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Topic: SIM900 - LM2596 Step-down 5V>4V adjustable output voltage (Read 1 time) previous topic - next topic


Nov 21, 2013, 09:10 pm Last Edit: Nov 23, 2013, 01:16 pm by karlok Reason: 1

for some IC I need an input voltage 4V. It has TTL 3V3 Levels so I have a 5V>3V3 regulator and powering this with some 5V constant voltage power supply. Then it will transform 5V to 3v3 to supply ATmega328P (on a breadboard) and this will communicate with the module using 3v3 TTL I/O.

SIM900A-module.JPG > http://www.ebay.com/itm/281194207422

LM2596 : How to adjust the output voltage? It says:
Input voltage: 4V-35V
Output voltage: 1.23V-30V

and is
Switching frequency: 150KHz
somehow concerning this ?


Thank you
using Arduino Uno Rev 3


Nov 22, 2013, 12:59 am Last Edit: Nov 22, 2013, 01:07 am by DVDdoug Reason: 1
On the bottom of page 12 of the datasheet it gives the formula:

Vout = 1.23 x  ( 1+ (r2/R1) )

It also referes to figure 20 on page 9.

It might be a good idea to print-out the datasheet. ;)   I usually do that for all but the simplest parts, and even for simple parts I usually print-out the "important" pages like the pinout diagram.


hello and thanks for the formula including the resistor values r1  and r2.
Unfortunately I do not know whether the ebay module linked
is there any possibility to adjust the output voltage because it also says "adjustable output voltage" ?
using Arduino Uno Rev 3


There is a potentiometer (the blue "thing") on the board that lets you adjust the output voltage. Take a good look at the datasheet of the LM2596, I don't think it's possible to get 4V from an LM2596 with an input voltage of 5V. You will need to find a regulator with a smaller drop out voltage.


Do you mean it   is not even possible because the voltages 5v (input)  and 4v (output) are too close together?
So my input voltage is 5v. Are there buck converters (step down) from 5v to 4v. I'd prefer some fixed ouput regulator, however think 4v is not that frequently used...
using Arduino Uno Rev 3

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