# L4978 voltage regulator feedback resistor values

I am trying to use a L4978 to create a 6 V regulator. I, however, am unable to find how they calculate the resistor divider in Figure 4 to get a given output voltage. Does anyone know how they are calculating this? The chip will be happy when the feedback voltage on pin 8 is 3.3 V. So for a 3.3 V output set the upper resistor, R3, to zero. Any other voltage can be obtained by simple voltage divider using R3 and R4.

The sample circuit uses a constant 4.7k for R4 and at 3.3 V there will be 0.7 mA flowing through it. For 6 V you will want the output 2.7 V higher than 3.3 V and since we know the current we can calculate R3.

R4's value will be mid-range. Too low and it will waste current. Too high and it may be loaded by pin 8 or susceptible to noise.

We can work out the formula. The relationship between $V_F$, the feedback voltage on pin 8, and $V_O$, the output voltage is given by

$$\frac {V_F}{V_O} = \frac {R4}{R3+R4}$$

Rearranging we get

$$V_O R4 = V_F(R3+R4)$$

$$(V_O - V_F) R4 = V_F R3$$

$$R3 = \frac {(V_O - V_F) R4}{V_F}$$

Sample test for 12 V out:

$$R3 = \frac {(12 - 3.3) 4k7}{3.3} = \frac {40k9}{3.3} = 12k4$$

The datasheet recommends the nearest E12 value of 12 kΩ so our maths looks right.

We could work out the error with this rounding from

$$V_O = \frac {R3 + R4}{R4}V_F = \frac {12k + 4k7}{4k7} 3.3 = 11.7 \, \mathrm V$$

You can keep on going with this by examining the conditions with max and min combinations of resistor tolerance and $V_F$ for the chip but most of us would be happy enough with 11.7 V.

• I don't believe this answers my question, or perhaps I worded it incorrectly. But a resistor divider requires 3 variables in order to calculate the resistors values, which three variables do I use to calculate R3 and R4? Or in this case just R3, as R4 seems constant. I'm looking to generate 6v.
– Alex
Sep 25, 2016 at 22:25
• Any better now? Sep 25, 2016 at 22:34
• ah so if im understanding correctly, we just want to add voltage to the 3.3v to get to the desired output. such that the voltage source in a way is 3.3, R3 = n, R4 = 4.7, and output will be the difference between desired and 3.3?
– Alex
Sep 25, 2016 at 22:42
• Not sure exactly what you mean, but given those numbers, the output voltage will be ((3.3/4.7)*n)+3.3) Where n is in kOhms Sep 25, 2016 at 23:13
• @Alex. Answer expanded further. Let me know if anything is unclear. Thanks for accepting the answer. Sep 26, 2016 at 6:44