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LP3878 difficulty in choosing CFF and Vin

Divya jyothi
Intellectual 810 points
Other Parts Discussed in Thread: LP3878-ADJ

Hi,

  • I am facing the difficulty with LP3878MR-ADJ/NOPB LDO regulator
  • We not found simulation circuit design in WEBENCH for the below requirement using the above Part

                 Vin =5V

                 Vout =1.8V

                 Iout =0.8A

  • But we are getting simulation circuit for the Vin= 3.3V & Vout=1.8V with the same part.
  • But as per the datasheet Vin > Vout +2V, hence Vin should be equal or greater than the 2+1.8=3.8.
  • Then How the WEBENCH is generated for  Vin =3.3 ?
  • why it is not generating circuit for Vin =5V ?
  • For the same circuit, from WEBENCH Cff= 1nF. If we are substituting this value in Zero frequency calculating formula, then Fz= 41.55khz which  is not in the Zero frequency range.

Please guide me how to proceed.

 

Thanks & regards,

Divya

  • 0
    Intellectual 515 points

    Hello Divya,

    For Vin=5V, Vout=1.8V and Iout=0.8A Webench Designer indicates that LP3878-ADJ is not selected because the device junction temperature is too high. This is due to device power dissipation in the package. Reducing Vin from 5V to 3.3V reduces the dissipation to an acceptable value.

          Pdiss = ( (Vin - Vout) x Iout)

          Junction Temperature = ( (Pdiss x Rθja) + Ta)

    For the Cff value, that would be an issue for the Webench modeling team to resolve with the component calculator.

    That said, the 1nF value shown with Rfbt = 3.83kΩ produces a Fz below the suggested 50 kHz minimum which may reduce load transient response, but will not cause oscillation.

  • 0 in reply to Daddio
    Intellectual 810 points
    Hi Daddio,

    Thank u for you quick reply.
    If i choose CFF= 330pF then FZ= 125KHZ.
    Can i choose this value ?
    What about the transient response for this cap ?
    can i get the information about zero frequency & plole frequency , how it is affecting to the stability of the regulator ?

    Thanks & regards,
    Divya
  • 0 in reply to Divya jyothi
    Intellectual 515 points

    Hello Divya,

    Yes, the 330pF value is good.

    Transient response will good.

    Unfortunately, the internal poles and zeros details are proprietary.

  • 0 in reply to Daddio
    Intellectual 810 points
    Thanks Daddio.
  • 0 in reply to Divya jyothi
    Intellectual 810 points
    Hi Daddio,

    In our board we are not having 3.3V supply input. if the input supply voltage to LP3878-ADJ is 5V then it will cause any problem ? only the heat dissipation is more right ? Or can you please suggest us to select the regulator which having below specification.
    Iout =1mA
    Vin=5V
    Vout =1.8V

    Thanks & regards,
    Divya
  • 0 in reply to Divya jyothi
    Intellectual 515 points

    Hello Divya,

    You are correct that increasing LP3878-ADJ Vin from 3.3V to 5.0V will increase heat dissipation (from 1.5mW to 3.2mW, with a 1mA load current).

    The 5V is well within the LP3878-ADJ Operating Vin range.

    From your description, junction temperature rise will be less than 1 degree C and there should not be any problem.

    .