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Original question:

LM5109B: Output impedance

LM5109B: Output impedance confirmation

Timur Haliulin
Intellectual 390 points
Part Number: LM5109B

Hello,

Following this thread:

https://e2e.ti.com/support/power-management/f/196/t/807812?tisearch=e2e-sitesearch&keymatch=LM5109B

I was asking about the output resistance of the driver.

I went through it and have a additional question:

If the output current, for instance, is going to be 500mA, what resistance values going to be then? As datasheet states test conditions at 100mA

Thank you

  • 0
    TI__Mastermind 20950 points

    Hello Timur,

    I think you are referring to the post response below:

    "Thank you for the interest in the LM5109B. I am an AE with the High Power Drivers group and will address your question. The driver output resistance in the high state and low state can be determined by the VOLH, and VOHH for the HO output and the VOLL and VOHL for the LO output. These voltages are tested with a 100mA test condition, so the resistance is 10x the voltage specification. The parameters for the LO and HO are the same for each driver, with typical pull down resistance of 3.8 Ohms, maximum of 6.5 Ohms. The pullup resistance is 7.2 Ohms typical and 12 Ohms maximum"

    If the driver current is 500mA the resistance of the driver output stage will still apply. For example if the pull up resistance is at the maximum 12 Ohms, which will occur at high temperature, the voltage drop across the pull up FET can be the 0.5A x 12 Ohms or 6V.

    Confirm if this addresses your question, or you can post additional questions on this thread.

    Regards,

  • 0 in reply to Richard Herring
    Intellectual 390 points

    Hello Richard,

    So the resistance is constant at any output current and the changing bit is voltage across it?
    Is it correct?

  • 0 in reply to Timur Haliulin
    TI__Mastermind 20950 points

    Hello Timur,

    The resistance of the driver will determine the voltage drop across the driver up to the point of the driver peak current rating.

    When the driver reaches the peak current limitation, there can be the full bias supply voltage across the driver switch at the peak current limitation.

    For example, if you have a large cap on the driver output, the instant you turn on there is full voltage across the driver, but the current is limited to the driver peak current rating.

    Confirm if this answers your question, or you can post additional questions on this thread.

    Regards,