow the 5V regulator, which disables the outputs (100% dead time). As the capacitor charges through R6, the output pulse duration slowly increases until the control loop takes command. With a resistor ratio of 1:10 for R6 and R7, the voltage at pin 4 after start-up is 0.1 × 5V, or 0.5V.

The soft-start time generally is in the range from 25 to 100 clock cycles. If selecting 50 clock cycles at a 20kHz switching rate, the soft-start time is:

Equation 12. t = 1 f = 1 20 k H z = 50 μ s   p e r   c l o c k   c y c l e

Calculate the value of the capacitor using:

Equation 13. C 2 = s o f t - s t a r t   t i m e R 6 = 50 μ s × 50   c y c l e s 1 k Ω = 2.5 μ F

This helps eliminate any false signals created while applying power to the control circuit.