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[LTC4417] [Internal Structure] How internal hysteresis circuit inference on UV and OV function?


I would like to know how internal hysteresis circuit inference on UV and OV function? 

Because there's the description says "Now set the UV hysteresis value using R3", 

but I don't really understand how this work internally. 

  • When input is rising (from say 0V) and is near the OV rising threshold, the OV hysteresis current is off (by the way, UV hysteresis current is also off).  So input OV rising threshold is simply:

    OVR = 1V * (R1 + R2 + R3) / R1


    As input goes above OVR threshold, the OV hysteresis current turns on (UV hysteresis current is still off). Now when input starts falling and reaches the OV falling threshold, the OV pin is at 1V (by definition because that is when comparator trips). So current in R1 resistor is 1V / R1. But current flowing in R2 and R3 resistors is (1V / R1) – Iovhys (using current summation at OV pin). Therefore, voltage drop across R2 and R3 resistors is (1V/R1 – Iovhys) * (R2 + R3). Add this to 1V OV pin voltage to get the total Vin voltage at which OV falling threshold occurs:

    OVF = 1V + (1V / R1 – Iovhys) * (R2 + R3) = 1V * (R1 + R2 + R3) / R1 – Iovhys * (R2 + R3)


    Do OVR – OVF to get OV hysteresis = Iovhys * (R2 + R3).


    (where Iovhys = Iuvhys = Ihys/8)


    The reason is that the OV hysteresis current subtracts from the normal current that flows through R2 and R3 while the current through R1 is always 1V / R1 at both rising and falling thresholds. Similarly, the UV hysteresis current adds to the current flowing in R3 while current in R2 and R3 is always 1V / (R2 + R3) whether at rising or falling threshold.

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