SLUS224H September   1994  – October 2024 UC1842A , UC1843A , UC1844A , UC1845A , UC2842A , UC2843A , UC2844A , UC2845A , UC3842A , UC3843A , UC3844A , UC3845A

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
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Pulse-by-Pulse Current Limiting
      2. 6.3.2 Current Sense Circuit
      3. 6.3.3 Error Amplifier Configuration
      4. 6.3.4 Undervoltage Lockout
      5. 6.3.5 Oscillator
    4. 6.4 Device Functional Modes
      1. 6.4.1 Normal Operation
      2. 6.4.2 Undervoltage Lockout (UVLO) Start-Up
      3. 6.4.3 UVLO Turnoff Mode
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1 Custom Design With WEBENCH® Tools
        2. 7.2.2.2 UC2842A Design Procedure
      3. 7.2.3 Application Curves
  9. 8 Power Supply Recommendations
  10. 9 Layout
    1. 9.1 Layout Guidelines
    2. 9.2 Layout Example
  11. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Development Support
        1. 10.1.1.1 Custom Design With WEBENCH® Tools
      2. 10.1.2 Device Nomenclature
    2. 10.2 Documentation Support
      1. 10.2.1 Related Documentation
    3. 10.3 Related Links
    4. 10.4 Receiving Notification of Documentation Updates
    5. 10.5 Support Resources
    6. 10.6 Trademarks
    7. 10.7 Electrostatic Discharge Caution
    8. 10.8 Glossary
  12. 11Revision History
  13. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

6.3.4 Undervoltage Lockout

The UCx84xA devices feature undervoltage lockout protection circuits for controlled operation during power-up and power-down sequences. Undervoltage lockout thresholds for the UCx842A, UCx843A, UCx844A, and UCx845A devices are optimized for two groups of applications: off-line power supplies and DC-DC converters. With a wider VCCON to VCCOFF range, the UCx842A and UCx844A devices are ideally suited to off-line AC input applications. The UCx843A and UCx845A controllers have a much narrower VCCON to VCCOFF hysteresis and can be used in DC to DC applications where the input is considered regulated.

During UVLO the IC draws typically 0.3 mA of supply current. This VCC current is considerable less than the UCx84x family and results in lower power drawn from the line. The reduced start-up current is of particular concern in off-line supplies where the IC is powered-up from the high-voltage DC rail, then bootstrapped to an auxiliary winding on the main transformer. Power is then dissipated in the start-up resistor which is sized by the IC start-up current. Lowering this by 50% in the UCx84xA version family, as compared to the UCx84x family, reduces the resistors power loss by the same percentage. Once crossing the turnon threshold the IC supply current increases typically to approximately 11 mA, During undervoltage lockout, the UCx84xA series of devices prevent the power MOSFET from parasitically turning on due to the Miller effect at power-up. This improved design to the lower totem-pole transistor operation during undervoltage lockout allows the IC to sink higher currents, up to 10 mA, at saturation voltages as low as 0.7 V, compared to the UCx84x devices which only sinks up to 0.2 mA under the same conditions.

UC1842A UC1843A UC1844A UC1845A UC2842A UC2843A UC2844A UC2845A UC3842A UC3843A UC3844A UC3845A Undervoltage LockoutFigure 6-3 Undervoltage Lockout