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LM5140-Q1: LM5140-Q1

PowerSystemsEngineer
Prodigy 90 points
Part Number: LM5140-Q1

I have got Troubles with the VCCX pin, pin 6 of LM5140-Q1. When I leave this pin Floating, the converter works. So far so good. In the datasheet it is written on page 3:

"6   VCCX   I   Optional input for an external bias supply. If VCCX > 4.5 V, VCCX is internally connected to VCC and the internal VCC regulator is disabled. If VCCX is unused, it must be grounded."

So if I do not want to use VCCX (an external 5V supply), then I have to connect the VCCX pin to Ground. I did so. After this Change, the converter did not switch anymore. After removing pin 6 (VCCX) from GND, it did work again.

Is this a flaw in the datasheet or am I doing something wrong? Help is greatly appreciated. Many thanks in advance.

Patrick

  • 0
    TI__Expert 8245 points

    Hi,

    Could you please leave your email address and I will forward your question to the dedicated engineer and copy you at the same time, thanks.

  • 0 in reply to Iven Su
    TI__Mastermind 49597 points

    Hi Patrick,

    Our general guidance is to tie the VCCX pin to GND if it is not used. The reason is that if the VCCX voltage floats up, it may exceed the changeover threshold and then the VCC sub-regulator is switched off, resulting in a loss of bias.

    No switching when you tie VCCX to GND is not expected behavior. One possibility is that the FETs have a very high gate voltage Miller plateau. Is the Rdson rated at Vgs = 4.5V? For further support, please forward the schematic and a completed quickstart file (downloadable from the product folder.

    Regards,

    Tim