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LMH6553: LMH6553 problem

Giora Eytan
Expert 1325 points

Part Number: LMH6553
Other Parts Discussed in Thread: LMH6554

I am using the LM6553 as in the picture attached. In TINA simulation it works OK - but in the  real world

I see that the three NCP114 does not output their voltages (1.1 on VCM, 1.35V on CLAMP and 0.75V to the resistor that goes to the minus input) - the voltages are always greater.

Do you have any explanation and how to correct?

Regards,

Giora

LMH6553.jpg

  • 0
    TI__Prodigy 190 points

    Hi Giora,

    There is nothing that looks obviously wrong with your schematic as far as I can tell. However, according to the NCP114 datasheet, it is recommended to connect at least a 1uF capacitor to the IN pin of your LDOs. This cap is necessary because it will provide a low impedance path for unwanted AC signals or noise modulated onto constant input voltage, and they are needed for stability. Have you tried disconnecting the LMH6554 from the NCP114's. If the same issue occurs with it disconnected, it will most likely have to do with the NCPs rather than the FDA itself. Also, is there a reason behind the R14 and R12 in your circuit. They could be an issue as it might cause potential asymmetry and imbalances in your network.

    Best,

    Amber

  • +1
    Expert 1325 points

    Amber,

    Thanks.

    1. We added the 1uF caps.

    2. The most important - that solve the problem  - was to add a 1K resistor on the NCP114 output.

    Regards,

    Giora