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Original question:

TUSB8041: GRSTz with just putting capacitance case

TUSB8041: GRSTz Pin Capacitance

Darren (FAE)
Expert 5483 points
Part Number: TUSB8041

I have a customer considering the TUSB8041 for one of their designs. They want to know what the GRSTz pin capacitance is, to help decide the value of the external cap (Like Figure 11 in the DS, they plan on not having an MCU/Processor control the pin, and just connect it to a capacitor for the RC time constant)

Also, can I get the value of the internal GRSTz pullup resistor to VDD33?

Thanks & Regards.

Darren

  • 0
    TI__Guru 60865 points

    Hi Darren,

    The pullup resistor value is maximum of 250 uA (~13.2K), nominal is 150 uA (22K).  The optimal external capacitor value will vary depending on power supply ramp timing in the system.  If they are unsure of the ramp timing, 2 uF should provide margin for typical use cases.

    Regards,

    JMMN

  • 0 in reply to JMMN
    TI__Expert 5483 points

    Hi JMMN, 

    Thanks for the info. Could you tell me the pin capacitance as well...? (I am assuming the pF range, but just want confirmation for the customer)

    Regards,

    Darren

  • 0 in reply to Darren (FAE)
    TI__Guru 60865 points

    Correct, pin capacitance is in the pF range and won't have a noticeable impact on GRSTz timing.  Please make sure if the customer is using the passive reset circuit, the 1.1V ramps before or very close to the 3.3V ramp.  Since the internal pullup is connected to 3.3V if the 3.3V ramps well before 1.1V, the pullup is enabled before the device is completely powered preventing a proper reset.

    Regards,

    JMMN