SNVAA44 july   2023 LM5155 , LM5155-Q1 , LM51551 , LM51551-Q1 , LM5156 , LM5156-Q1 , LM51561 , LM51561-Q1 , LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581

 

  1.   1
  2.   How to Implement the Soft Start in a Flyback
  3.   Trademarks
  4. 1Overview
  5. 2Design Considerations
  6. 3Soft-Start Time Calculation Considerations
    1. 3.1 Soft-Start Capacitor Selection for the LM5155x
  7. 4Soft-Start Time Within Different Topologies
    1. 4.1 Buck Topology
    2. 4.2 Boost Topology
    3. 4.3 Sepic Topology
    4. 4.4 Flyback Topologies
      1. 4.4.1 PSR Flyback: Bipolar Transistor as Pull Down for COMP Pin
      2. 4.4.2 Secondary Side Soft-Start Components
  8. 5Fixing Soft-Start Issue
  9. 6Summary
  10. 7References

3.1 Soft-Start Capacitor Selection for the LM5155x

At start-up of the LM5155, the internal 10-μA soft-start current source (ISS) turns on 50 µs after the VCC voltage exceeds the 2.85-VCC UV threshold, or if the VCC voltage is greater than 4.5 V, whichever comes first. The soft-start current gradually increases the voltage on an external soft-start capacitor connected to the SS pin. The SS pin is pulled down to ground and the capacitor on the SS pin is discharged by an internal switch if the VCC is less than VCC UVLO threshold, the UVLO is less than the UVLO threshold, during hiccup mode off-time or during thermal shutdown.

The soft-start capacitor is used to minimize and overshoot on the load voltage during the start-up of the regulator. Equation 3 is used to calculate the minimum recommended soft-start capacitor value.

Equation 3. C S S >   10 μ A   ×   V L O A D × C O U T I L O A D

In a secondary side isolated Flyback, the soft-start mechanism is taking over right at the start up, when the optocoupler is not yet conducting (meaning VLOAD < approximately 1.5 V).