Electronic – Adjustable Voltage Regulator Resistor Values

resistorsvoltage-regulator

I'm working with an adjustable voltage regulator and am having a hard time understanding my equation for determining resistor values.

Datasheet: http://www.chinaeds.com/zl/20103612559506780_AS2850YU-X,AS2850YT-X,AS2850AUPDF.pdf

Figure 2

I'm able to get ~5V (my goal) on Vout when using the following values:

  • Vin = 12V
  • R1 = 22K
  • R2 = 1.47K (1K and 470 in series)

As soon as I put a load on Vout it drops to ~3.7V

What am I doing wrong?

Edit: turns out I had multiple regulators that were bad. Once I found a good one everything works as expected (with the proper resistance values).

Best Answer

The current \$I_{ADJ}\$ is very small and constant. Therefore its contribution to the overall output voltage is very small and can generally be ignored.

\$I_{ADJ}\$ is 40-80\$\mu\$A.

So how are you getting 5V? Calc gives:

\$ V_{OUT} = V_{REF} \times \left({1 + \frac {R_2} {R_1}} \right) = 1.25V \times \left({1 + \frac {1.47k\Omega} {22k\Omega}} \right) = 1.333V \$

I'd replace \$R_1\$ with 120\$\Omega\$. Standard value from LM337 calcs.

\$ R_{2} = \left( {\frac {V_{OUT}} {V_{REF}} - 1} \right) \times {R_1} = \left( {\frac {5V} {1.25V} - 1} \right) \times {120\Omega} = 360\Omega\$

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