# Forum: Analog Circuits Stepdown converter 7,2V to 5V

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Hi guys
i am searching for stepdown converter. I need 5V 12 A and I would like
to use an akku 7,2V 30A 6000mAh LI Ion. Its a hexapod with 18 servos
which needs 5V 12A
So i need to convert the voltage to 5V but I don't know which converter
I should use. I thought about using the LM2596S with 3A max. So i would
use 4 or 5 of them to provide my voltage.
I am not sure about my decision! Maybe some of you might know better
stepdowns or have completly other ideas. Maybe using 12 V Akku
(You can answer also in german if you want)

Cheers Max

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A 7.2V battery has ~6V minimum, so you'd better choose a step down,
having a correspondingly low dropout.

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You should look for two baords with LTC3611.

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Hi what do you mean? The LM2596S is a step down converter.

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a) 12A switching regulators with integrated MOSFETs are rare.
b) is 12A really the highest current that ever occurs, for instance if
something is stuck ?
c) as you have multiple servos, you can use multiple step down
converters, but you should not connect their outputs together (in
parallel) but each one supplies his set of servos.
d) if you use for instance 1 converter for 1 servo, the converter needs
to handle the current that is required when the servo is stuck, which
may easily be 3A. In the sum the converters will of course not draw
18*3/(5/12)A
e) if 5V/15A is really all you need https://www.pololu.com/product/2881

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Max A. wrote:
> Hi what do you mean? The LM2596S is a step down converter.

There's not much left to step down when the input is at 6V, the output
at 5V, and the saturation voltage is 1,5V.

: Edited by User

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Use servos designed for up to 8.4V.

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Thank you for your answer. I think this Pololu is a good idea. Did you
use it before? Do you think I need a cooling?
The problem  is that I use a PCA 9685 so I would have to use some
stepdowns in parallel. So I think this Pololu is the best.

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Max A. wrote:
> think this Pololu is a good idea

Because it requires 6.5V at least, and you supply is not 7.2V as you
told but 5V to 8.4V, it will not allow full use of the battery.

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Sorry I don't know what you mean

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Max A. wrote:
> Hi what do you mean? The LM2596S is a step down converter.

Yes, but it has too much drop-out voltage. The efficiency at the minimum
input voltage of 7V is about 70%. But your battery goes down to 6V or
even less.

I would use a linear low-drop regulator with a drop-out voltage of 600mV
or less. However I cannot recommend any LDO because all that I used are
for the range below 1A.

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Do you have a link for me. Cause that what I found can only handle with
1A but I need 12A. And I don't want to use so many

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Max A. wrote:
> Do you have a link for me.

No, as I wrote.

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You might want to try a buck-boost converter.

there are modules available, like this one:
https://www.ebay.de/itm/DC-DC-10A-Buck-Boost-Converter-Step-Up-Down-regulator-Module-for-LED-Driver-US/254169431168

some further information about it is available here:
https://www.eevblog.com/forum/projects/reverse-engineeringdeciphering-mystery-buck-boost-circuit/

You might want to replace the Mosfets, as they are not logic level but
only driven with 5V at the gate (at least in the one I got). The FETs
will begin to enter the linear mode (and heat up quickly) when the input
voltage drops slightly below 5V, so make sure this never happens. At 12V
input (this is what I tested) and a few amps this module runs fine.

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In case your 5V range is 4.9 to 5.2 you could simple built your own low
drop linear regulator with discrete transistors.

Two out of three mostly do not need all power regulated because PWM
supplied consumer can live without it.

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I think I will use 3 Li Ion Akkus in Serie for 10,8 V and convert them
with 2 buck converter to 5V. Each of them has to handle with 6Amps and
they are built to handle with 9Amps so I think this will work.
Also I won't have the problem with too low drop-down voltage.
Do you think this is a good Idea?

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Max A. wrote:
> Do you think this is a good Idea?

Better than using 2 cells. With 2 cells you need a SEPIC converter. They
are not so common and less choices.