Electronic – Resistor values for adjustable voltage regulator

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I'm using a LT1529 voltage regulator and I'm having a hard time understanding the process for determining the resistors to use in the voltage divider. The datasheet provides the following schematic and calculation:

LT1529 Adj Version Schematic

ADJ (Pin 2): Adjust Pin. For the LT1529 (adjustable
version) the ADJ pin is the input to the error amplifier. This
pin is internally clamped to 6V and – 0.6V (one VBE). This
pin has a bias current of 150nA which flows into the pin.
See Bias Current curve in the Typical Performance Characteristics.
The ADJ pin reference voltage is equal to 3.75V
referenced to ground.

I want an output voltage of 4.0V. The typical ADJ pin bias current (which I think is the desired current for the adj pin) is 150nA. I'm not sure if I should use these values to determine the R2 resistor. It gives a rather large resistance value (like 26.6M). Once I know R2, R1 should be easy to solve for but I would appreciate confirmation on that value too.

Best Answer

Turn the problem around: Once you know R1, then R2 is easy to solve for. R1 is pretty easy to decide upon.

It is a bit of a balancing act, really, but here's how it goes:

  • The datasheet suggests R1 be kept below 400 kOhms for stability. Lower the value of R1, higher the quiescent current required by the voltage splitter R1+R2. Higher the value of R1, higher the instability of the output.
  • We know that the upper leg of R1 in the diagram is biased at 3.75 Volts for steady state.
  • Hence, let us start with the maximum standard E12 series resistor value for R1 within the datasheet constraints, i.e. 390 KOhms
  • IR1 can be calculated thus: I = V / R = 3.75 / 390,000 = 9.61538 uA
  • Current through R2 is given as the sum of current through R1, and bias current 150 nA. IR2 is thus 9.61538 - 0.15 = 9.46538 nA
  • For a desired output voltage of 4.0 Volts, R2 must thus develop 4.0 - 3.75 = 0.25 Volts for the above current.
  • Therefore R2 = 0.25 / 9.46538e-6 = 26412 Ohms. Closest E12 value = 27 kOhms.
  • Vo with R1 = 390 k and R2 = 27 k is 4.01367 Volts, less than 0.5% deviation from target voltage (assuming perfect resistor values, of course).

If stability is more desirable than saving quiescent current, try the above sequence with a starting value of R1 as 22 kOhms.

  • IR1 = 170.455 uA
  • IR2 = 170.305 uA
  • R2 = 1468 Ohms, nearest E12 value 1.5 kOhms
  • Vo = 4.00591 Volts.

Using the above calculation steps, choose any value for R1 as long as it is less than 400 kOhms, to obtain the value of R2.

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