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


von Max A. (max_mma)


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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
Thank you for answers
(You can answer also in german if you want)

Cheers Max

: Moved by Moderator
von (prx) A. K. (prx)


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

von Dieter (Guest)


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You should look for two baords with LTC3611.

von Max A. (max_mma)


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

von MaWin (Guest)


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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

von (prx) A. K. (prx)


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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
von hinz (Guest)


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Use servos designed for up to 8.4V.

von Max A. (max_mma)


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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.

von MaWin (Guest)


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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.

von Max A. (max_mma)


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Sorry I don't know what you mean

von stefanus (Guest)


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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.

von Max A. (max_mma)


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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

von stefanus (Guest)


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

No, as I wrote.

von K.S. (Guest)


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You might want to try a buck-boost converter.

there are modules available, like this one: 
Ebay-Artikel Nr. 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.

von Dieter (Guest)


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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.

von Max A. (max_mma)


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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?

von Dieter (Guest)


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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.

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