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UCC2897A common line and VDD rail?

Olli Mannisto
Expert 1750 points
Other Parts Discussed in Thread: UCC2897A

Is it possible to use the UCC2897A controller using +15V voltage for both line and VDD? I suspect there might be a gotcha with regards to the relative voltage levels. Datasheet suggests +18V minimum for line voltage.

I'm intending to use the UCC2897A as a high-voltage flyback controller. High voltage from low input voltage that is. Specification is 24Vin, 600Vout. I have a prototype 1:10 turns ratio flyback transformer for this which would be run in DCM.

I'm planning to use an intermediate 24-to-10V buck converter to provide the VDD rail for the UCC2897. It should be possible to change 24V supply to 15V in future applications but is it still necessary to keep line voltage at least +2V above VDD? 

  • 0
    TI__Mastermind 25130 points

    Olli,

    From the block diagram and the recommended operating conditions at 15 volts you would be under the maximum operating voltage for the VDD voltage but also under the minimum operating voltage for the Vin voltage.

    Is it possible to use the UCC2897A controller using +15V voltage for both line and VDD?   It should be possible to change 24V supply to 15V in future applications but is it still necessary to keep line voltage at least +2V above VDD? 

    We have tested the IC with power applied to VDD and Vin open. That is what we would recommend as an operating condition.

    John

  • 0 in reply to John Bottrill
    Expert 1750 points

    John Bottrill said:

    Olli,

    From the block diagram and the recommended operating conditions at 15 volts you would be under the maximum operating voltage for the VDD voltage but also under the minimum operating voltage for the Vin voltage.

    We have tested the IC with power applied to VDD and Vin open. That is what we would recommend as an operating condition.

    John

    Thanks. Looking again at the block diagram and the VIN pin description, this appears to be true. VIN is only used to get the SMPS started so the aux coil with low voltage becomes available. In my case there is no aux coil or line voltage so connecting the 12VDC SMPS to VDD and PVDD with VIN would simplify things. 

  • 0 in reply to Olli Mannisto
    Expert 1750 points

    I have finally a prototype in the lab. It appears there is indeed a catch with the VDD and VIN. 

    I have 24->12V buck SMPS supplying VDD and VIN left floating as recommended. The SMPS does not start as at least 12.7V voltage is required to start the device. For prototype I can adjust the VDD voltage to 13.8V without problems.

    For similar functionality as in the "intended" setup I'd connect 24V to VIN and 12V to VDD. After VIN has exceeded 12.7V, VDD threshold drops to <9V.