How to calculate these resistor values?

Discussion in 'The Projects Forum' started by ChrisChemist116, Feb 13, 2011.

  1. ChrisChemist116

    Thread Starter Active Member

    Mar 13, 2009

    I've been reading through TL431 datasheet, but i find a little bit confusing Figure 20. Could anyone give me a hand on this?

    I would like to know how to calculate the values for both resistors used along the transistor to be used with high current shunt regulator.:confused:
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    The resistor in parallel with the transistor base should be chosen so that the sum of the minimum TL431 operating current plus the maximum collector-base leakage current gives a voltage less than the VBE for the minimum collector current required. Probably in the range of a few hundred ohms, if the IC needs 1mA minimum. You need to be sure of how hot the transistor will get though, as VBE reduces with temperature, and leakage increases.

    The input series resistor RS (horizontal in your picture) is chosen by considering the range of input voltage VINmin to VINmax (must always be greater than the output), the output voltage VOUT, the required load current ILOAD, and the minimum current required by the TL431/transistor combination.

    For the resistance value, RS = (VINmin-VOUT)/(ILOADmax+ITL431)

    For the resistor power rating P, you need to take into account the maximum input voltage P = ((VINmax-VOUT)^2)/RS
  3. ChrisChemist116

    Thread Starter Active Member

    Mar 13, 2009
    Alright. It seems to me somewhat clear. I still find a little bit bogus how to calculate the parallel resistor with the transistor base.
    I know maybe i'm asking too much, but i usually get lost with words when they refer to calculations. Equations and examples are okay.
    If would be possible,
    how to calculate the values for the resistors to be used with MJE2955?. :confused: