# How to calculate the feedback resistors for an SMPS

I have designed an SMPS circuit for my host board. As the datasheet, the feedback resistors are calculated as

As per the datasheet, Feedback Voltage(Vref) is 0.6V and I want 4V Output Voltage. Therefore, feedback resistors are chosen like below after the calculation.

R15 = 5.6K

R17 = 1K

I measured the output voltage from the board; it is 1.2V.

I also measured the feedback voltage and it is 0.6V, same as in the datasheet. So, what can be the problem? Am I doing something wrong? Calculations were done with an oscilloscope.

(I can not add the datasheet link as I don't have enough reputation. The SMPS regulator is L7986 from STmicroelectronics.)

Update1: I changed feedback resistors to get the 4V output by changing the resistors ratio. Measured output was 1.2V with initial resistors. So I changed the resistor rate by multiplying it with 2.3 (4/1.2V) to get 4V output. After this change, the output voltage is about 7.8V. I had thought that I can get the desired output voltage if I modify the ratio of feedback resistors. So, any thoughts?

Update2: I guess I broke the regulator while trying to measure the output voltage with an oscilloscope. I don't know what caused this, but now the regulator outputs the input voltage directly to the output pin. I was very careful with the connections, I just connected it to voltage supply as the same before and measured voltages from the scope.

• What does "I also measured the feedback voltage and it is true." mean ? Do you see 0.6V or do you something else ? If so, what ? – efox29 Mar 23 '16 at 15:42
• But when I tried to measure the output voltage from board, It was calculated as 1.2V That is a confusing sentence. Do you mean: I measured the output voltage, it is 1.2 V ? – Bimpelrekkie Mar 23 '16 at 15:46
• 1) The feedback resistor values look OK to me. 2) It would help if you linked to a datasheet of the switcher IC you're using. 3) just because the output voltage is incorrect, do not assume that the feedback circuit is wrong. There can be plenty of other reasons why this circuit is not behaving as expected. – Bimpelrekkie Mar 23 '16 at 15:50
• @FakeMoustache ,efox29 Thanks for answers. Yes, I measured the output voltage from board, it is 1.2V and I also measured the feedback voltage from board, it is 0.6V. Here is the datasheet for IC – abdullah cinar Mar 23 '16 at 16:48
• I double checked the board for short circuits, there is no problem with that. I'm new with regulators and power circuits, I wanted to ask if there is another problem that I couldn't figure out. – abdullah cinar Mar 23 '16 at 16:50