LM2576 ADJ buck converter

[ejazbeyond]

hello, i m trying to power my sim800 from a 12v dc supply, as sim800 work in 3.4-4.4v. i m using LM2576 ADJ to step down 12v to 4.2v. i have calculated value of resistor using a online calculator https://www.changpuak.ch/electronics/FeedbackCalculator.php , R1=8.2k & R2=3.3k. when i am using it on hardware its giving 5.16v instead of 4.2v. i have not connected any capacitor or diode as i just want to check voltage. Please suggest any simulation or provide proteus library for lm2576 as i am unable to find it.

 

[ChrisP58]

I suggest using TI's WEBENCH® Designer on this page:
https://www.ti.com/sitesearch/docs/...rchTerm=lm2576#q=lm2576&t=everything&linkId=1

It will give you recommended values for all components for the design.

Be aware, the LM2576 is a switching converter, NOT a linear regulator. It works by turning the output on an off rapidly. To properly work, it needs an inductor, a freewheeling diode and an output capacitor.

 

[ejazbeyond]

I suggest using TI's WEBENCH® Designer on this page:
https://www.ti.com/sitesearch/docs/...rchTerm=lm2576#q=lm2576&t=everything&linkId=1

It will give you recommended values for all components for the design.

Be aware, the LM2576 is a switching converter, NOT a linear regulator. It works by turning the output on an off rapidly. To properly work, it needs an inductor, a freewheeling diode and an output capacitor.

Thanks Chris

 

[crutschow]

i have not connected any capacitor or diode

It likely won't work properly without all the components connected.
Why would you think it would?

 

[ejazbeyond]

It likely won't work properly without all the components connected.
Why would you think it would?

because dont have all the component, also i think other components are for smoothing purpose. my bad.

 

[ejazbeyond]

Thanks Chris

to change voltage i just need to change resistor value am i correct?
my existing circuit give me 4.48v now i need 4.18-4.2v max.

 

[ChrisP58]

Yes. The output voltage is set by the ratio of the two feedback resistors, factoring in the internal reference voltage. See the datasheet for the exact equation, or use TI's WEBENCH® Designer.

But you must have ALL of the components needed for the complete circuit.

 

[eTech]

Hello,

This is a LTspice sim. The component values were taken from a Winbench design, 12V in, 4.2v@3A out

eT

 

[dr pepper]

The sim circuit has a load resistor, dd you have a load resistor on the practical circuit, if not thats probably why its on the high side.

 

[ejazbeyond]

thanx for your inputs. with help of webbench tool i got the values of all component. tested on hardware working fine.

 

[hesam_m]

You can try to build this