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LM5116: Unstable output when the VCCX is supplied externally

Danilo A.
Mastermind 28680 points
Part Number: LM5116
Other Parts Discussed in Thread: LM5576, LM5164, LM76005

Hi Team,

Our customer is using LM5116MH. The device works fine using the internal LDO, however, when he supplied external power to VCCX the output voltage is unstable and below the nominal value. He built 3 boards with the same issue. In the schematic diagram, the VCCX has a 0 ohm resistor (R16) to ground which he replaced with 100nF and connect to 10VDC. The RC loop values on FB pin were configured like the values from the Webench schematic diagram below. What could be the cause of the instability? 

WBDesign12 (4).pdf

Regards,

Danilo 

  • 0
    TI__Guru 73652 points

    Hi Danilo,

    Please check the compensation values using the LM5116 quickstart calculator. Also, remove L1 in series with the HB pin. Also, make sure the input caps are very close to the FETs (app note snva803 has more detail on this).

    The external VCC rail provides a higher gate drive voltage, so the FETs switch faster, possibly increasing noise. A 4.7 Ohm resistor on the high-side gate (or where L1 is located in the boot circuit) slows the FET turn on and may alleviate the increased switching noise.

    --

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Guru 73652 points

    By the way, please confirm the output current requirement here. The design looks much different to the WEBENCH file attached.

    Note that we have high-current converters (with integrated FETs) that provide a smaller solution size that an equivalent controller design - see the LM76005 60V/5A converter for example. Also... LM5576 75V/3A and LM5164 100V/1A are other synchronous buck converters rated for 75V+ Vin.