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TPS562200 duty cycle limit

user4292685
Intellectual 945 points
Other Parts Discussed in Thread: TPS562200

Hi Engineer,

I would like to use TPS562200 under the following condition.

Vin min. :  4.9V

Vin max. : 5.5V

Vout : 3.3V

Iout : 1A

When I tried to simulate the above condition by WEBENCH, an "Duty cycle limit error" occurs and it looks like TPS562200 can not use in case of less than 5Vin when output voltage sets 3.3V.

Is this correct result?

If so, how do I workaround this problem? The only way to solve this problem is to change DCDC device which duty cycle is more than 80%?

Thanks,

  • 0
    TI__Guru**** 191445 points
    The TPS562200 maximum duty cycle is limited by the minimum off time of 310 nsec. So max duty cycle is about 78.92 % with Fsw approx. 680 kHz for Vin < 5 V and Vout = 3.3 V. That is the "hard limit". It will not go above it. It has a soft approach though. As Vin lowers toward Vout and duty cycle increase towards the hard limit voltage regulation starts to degrade above around 70-72%. Also bear in mind that good transient response requires that duty cycle can increase rapidly. If you are already operating near the limit there is no room for duty cycle to expand and transient response worsens somewhat.

    For Vout = 3.3 and Vin = 4.9 V requires about 67.34% duty cycle typical. You are near the range where regulation may start to roll off, but TPS562200 will regulate to 3.3 V at that minimum input voltage of 4.9 V. Not a lot of headroom for transients though.

    Webench has a hard limit. I think it may be 65%. If you want to use webench, raise your minimum input voltage slightly. Otherwise just follow the datasheet recommended design values.
  • 0 in reply to JohnTucker
    Intellectual 945 points

    Hi Tucker,

    Thank you for your reply. It sounds like there is not a lot of headroom. Off course it depends on the system though, usually how much headroom(%) do you have?

    Moreover one point I can not understand is the relationship between "duty cycle close to the hard limit" and "voltage regulation degradation".

    Could you explain this relationship in detail?

    Regards,

  • 0 in reply to user4292685
    TI__Intellectual 2590 points

    TPS562200 can support 5V to 3.3V application, as we write on the data sheet, please make sure the duty cycle not larger than  75%.

    thanks!