SLUU182A January   2004  – March 2022 TPS5124

 

  1.   Trademarks
  2. 1Introduction
  3. 2Features
  4. 3Schematic
  5. 4Design Procedure
    1. 4.1 Frequency Setting
    2. 4.2 Inductance Value
    3. 4.3 Output Capacitors
    4. 4.4 Input Capacitors
      1. 4.4.1 Case One: D1, D2 < 0.5
      2. 4.4.2 Case Two: D2 < 0.5 < D1
    5. 4.5 Compensation Design
    6. 4.6 Current Limiting
    7. 4.7 Timer Latch
      1. 4.7.1 Undervoltage Protection
      2. 4.7.2 Short Circuit Protection
      3. 4.7.3 Overvoltage Protection
      4. 4.7.4 Disabling the Protection Function
        1. 4.7.4.1 Disabling the Overcurrent Protection
        2. 4.7.4.2 Disabling the Overvoltage Protection or Undervoltage Protection
  6. 5Test Results
    1. 5.1 Efficiency Curves
    2. 5.2 Typical Operating Waveform
    3. 5.3 Start-Up Waveform
    4. 5.4 Output Ripple Voltage and Load Transient
  7. 6Layout Guidelines
    1. 6.1 Low-Side MOSFET
    2. 6.2 Connections
    3. 6.3 Bypass Capacitor
    4. 6.4 Bootstrap Capacitor
    5. 6.5 Output Voltage
  8. 7PCB Layout
  9. 8List of Materials
  10. 9Revision History

4.6 Current Limiting

The current limit in the TPS5124 is set using an internal current source and an external resistor (R13 and R14). The current limit protection circuit compares the drain-to-source voltage of the high-side and low-side drivers with respect to the set-point voltage. If the voltage exceeds the limit during high-side conduction, the current limit circuit terminates the high-side driver pulse. If the set point voltage is exceeded during low-side conduction, the low-side pulse is extended through the next cycle. Together, this action has the effect of decreasing the output voltage until the undervoltage protection circuit is activated and the fault latch is set and both the high-side and low-side MOSFET drivers are shut off. Equation 16 should be used for calculating the external resistor value for current protection set point.

Equation 16. R C L =   1.3   ×   R D S ( o n )   × ( I L I M +   I R I P P L E 2 ) I T R I P

where

  • RCL is the external current limit resistor (R13 and R14).
  • RDS(on) is the on-resistor of the low-side MOSFET (Q2 and Q4).
  • 1.3 is the temperature coefficient of RDS(on).
  • ILIM is the required current limit.
  • ITRIP is the internal current source with a typical value of 13 μA at 25°C.