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UC3834: Compensation of LDO

Prashant Surana
Intellectual 840 points
Part Number: UC3834

Dear e2e members,

I am designing a linear regulator using UC3834 with PMOS pass device. I am going through this document for compensation.

www.ti.com/.../slyt194.pdf

Kindly let me know, What is output resistance of error amplifier in UC3834? page number 2 uses Roa as 300kOhm. What is this value for UC3834?

  • 0
    TI__Expert 4035 points
    Hi Prashant,
    It should have a range between (Vin-1.5V)/100mA.

    BR,
    Haidar
  • 0 in reply to Haidar Hamoo
    Intellectual 840 points

    Hi Haider

    So as per formula given by you (I dont know how it came), Vin should be 30001.5 V (almost 30kV) to get 300kOhm Resistance. I Think it is not practical to drop almost 30kV across LDO. please correct me if I am wrong.

  • 0 in reply to Prashant Surana
    TI__Expert 4035 points

    Hi Prashant ,

    My apology for confusing you with a wrong answer, the error amp output impedance is a function of load and frequency as stated in UC3834 datasheet page 4 and 5, here are a screen captures of how such impedance can be estimated:

     

    So by using the equation for the output impedance of pin 14 (error amp output impedance) on page 5 (first screen capture) and error amplifier gain @ 1 kHz from from page 4 (second screen capture approx. 50 dB) we get the impedance to be approx.  ROA = 700 x 10^(50dB/20dB) = 221.4 kOhms.  again

    BR,

    Haidar