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[FAQ] TPS3840: PFET based disconnect on overvoltage

Part Number: TPS3840

In case of an overvoltage situation, how can the downstream power tree be disconnected to protect it from such overvoltage? Can reset ICs not only sense the overvoltage but also drive a PFET switch to turn it off? 

  • Hi Ajay,

    Yes, a reset IC or a supervisor can be used to control the gate of a PFET to disconnect the output from the downstream power tree.  Depending on the speed requirements, a supervisor with a FET driver may be needed to drive the PFET harder for a faster turn off/on transient response.

    An overvoltage supervisor with an active-high output can be used to turn off the PFET once an overvoltage fault occurs.  When the overvoltage supervisor is not detecting a fault condition, the output will be low turning the PFET ON.  Once an overvoltage event is detected, the output of the supervisor will go high shutting the PFET off and disconnecting the monitored rail voltage from the downstream power tree.

    If a particular overvoltage supervisor is not available, setting up an undervoltage supervisor with an active-low output can be used to detect an overvoltage fault condition.  Assume a monitored voltage rail of 3.3V and an OV fault condition set at 3.6V.  If we select an undervoltage supervisor with a voltage threshold around (VIT- + VHYS) = ~3.6V, then we can "detect" an overvoltage event.  How?  When the monitored rail voltage is below the threshold voltage of 3.6V, the output of the undervoltage supervisor is low turning on the PFET and allowing the downstream power tree to be on.  If the monitored rail voltage goes above the 3.6V threshold, the output of the undervoltage supervisor goes high and effectively turning off the PFET and disconnecting the monitored voltage rail from the downstream power tree.   

    Ben

  • Thank you for explaining how an active high as well as active low supervisor can be used for this purpose. What supervisor and PFET specifications need to be reviewed to check if a FET driver is needed? What supervisor specifications need to be reviewed to check if it is right for the voltages monitored?

  • Thank you for explaining how an active high as well as active low supervisor can be used for this purpose. What supervisor and PFET specifications need to be reviewed to check if a FET driver is needed?

    For the supervisor, you would want to know the propagation time of when the supervisor senses a fault condition and when the output is asserted or deasserted.  The important aspect is how fast the PFET device can turn off.  A simple resistor pullup may work but may not be fast enough to turn off the PFET.   The PFET has input capacitance that affects the speed of turning on or off.  It is best practice to find a PFET device that has very low input capacitance for faster turning on or off.

    What supervisor specifications need to be reviewed to check if it is right for the voltages monitored?

    I would make sure that the monitored voltage does not exceed the abs max ratings shown in the datasheet.  I would check to make sure that the hysteresis plus the part's threshold voltage does not exceed the actual threshold voltage of the monitored rail.  Also, the correct output polarity will need to be checked to ensure proper operation of the PFET device.

    Ben