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Original question:

LM5141: No output

LM5141: Does 0 V at VCC indicate a damaged chip?

Matt Bruensteiner67
Prodigy 220 points
Part Number: LM5141
Other Parts Discussed in Thread: CSD19534Q5A, CSD18563Q5A

I have made the following circuit, based on a design recommendation from WebBench:

Some changes have been made since the schematic was printed:

* Switching FETs changed to CSD18563

I have 2 samples of this circuit. One is working as expected, the 2nd one is producing no output voltage.

* 24 V is present at VIN (the label as VCC48 doesn't mean 48 V is expected)

* 3.3 V is present at EN

* No switching activity is seen on the FET gates. 

* No voltage is seen at SS

But the thing that makes me think the chip might be damaged is that 0 V is seen at the VCC pin.

There is not much detail of what is attached to this pin internal to the chip. But I suspect it is just a linear regulator. I don't see any mention of any circumstance that would produce 0 V at VCC if the chip is operating normally. VCCX is shorted to ground as shown in the schematic.

So, can I take the 0 V at VCC as an indication the chip is damaged and needs to be replaced? Are there any operating conditions that should be expected to produce 0 V at VCC?

(I had previously posted another question about the same design, but this is a different problem. In that situation I always saw 5 V at VCC.

Thanks,

Matt

  • 0
    TI__Guru 59937 points

    Hi Matt,

    In general, if the EN voltage is high and VIN is up, then VCC should also come up.

    Here are a few comments on the schematic:

    1. Good that you changed to the CSD18563Q5A 60V NexFET. The gate voltage Miller plateau of the CSD19534Q5A 100V FET is too high for a 5V gate drive amplitude. We need Rds(on) rated at Vgs = 4.5V.
    2. Add more 1uF ceramic input capacitors (the 47uF input electrolytic may not provide much filtering of the switching frequency currents). C20 local decoupling cap for the VIN pin of the IC can be 0.1uF/0805.
    3. Use the new quickstart file (available by download from the product folder) to check compensation. C97 22nF seems high and will cause a slow transient response settling time.
    4. Change the boot diode D8 to a 60V/0.2A Schottly diode. The 100V/1A SMB package diode currently specified looks really big.
    5. You likely will not need the gate resistors on the low-side FET, especially R57 that just slows the turn-on speed of M7, forcing the body diode to conduct longer and increasing conduction losses.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 220 points

    Hi Tim,

    Thanks. 

    After some more investigation, I'm seeing signs that the VCC pin was shorted to ground. Do you know if there is any protection circuit that would save the VCC regulator from being damaged with its output shorted?

    Regards,

    Matt

  • 0 in reply to Matt Bruensteiner67
    TI__Guru 59937 points

    Hi Matt,

    There is short-circuit protection on VCC (see EC table spec in the datasheet). I recommend changing the boot diode as recommended as the existing MBRS1100 has a large reverse leakage current, particularly at hot.

    Regards,

    Tim