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TLV1822-Q1: Should I consider open drain maximum Low output of 250mV when setting a hysteresis?

Tony Zhang
Prodigy 165 points
Part Number: TLV1822-Q1
Other Parts Discussed in Thread: TLV1822

Dear Friend

When I tried setting a narrow hysteresis (400mv) with TLV1822, I wondered if a non-zero Low output of TLV1822 should be countable, the difference seems quite big.

I have Vcc = 3.3V, Vth = 1.5V,  I want a 400mV hysteresis which means Vthh = 1.7V, Vthl = 1.3V.  Assuming R3 is a pull-up resistor.

From the equations given by the datasheet, I got R1 = 0.1333 * R2,  R3 = 0.2 * R2, if I let output Low equal to zero.

While I will get R1 = 0.16 * R2,  R3 = 0.44 * R2,  if I count 0.25V as the device output low.   The two results showed a quite big difference.

Considering the datasheet said: "the pull-up resistor be at least 10 times less than the feedback resistor value", neither result will allow me to choose R3 like that...where should I go? What is the correct selection right now?

is there something wrong with my calculation?  Or is it because 400mV hysteresis is too narrow with 250mV low output counted?

Anything enlightening me will be greatly appreciated.

Regards 

Tony

  • 0
    TI__Genius 11701 points

    Hi Tony,

    Thanks for reaching out. What is your pull up voltage? Please keep in mind that the 250mV vol max is only when you are sinking 4 mA; so assuming VPU is 3.3V, then youd only get the vol max of 250 mV when the pull up resistance is roughly (3.3V/4mA) ~800 ohm. I would use the graphs in Figure 7-3 to 7-9 to better estimate what your typical VOL would be (at a chosen pull up resistance value) and using those values to calculate the hysteresis thresholds. So starting with a select pull up resistor and working from there to find the feedback and input resistor values.

  • +1
    Prodigy 165 points

    Hi Chi

    Thank you for your reply.

    I didn't notice Figure 7-3 to 7-9 giving the detailed low output level that approaching zero when the sinking current is less than 100uA.

    My pull-up resistor is R3 and pull-up voltage is 3.3V. I have recalculated based on 500mV hysteresis and got R1/R2 = 0.16, R3/R2 = 0.17.

    if I choose R3 = 34K, then R2 = 200K, R1 =33.2K.  I think that should be ok even though R3 is not 10 times less than R2.

    Thanks a lot.

    Tony