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Original question:

LM5116: LM5116MHX/NOPB

LM5146-Q1: DC output Voltage is not building.

Amit Kumar
Prodigy 10 points
Part Number: LM5146-Q1
Other Parts Discussed in Thread: LM5146

Tool/software:

In one of our PCBA board, which is based on LM5146 IC, DC output voltage is not building. It stays close to 4-5V. After further investigation, we found that SS pin is not able reach its full voltage, it is reaching to 0.3 ramp and then dropping down to zero. Would you please let us know the potential root cause?

regards,

Amit

  • 0
    TI__Guru 67402 points

    Amit,

    Please send the schematic and a completed quickstart file. You may be hitting current limit at startup - try increasing the SS time or reducing Cout and the load.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    Hello Tim,

    I tried increasing the SS time but result is intact. I am attaching waveforms for your reference wherein VCC is experiencing glitches every 5ms cycle and SS voltage is not building up properly. 

    DC-DC converter (48V to 14.5V) is operating at 150kHz switching frequency and feedback is (Output voltage/18).  Would you please address below questions

    1) Why VCC experiences glitches 

    2) Why boot cap is not able to charge beyond 4V?

    3) Why SS voltage is dropping? 

     

  • 0 in reply to Amit Kumar
    TI__Guru 67402 points

    Any files to review here?

    VCC should be a steady 7.5V DC voltage, so make sure the VCC cap is located close to the VCC and PGND pins.

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    Hello Tim,

    VCC cap is already present close to pin but still we are facing above mentioned issue. I am attaching schematic as well one more waveform data wherein HO and LO signals are also captured, upon analysing the waveforms, it can be observed that FB is not following DC-DC output voltage waveform though FB is simply resistor divider network. Does that mean FB is governed by controller IC and not the DC-DC output signal?

    In what scenario, captured HO and LO switching can happen?

  • 0 in reply to Amit Kumar
    TI__Guru 67402 points

    Hi Amit,

    There should be no cap from FB to GND - this just creates a pole and possibly the behavior you're seeing. Also, it's important to keep the lower feedback resistor right at the FB and AGND pins, so just use one resistor here. FB should follow Vout.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Guru 67402 points

    Further points:

    1. Complete the quickstart calculator and verify stability.
    2. 1uF is really high for the boot cap...try 100nF.
    3. No need for antiparallel diode D4 - the deadtime is very short and this diode will not conduct.
    4. An electrolytic damping cap should be placed on the right side of the input filter inductor.
    5. One VCC cap is sufficient, but keep it close to VCC and PGND pins.
    6. Remove caps on the gates of the FETs.
    7. Connect the NC pins to GND for better heatspreading.