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LMP8646: leakage current

Frederic LE DAULT
Prodigy 70 points
Part Number: LMP8646
Other Parts Discussed in Thread: LMZM23601

Hi TI Community,

To the componant TI LMP8646, can you give us the input leakage current of the output pin Vout ?

Cheers,

Frédéric

  • 0
    TI__Mastermind 27500 points

    Hi Frederic,

    Normally leakage is not specified for low impedance output pins. What is the reason behind your request for such information?

    Regards, Guang

  • 0 in reply to Guang Zhou
    Prodigy 70 points

    Hi Guang

    We need this information to calculate the power supply accuracy. We use the LMP8646 to limit the output current of the lmzm23601.

    To calculate the output current, we need the input current (Ifb) of the feedback pin (lmzm23601) and the leakage current of the lmp8646 when no limitation.

    Regards, Frédéric

  • 0 in reply to Frederic LE DAULT
    TI__Mastermind 27500 points

    Hi Frederic,

    When LMP8646 is active, its output is similar to a voltage source, ie it will source/sink necessary current within its capability to maintain its output voltage level. If you use LMP8646 output to drive FB pin of the DCDC, it will supply required leakage current of the FB pin.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Prodigy 70 points

    Hi Guang,

    I write a mistake. "To calculate the output current, we need the input current (Ifb) of the feedback pin (lmzm23601) and the leakage current of the lmp8646 when no limitation." => "To calculate the output voltage accuracy, we need the input current (Ifb) of the feedback pin (lmzm23601) and the leakage current of the lmp8646 when no limitation."

    We need to define all voltage accuracy.

    Regards, Frédéric

  • 0 in reply to Frederic LE DAULT
    TI__Mastermind 27500 points

    Hi Frederic,

    Leakage is not a valid definition for output pin.

    If you post a schematic, I’ll be happy to provide additional comments regarding your concerns.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Prodigy 70 points

    Hi Guang,

    Below you can find my schematic .

    Regards, Frédéric

  • 0 in reply to Frederic LE DAULT
    TI__Mastermind 27500 points

    Hi Frederic,

    You can simply regard LMP8646 output node as a voltage source, together with Vout (of DCDC), it provides necessary current to maintain the voltage at node FB. The current provided by LMP8646 is NOT leakage current. And this current is determined by the circuit itself, not LMP8646.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Prodigy 70 points

    Hi Guang,

    Our aim is to define the output voltage accuracy.

    To obtain it, we need to know all component imprecision.

    In this case, the output voltage accuracy is function of LMZM23601 IFB (Input current from FB to GND) and the input current of the output pin of the LMP8646 when no current limitation.    

    The input current of the output pin of the LMP8646 correspond of the output diode leakage current.

    Regards, Frédéric

  • 0 in reply to Frederic LE DAULT
    TI__Mastermind 27500 points

    Hi Frederic,

    Maybe you can show me a test setup schematic how you would measure the leakage current on an output pin?

    Regards, Guang

  • 0 in reply to Guang Zhou
    Prodigy 70 points

    Hi Guang,

    It's not necessary to use a test setup schematic. If you have internal output diode characteristic, you can give us the leakage current.

    Regards, Frédéric

  • 0 in reply to Frederic LE DAULT
    Guru 140200 points

    Bon jour Frédéric,

    I don't know the leakage current of the output when the output OPamp of LMP8646 is out of regulation either. But I would assume a standard diode's behaviour like the 1N4151 or so. This means a leakage current of up to 50nA at room temperature and up to 1µA at elevated temperatures (80°). So, I would decrease Rbb and Rtx in your schematic as much as possible (as much as the drive capability of the LMP8646 allows). This minimizes the error due to the voltage drop of the leakage current across the feedback resistors. And don't forget the leakage current of the FB pin of the switcher...

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 27500 points

    Hi Frederic,

    Under the rare situation where LMP8646 output pin becomes high impedance due to some reason, of course there will be certain amount of leakage. But this leakage is not characterized as it is not a normal device working mode. I don’t have the characteristics of the internal diode or the opamp input, both of which will contribute to leakage among others such as ESD cells.

    The next best is to measure a few devices under the exact condition that you care about. That’s one of the reasons why I asked for the test setup. Of course you could easily measure it yourself.   

    Regards, Guang