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I'm looking for an adjustable or fixed 6V regulator, performing up to 6A peak current. The continuous consumption will be much smaller. It needs to be a LDO regulator, since I will be powering the regulator from a 2S LiPo batery (7.4V nominal). The drop-out would need to be about 1V or so.

Could I use a switching regulator for this kind of thing? I'm afraid of noise on the power rail that would be bad for the motors and would make it harder to control them. They will be used on a robot for robocup rescue B. I'm using a 9.7:1 Metal Gearmotor motor with a DRV8432 motor driver.

I will be driving 4 motors, so I'll probably get a separate regulator for each one.

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    \$\begingroup\$ Do you really need to add a 6v regulator? To quote from the specs of your motors (my emphasis) - "These motors are intended for use at 6 V. In general, these kinds of motors can run at voltages above and below this nominal voltage, so they should comfortably operate in the 3 – 9 V range, though they can begin rotating at voltages as low as 1 V." Sounds like they'll be quite happy with the 7.4V. \$\endgroup\$ – JIm Dearden Aug 13 '13 at 14:29
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    \$\begingroup\$ @JImDaerden: I think you could post that as an answer. \$\endgroup\$ – Johan.A Aug 13 '13 at 14:30
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    \$\begingroup\$ Sounds like an answer to me \$\endgroup\$ – Andy aka Aug 13 '13 at 14:37
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Let me start by saying that @JImDearden's answer is the correct one. You don't need a regulator for this motor. The answer I'm going to give assumes that you, for some reason, still need a regulator. While this doesn't exactly directly apply to this question, it brings up some things to consider that are worth knowing.

Don't Use A Linear Regulator:

Going from 7.4V to 6V will require a voltage drop of 1.4 volts. At six amps, that works out to be 8.4 watts. This means that your linear regulator will be outputting 8.4 watts of heat. That might not sound like a lot, but when it is concentrated into a small area it is enough to break things. You would certainly need a reasonable heat sink on this, just to get rid of the heat. But more importantly, that 8.4 watts of power will be wasted and since you are running on batteries this is not a good thing.

You Could Use A Switching Regulator, But:

A switching regulator can easily handle the 6 amp output that you require. However, you would want to carefully design it for maximum efficiency. If you don't take care in designing it, it would be about 80% efficient. With 36 watts of output, an 80% efficient switching regulator would waste 7.2 watts-- not significantly better than the linear regulator. With care, a switching regulator can be 90-95% efficient, and only waste 1.8 to 3.6 watts.

Switching regulators are also complex, and is beyond most hobbyist-level EE's. But buying a switcher module is an easy way to do it, without the difficult part of designing a proper PCB.

But PWM or Current Limiting is a good alternative:

Motors rarely care a lot about the voltage, it is total power (and thus, heat) that they care about. (There are some important caveats that I'll cover later.) It is possible, and common, to run motors at a higher voltage than what they are rated for. Two ways to achieve this is by using PWM or current limiting be within the rated power.

Let's say that you run the motor at roughly double the rated voltage. In that event, you can run the motor with a 50% duty cycle. Total power is essentially the same as if you ran it at the rated voltage but with a 100% duty cycle.

Alternatively, you can somewhat ignore voltage but run the motor at the rated current. Many motor driver chips can automatically measure and limit the current to the motor for just this purpose.

Caveats: Of course things are rarely that easy, and there are many factors to consider. I've just given a super quick overview. Here are some things to be aware of... Brushless DC motors (like muffin fans) have IC's in them that often can't handle voltages that are too high. Brushed-DC motors might wear out sooner due to increased arcing on the brushes at higher voltages. A voltage that is very high could cause the insulation to break down, so don't run a 6v motor off of 100 volts. Motors are highly inductive, which could help or hurt you when PWMing. And motor control is a complex subject, and you can make it as simple or as difficult as you want. But the closer to the edge that you push your motors, the more you have to pay attention to the details.

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  • \$\begingroup\$ Thank you very much for you in-depth explanation! I have another question though, if you don't mind. On the evaluation board for the drv8432 ti.com/tool/drv8432evm datasheet page 6, they connected the motor channels through 2 4.7uH inductors. Why is that good? \$\endgroup\$ – Domen Aug 13 '13 at 17:55
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    \$\begingroup\$ @Domen The inductor plus caps form a low-pass filter (an LC filter, instead of an RC filter). In some applications this may be needed to smooth out the motor rotation or to reduce EMI. Odds are high that you do not need this. \$\endgroup\$ – user3624 Aug 13 '13 at 18:06
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Do you really need to add a 6v regulator? To quote from the specs of your motors (my emphasis) - "These motors are intended for use at 6 V. In general, these kinds of motors can run at voltages above and below this nominal voltage, so they should comfortably operate in the 3 – 9 V range, though they can begin rotating at voltages as low as 1 V." Sounds like they'll be quite happy with the 7.4V

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  • \$\begingroup\$ But maybe I would have problems tuning the PID? I think it wild be easier to control them if they would be at a constant voltage ... Thanks for the answer. \$\endgroup\$ – Domen Aug 13 '13 at 17:46
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    \$\begingroup\$ @Domen Not really - the motor has an inbuilt quadrature encoder so you can monitor the speed and adjust accordingly. Also the motor will not suddenly die if the battery voltage dips below 6V + drop-out so as the battery discharges it will keep moving. \$\endgroup\$ – JIm Dearden Aug 13 '13 at 18:07
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    \$\begingroup\$ @Domen Power "smoothness" is probably not a primary issue for you. There are many other issues that can affect your PID stuff that are more important or more significant than power. \$\endgroup\$ – user3624 Aug 13 '13 at 18:08

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