Author Topic: Power Regulation  (Read 1321 times)

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Offline vivek9856Topic starter

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Power Regulation
« on: February 08, 2009, 09:19:38 AM »
I have to build an RC car that runs off of 4 AA cells. Sometimes, the person using the car may put in 4 alkaline cells, resulting in a source voltage of a bit more than 6V (When the batteries are fully charged), or they might put in 4 NiMH cells which would result in a source voltage of 5V (Or maybe a little less).

On top of that, I have to drive two motors that would be pulling a fair amount of current. So when the motors begin to operate, the source voltage will drop even further.

My problem is that I have some logic circuits and controllers that need a clean 5V power source. I can't use a LDO voltage regulator because my power source will dip below 5V when operating.

I wanted to know how I can go about making some circuit that will always provide my controllers with a nice clean 5V source.

I thought about using the TPS61027 (http://focus.ti.com/lit/ds/symlink/tps61027.pdf), as it can boost up as well as down-regulate the power source. Can I use this and power the logic circuits directly (without a separate external voltage regulator)? Or is this even a good choice for me? Or the MAX770 (http://datasheets.maxim-ic.com/en/ds/MAX770-MAX773.pdf)

Or should I just boost the voltage to 7V with another DC/DC converter and down-regulate the power to 5V using a LDO voltage regulator?
« Last Edit: February 08, 2009, 09:47:30 AM by vivek9856 »

Offline Soeren

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Re: Power Regulation
« Reply #1 on: February 08, 2009, 03:33:25 PM »
Hi,

The TPS61027 sounds like a very good choice. You could even drive it by one or two separate cells if you wanted to completely separate the two voltages. However, it only comes in a 10 pin QFN package - can you handle such a beast?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline vivek9856Topic starter

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Re: Power Regulation
« Reply #2 on: February 08, 2009, 04:58:03 PM »
Hi,

The TPS61027 sounds like a very good choice. You could even drive it by one or two separate cells if you wanted to completely separate the two voltages. However, it only comes in a 10 pin QFN package - can you handle such a beast?


Haha. I don't really know what a 10 pin QFN package is. I'm an amature pretty much, so do you think I can handle "the beast"? :D

Also, I will be running an ooPic R off of the boosted voltage. Can I connect the 5V output of the TPS61027 directly to the ooPic, or would I need to use a voltage regulator on top of the TPS output? Basically, is it safe to directly use the TPS output?
« Last Edit: February 08, 2009, 04:59:08 PM by vivek9856 »

Offline vivek9856Topic starter

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Re: Power Regulation
« Reply #3 on: February 08, 2009, 05:12:09 PM »
Okay, the TPS package is 3mmx3mm. I can't solder that myself.

So can someone recommend a way of getting this chip to work (eg breakout board or something), or some other booster/regulator combination?

Offline cosminprund

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Re: Power Regulation
« Reply #4 on: February 09, 2009, 01:13:41 AM »
Have you considered running your ooPIC at lower then 5V? Looking at the datasheets from some of my PICMicro MCU's they can operate from 2V to 5.5V - not strictly 5V; You probably need a fixed 5V supply while programming them but that's it! I'm not sure how this would affect other aspects of the MCU operation, like oscillator stability or ability to run at maximum oscillator speed but an compromise can be made.

Back to your issue. Since you're going to run off 4xAA batteries, you're going to have a maximum of 6V (4x1.5V for alkaline primary cells) or 4.8V (for a pack of 4 NiMh cells).

About the minimum volts in those cells: I had a hard time locating some battery datasheets that would give exact numbers. I found something like this in a Duracel datasheet (alkaline AA):
Quote
If the battery voltage is greater than or equal to 1.1 volts, the battery has approximately 20% service left;

NiMh cells can apparently discharge all the way to 0V - but it's not safe to do so, one's supposed to stop discharging at 0.9-1.1V in order to keep the cells from being damaged (when one cell in the pack reaches 0V and the others still have some power left in them). So the practical minimum for both types of batteries would be around 1V per cell, 4V per pack.

So you want a power regulation solution that can feed your electronics from a power supply of 4V to 6V. I'd suggest a LDO linear voltage regulator with "voltage following" capability (so the regulator would feed your electronics whatever it gets from the batteries when the voltage drops under 5V). Or no regulator at all! If someone loads 4 fresh alkaline batteries in the car the voltage will be over the maximum 5.5V accepted by the PIC but not for a long time: once the electric motors in the car start spinning I'm pretty sure there will be a drop in voltage!

 


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