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LM25019: LM25019 under voltage hysteresis value

user1312478
Prodigy 60 points

Part Number: LM25019

hello,

i have a question for the LM25019 under voltage hysteresis value, see below description that from datasheet chapter 7.3.9 (Undervoltage Detector)

"UVLO hysteresis is accomplished with an internal 20-μA current source that is switched on or off into the impedance of the set-point divider. When the UVLO threshold is exceeded, the current source is activated to
quickly raise the voltage at the UVLO pin. The hysteresis is equal to the value of this current times the resistance RUV2."

My question is what is the means of "hysteresis", how to use this "hysteresis value" for calculation? Thanks!

  • 0
    TI__Mastermind 40410 points
    Hello,

    Hysteresis enables the designer to set the turn on threshold voltage (at the input) and the turn off threshold voltage (at the input) at different levels, the turn on threshold obviously being higher than the turn off threshold voltage.
    Additionally hysteresis also prevents the possibility of "chatter", by chatter, I mean the device would randomly turn on and off should the input voltage level be at exactly the turn on/turn off threshold where hysteresis is not present.

    The link below provide more detail on this subject.

    www.ti.com/.../tidu020a.pdf


    Hope this helps?

    David.
  • 0 in reply to David Baba
    Prodigy 60 points

    hello David,

    Thanks for you feedback.

    For LM25019, the UV Threshold range 1.19(min) 1.225(typ) 1.26(max), the UV Hysteresis Input Current  –10(min) –20(typ) –29μA(max).

    chapter 8.2.2.8 calculation

    VIN(HYS) = IHYS x RUV2

    VIN (UVLO,rising) = 1.225V x (RUV2/RUV1 + 1)

    So VIN (UVLO,rising) is turn off threshold, VIN (UVLO,rising) + VIN(HYS) is turn on threshold, my understanding is correct or not?

    the IHYS (hysteresis input current) is negative value, i  think the current from ULV pin to RUV1 (simulation verify my understanding), why VIN(HYS) calculation use IHYS times RUV2 not RUV1?

    Thanks!

  • 0 in reply to user1312478
    TI__Mastermind 40410 points

    Hello, 

    Vin (Hyst, Rising) is the turn on (not turn off) threshold measured at Vin.

    Ihyst *RUV2 is the voltage hysteresis measure at Vin.  The Ihyst *Ruv1 would be the hysteresis voltage measured at the UVLO pin, however this is gained up by the top UVLO resistor, hence the equation.  In effect we reference this thresholds at Vin, not to the UVLO FB divider point.

    hope this helps?

    David.