In my earlier post, I was using LM317 for SIM900 but due to some voltage drop problem, I decided to go with LM2576. I am trying to step down the voltage from 12v to 4.5v using LM2576. I have studied the voltage calculation formula described on page 21. For the time being, I am just testing it on breadboard so have not connected the capacitors and diode at the output. Following is the schematic
According to the voltage calculation formula:
O/P Volts = Vref(1 + R2/R1)
O/P Volts = 1.23(1 + 4700/2000)
O/P Volts = 1.23(1 + 2.35)
O/P Volts = 4.12v
So the calculation shows output voltage of 4.12v but in practical it is showing 5.64v.
Is my calculation wrong or I am doing something wrong in the circuit. Please help.?
Best Answer
That's just plain ignorance. All bets are off if you don't have the flyback diode because it IS an integral part of the energy reclamation into the output capacitor (also not fitted) from the inductor.
The diode also prevents (as a side issue) back emfs from the inductor harming the chip.
The other issue is that without the output capacitor the voltage across the potential divider will be a somewhat distorted square wave and not representative of anything when measured with a multimeter.
It's like trying to drive a car with the tyres removed.