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LSF0108-Q1: Pull up resistor

Part Number: LSF0108-Q1

I have two questions about pull up resistor.

1. Can we change resistor value of VB pull up resistor that is 200k?

If we can, how can we calculate that value?

If not, why cannot we change that value?

2. As app note mentinoed at down translation, we can remove pull up resistor connected with VA when leakage current is less than 1mA. Why is criterion of leakage current 1mA? Is there any criterion?

  • 1. The resistor limits the current that flows into Vref_B, through the reference transistor, and out of Vref_A, and allows the voltage at Vref_B to be at Vref_A plus the gate threshold voltage:

    If the resistor is too small, the current flowing into Vref_A might be too large (power supplies usually are not able to sink current).

    If the resistor is too large, not enough current might flow for the gate voltage to become large enough to open the channel.

    Also see Understanding the Bias Circuit for the LSF Family.

    2. Section 5.2 of Voltage-Level Translation With the LSF Family says that you should add the pull-up when "there are signal integrity problems". So the value of 1 mA sounds more like a rule of thumb, and you always have to measure whether it actually works.