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No Rail 2 output, Incorrect Rail 1 output - Please HELP!!

Pawan Ghotikar
Expert 1140 points

Hi,

I am trying to set 3.45V on both Rail1 and Rail2 of UCD9222 with UCD7242 with SW-A and SW-B connected as in 20A Design described in ucd7242 datasheet. All I see is 6.4V on Rail1 and no output on Rail2. I also noticed that PHASE_INFO command gets a NACK. Please suggest

Thanks,

Pawan Ghotikar

  • 0
    TI__Genius 9845 points

    In the 20A design (UCD7242 datasheet Fig18), the SW-A and SW-B are not connected. PWM-A and PWM-B are driven by the same PWM signal. The other end of the two inductors are connected to the same output cap. Is it how you connected? 

  • 0 in reply to Zhiyuan Hu
    Expert 1140 points
    Hi Zhiyuan Hu,
    Yes. It is connected the way you have described. The Vout1 is fed back to EAP1 via voltage divider ratio of 0.242 (1.5K and 4.7K).

    Thanks,
    Pawan Ghotikar
  • 0 in reply to Pawan Ghotikar
    TI__Genius 9845 points
    So you have connected output caps of rail 1 and rail 2? Then they are in parallel and should have the same voltage.
  • 0 in reply to Zhiyuan Hu
    Expert 1140 points
    I do have two identical circuits one connected to PWM1 output to ucd7242-1 and other connected to PWM2 output to ucd7242-2. And in this case, is the Vout1 considered as one of the two rails I am trying to control? I have similar circuit which gives Vout2 and fed back using the same voltage divider ratio to EAP2 from ucd7242-2.
    - Pawan
  • 0 in reply to Pawan Ghotikar
    TI__Genius 9845 points
    What Fig18 suggests is that you need to connect PWM1 and PWM2 of UCD7242 to the same PWM pin of UCD92xx. Only one feedback is used. If you connect both UCD7242 PWM to rail1's PWM, you are using rail1's feedback, and vice versa.

    The output caps of the two rails must be connected together, and they should have the same output voltage. If one cap has 0V as you described, something is wrong. At least the connection between the two caps is open.
  • 0 in reply to Zhiyuan Hu
    Expert 1140 points

    Found shorts on the other rail. Many thanks Zhiyuan Hu. I should've gone the conventional method of checking the shorts first. But worthwhile brainstorming with you. Learnt a lot from you. After fixing the shorts if I have any problems I will be posting it. I am grateful for your assistance.

    Thanks,
    Pawan Ghotikar