I am trying to implement a switching voltage regulator from 7.2V to 5V with LM2596 by HTC Korea. Since I don't need an adjustable regulation, I chose typical application circuit from the datasheet which is for fixed output voltage (seen below).
I use 1N5817 (20V, 1A, for testing only) Schottky diode for D1, and different values for L1 including 27uH and 33uH. I also change Cout to 100uF, 270uF, 330uF, 470uF and 1000uF (16V and 25V). No matter what I did I couldn't change output voltage. It's always around 1.25 volts, the value does not change if I remove D1, L1 or both. Changing L1 or Cout also does not affect output voltage. If I remove Cout and measure output voltage without a load, it's around 4V.
What am I doing wrong? Is there anything that I am missing here? I also attach the pictures of my test configuration on breadboard below.
Answer
The fact you always get 1.25V sounds suspicious to me. This is often the Vref voltage for adjustable regulators. Checking the datasheet for the LM2596 says it's 1.25V also.
Are you sure you don't have the adjustable version?
One way of finding out easily is to set up your circuit as if it was and see if it works. Add the two resistors in the diagram below - R1 should be 1k and R2 3k for 5V.
The feedforward capacitor across R2 (Cff) shouldn't be necessary with this setup, but you can check TI's better datasheet for info on selecting one for situation when it is necessary (over 10V out or extra low ESR on output)
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