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TLC7701 - SENSE threshold voltage setting

Hello all,

 

I have a question regarding how to set the SENSE threshold voltage on TLC7701, Micropower Supply Voltage Supervisor.

The datasheet states that the SENSE threshold voltage on TLC7701 can be customized with two external resistors, but I cannot find any formular for threshold coltage customization.

VDD on my circuit is 3.3V, and I would like to set the SENSE threshold voltage to around 1.24V.

Can anyone clarify the relation between the external resistor values and the threshold voltage?

 

Thank you for your help in advance.

 

Best regards,

EJ

  • Hi EJ,

    It actually means the threshold voltage can be set using an external divider, same way as many of our other supervisors. Below is the calculation:

    Vthreshold(typ) = Vit(typ) * (1+R1/R2)

    where Vthreshold is the voltage you want to detect, Vit is the datasheet specification (1.1V typical for TLC7701), R1 is the resistor to the voltage being detected, and R2 is the resistor to ground in the divider.

    To design for this, first set your R2 value. A general rule of thumb to avoid DC accuracy errors is that the current going through the divider network should be 100 times less than the current into the pin (specified as Ii(sense)). A quick calculation for this is R2 < Vit(min)/Ii(sense, max). For the TLC7701 your maximum R2 value, by this rule, is 5.2kOhm.

    For the TLC7701 I would set R2 = 1kOhm to make the next calculations simple:

    R1 = (Vthreshold(typ)/Vit(typ) - 1) * R2

    R1 = ~127.3 ohm => 127ohm as a 1% resistor.

    This is an extremely small resistor as your sense voltage is only 1.24V. If you set your sense threshold to 2.5V, R2 = 1.272 kOhm => 1.27kOhm as a 1% resistor. Another way to reduce the size of these resistors is to use a part with much lower leakage into the sense node, such as the TPS3808G01.

    Regards,

    David