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UCC28631: Soft Start

Part Number: UCC28631
Other Parts Discussed in Thread: UCC28630EVM-572, , UCC28630, UCC28632, UCC28633


My customer has the following questions:

  1. Is there a soft-start mechanism? (Datasheet says yes but not much info)
  2. How does the soft start (SS) start up?
  1. Any specific frequency, timing, or anything we can mimic as a model?
  • Can SS be externally controlled?
  • Is Tstart (start-up time) an estimate from start to normal PWM operation?
  • We are using UCC28631, are there major difference if using  demo board UCC28630EVM-572?



  • Aaron,

    The UCC2863x family devices do not have an explicit soft-start. Instead, they make use of the internal CC-mode current limiting to ensure a controlled ramp-up of the output voltage. Effectively, the output current to charge the output capacitors is limited to a constant value by the internal CC-mode control loop – this constant charging current results in a well-controlled linear dv/dt as Vout rises across the output caps.

    When the output voltage reaches the regulation set-point, the CC loop will seamlessly transition control to the CV loop to regulate the output voltage.

    Since Vout is so well-controlled, with no startup overshoot, there is no need for a conventional soft-start. Typical startup plots for the EVM are shown in figs. 51 & 52 in the UCC28630 datasheet.

    Depending on your design turns ratio & Rcs value, you can calculate the Iocc current limit using eqn. 20 in the datasheet. The time taken to ramp Vout to the regulation level then depends on the size of the output cap, and time can be calculated from t = Vout(nom) * Cout / Iocc.

    Tstart is the delay to charge VDD to the startup level via the HV pin, so yes you are correct, it’s the delay to start of PWM activity.

    The UCC28630EVM-572 comes with UCC28630 installed – but it can be modified to use any of the other family devices UCC28631, UCC28632 or UCC28633 – all will work ok in the EVM. However, since the overload timer is not active in these devices, the EVM can operate all the way to the ~7-A CC limit without shutting down. At > 65 W load, the EVM transformer and power devices will get hot, since the heatsinking was sized for 65 W. At >65 W load, a cooling fan is recommended to keep the EVM from getting too hot.