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Problem with LM34922

Mayur Sutariya
Prodigy 95 points
Other Parts Discussed in Thread: LM34922

Hello,

I am using LM34922 switcher for two voltage rails. 5.4V@0.6A and 9.1V@0.4A. I designed the power supply using TI's webench tool and almost all the compoenets are same except some resistors due to availability. When I tested my first proto board, i was unable to get correct output on either supply voltage. I got 1.6V on 5.4 and 2.7V on 9.1. Currenty there is no load connected to it. Voltage at the FB pin is nearly 0.8V for both the supplies. Input voltage is 12V.

I tried with external load and varied it between 0.2A to 0.6A on 5.4V regulator. In this case output voltage tried to increase but it could reach up to 4.6V only and not to 5.4V. Also LM34922 chip was heating too much when I connected axternal load and also making some humming noise which inceased as the load increased.

  • 0
    TI__Genius 11490 points

    HI Mayur,

    I have replied to this question through email. But to repeat it seem PowerPAD is not connected. I will also check the current sense connections.

  • 0 in reply to Vijay86929
    Prodigy 95 points

    Hi Vijay,

    At first I also thought that this could be due to power pad. Then I tried to solder power pad using hot gun in lab on another board and did check again. But still same rsult.

    I checked all the connections but all connections are good. I looked at the waveform of SW node. it is not coming out pure square wave also not reaching up to 12V. It is clamping at 8-10V.

    Do you think it has anything ralated to On and Off time which is defined by Rt? I also cheked by changing current sense resistor Rs, but no luck so far.

    Below are schematic for the design.

    Appriciate your help to troubleshoot the problem.

    Thanks, 

    Mayur