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LSF0204: Minimum current of LSF series / Maximum Pull-up resister

JaeKyung Shin
Prodigy 230 points
Part Number: LSF0204

Hi, Every one.

I have a question on LSF series.

By the datasheet, "External driver must be able sink current on both side"

And Example of the datasheet was 3mA, 10mA, 15mA...

But my external driver device's maximum sink current is 1.6 mA

According to the datasheet, I must select pull-up resister for 0.8mA.

I think 0.8mA is so low value from 3mA~15mA.

I wonder mimimum current of LSF series or maximum pull-up resister.

I can design 0.8mA, or 0.1 mA by using LSF series?

And How does this affect the speed of the signal?

Thank you.

  • 0
    TI__Guru* 96591 points
    Hi JaeKyung,
    There is no minimum current for this device, however you need to take speed into account.

    The output rising edge is controlled by the RC circuit formed on the output when it goes high-impedance. For example, if your output has a large capacitive load (50pF of capacitance) and you choose a resistor for 0.8mA of current, the resistor value would be (at 3.3V): R = 3.3 / 0.0008 = 4125 ohms.

    Given these two values, you can determine the charge time of the output with tr = 4*tau = 4*R*C = 825ns. Typically I want the rise time to less than or equal to 1/3 of the total pulse width, so we can get the full pulse width as: tw = 3*825ns = 2.475us, which means (for this example) the maximum data rate would be around 800 kbps.
  • 0 in reply to Emrys Maier
    Prodigy 230 points
    Thank you! It is very helpful to me.