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LM5116 does not start-up reliably when feeding its 12V output into VCCX.

Olan Way
Prodigy 190 points
Other Parts Discussed in Thread: LM5116

We use an LM5116 Wide Range Synchronous Buck Controller to generate 12VDC.  The DC load is typically only 220mA.  The capacitive load is 200uF.  The soft-start capacitor is 0.1uF which provides a 12msec ramp-up to 12V on the output.  The design is extremely similar to the LM5116-12 Evaluation Board. Like the LM5116-12 Evaluation Board, the regulator output voltage is fed back into the VCCX input, to allow the gates of the external  Si7852DP FETs to be driven at 12V instead of the internally generated 7.4V. 

When the VCCX input reaches 4.5V on its way up to 12V, the LM5116 begins to use this voltage (minus the external diode voltage drop) to drive the gates of the external FETs.  Unfortunately on some of the boards, the resulting voltage at this point appears to be too low to reliably turn on the external FETs.  The LM5116 enters hiccup mode, as if an over-current were occurring.

More than a single date code of the LM5116 exhibit this behavior.  The LM5116 was replaced on at least one problematic board, correcting the symptom.  However, when that LM5116 was checked at TI, the chip tested good.

Is this a known problem?

  • 0
    TI__Guru 61151 points

    Hi

    Please try to connect 12V directly to VCC through a 1 ohm resistor similar with LM5116-12 evaluation board. You don't need to supply VCCX through a diode.

    Also, place bigger Cvcc & Cvccx capacitors.

    Regards

  • 0 in reply to EricLee
    Prodigy 10 points

    Hello:  I have same problem using VCCX pin.  With VCCX pin open, I can drive up to 85V input with  Vout=12.75V  up to 1A OK.  The problem is the heating of the LM5116.  I tried to use VCCX to reduce the heating issue. Connecting directly from Vout to VCCX would cause the LM5116 to fail, when Vin is >24V.  Tried with 4 ohm with 1uf cap, will fail  around 72V.  I am using 2uf at the Cvcc and Cboot. 

    After the failure, we measured 50 ohm short from SW pin to Ground. I don't think it is the low voltage issue as pointed out by Olan, it feels like a noise spike issue, which damaged the LM5116 and caused other behaviors.

    Does anyone has good luck with VCCX feeding back from Vout? 

    To Mr. Olan Way, did you ever solved your problem?

    I am not sure if there is a problem with the IC, but it is very fragile, when it comes to VCCX.

    casey-ca1@cox.net