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TPS3803: RESETn pin with capacitor

Shota Mago
Expert 8110 points
Part Number: TPS3803

Hi team,

My customer has been using TPS3803-01 and he considers to add 1nF capacitor(C802) on RESETn pin as shown in this schematic(TI internal link).

 When RESETn pin is pulled Low, discharge current will flow into RESETn pin from C802 and other stray capacitance.

Is current limit resistor(greater than 660Ω at 3.3V supply) needed to limit transient current less than 5mA rating?

(Is current limit resistor needed even if without C802 and only stray capacitance of MCU side?)

Best regards,

Shota Mago

  • 0
    TI__Guru 56345 points

    Hi Mago-san,

    No, there is no need for a current limit resistor with or without C802.  Base on the capacitor current equation [i = C * (dv/dt)], the min amount of time to meet the 5mA rating is 660nS.  If needed, the current limit resistor location is shown below:

    Ben

  • 0 in reply to Ben C
    TI__Expert 8110 points

    Hi Ben-san,

    Thanks for your quick reply.

    The customer doesn't want to add current limit resistor and it's helpful that it's not needed even if with C802.

    But let me confirm why current limit resistor is not needed more detailed.

     

    According to Fig6, ON resistance of RESETn seems around 40mΩ(25C) and at start of discharging, peak current will be 3.3V/40mΩ=82.5mA from VCC/R*e^-t/(R*C).

    I'm not sure how to limit discharge time and current less than 5mA w/o additional resistor.

    If there is misunderstanding, please let me know.

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Guru 56345 points

    Hi Mago-san,

    Figure 6 shows the resistance at a given current and results in a voltage.  The figure is more of a DC result. 

    In your situation, the current is a transient event where the discharging of a capacitor is time based.  Over time, the both the voltage and current will go lower as the capacitor discharges. 

    To answer your question, to limit the current, you can either reduce the capacitance or reduce the voltage on RESETn.  Is there a reason why the customer wants to add a capacitor to RESETn?

    Ben  

  • 0 in reply to Ben C
    TI__Expert 8110 points

    Hi Ben-san,

    Thanks for your reply.

    I understand the peak current is instant transient value, but I would like to check if it's okay to exceed 5mA since it's stated as Absolute Maximum Rating just in case.

    The capacitor on RESETn pin might be considered for decoupling noise.

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Guru 56345 points

    Hi Mago-san,

    Can you obtain a scope photo showing the RESETn signal?  Obtaining the scope photo will give us idea of how long the discharge time it takes for the 1nF capacitor to go from 3.3V to 0V.  The transient current depends on how fast the open-drain device turns on and discharges the capacitor.

    The 5mA ABS Max rating is for DC current.  Just a note, excessive current through a transistor for a period of time will damage the device.  It is the heat that is generated that damages the transistor.

    Ben 

  • 0 in reply to Ben C
    TI__Expert 8110 points

    Hi Ben-san, 

    Your explanation of "The transient current depends on how fast the open-drain device turns on and discharges the capacitor." make sense to me.

    Thanks for your kind support for this thread.

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Guru 56345 points

    Hi Mago-san,

    If I have answered your question to your satisfaction, please close the thread.  Thanks!

    Ben