Single cell boost help.

Hey guys, I am haven't quite a bit of trouble boosting a single nimh cell to 3.3V at 300mA. At first Maxim looked great and user friendly. So i order a few samples and built a few boost using the MAX1797, MAX1724 and MAX1674. I am getting 3.3V out but as soon as the current increases the voltage drops to 2V. I was just wondering someone could give me some pointers because im out of ideas. I was thinking about using mosfet driver boost regulator instead of boost ICs. But my search has been unsuccessful. Thanks for your time.

-Rich

Reply to
newsrichie
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So, you are trying to get 1W out of a single battery?

What is the input voltage?

Any booster circuit still need to obey the law of physics.

Reply to
linnix

The usable voltage will be around 1-1.4V. I understand there will be a extremely high current draw at the input.

Reply to
newsrichie

The usable voltage will be around 1-1.4V. I understand there will be a extremely high current draw at the input.

Reply to
newsrichie

But what was the actual measured input voltage when the output was 2V. I am trying to figure out if it's within the spec or not.

Reply to
linnix

Hello,

I do wish you wouldn't snip all the context from your replies - this will seem a strange response to 1.2V !

From the Maxim data sheet for the MAX1797 its is barely capable of 3.3V @

300mA with a 1.2V input. You will need to make sure that all the impedances in the supply to the chip are very low since you can't afford to lose even 50mV. The average current from your battery will be 0.8A so the peak will be much higher. The 1797 quotes a maximum switch current of 1A and switch resistance of 0.25R so its very marginal for your application. The other thing to check is that your inductor not too large.

If you mean to build many of these it would be worht looking for annother chip.

Michael Kellett

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Reply to
MK

I just need one working supply. Can anyone recommend a vendor or a series of ICs? I am out of ideas.

-Rich

Reply to
newsrichie

Sorry about the short replies, ill give more details after class today.

-Rich

Reply to
newsrichie

It's very difficult to work with 1A @1.2V. Can you settle with 0.5A @2.4V, using 2 cells?

Reply to
linnix

I really wish i could use more cells. But my design requires that i use

1 cell. I am building a robot that will sit under a light and charge it's single nimh cell when a certain charge level is reached it will go out and fine a patch of tin foil. I have to power 2 servos, some sensors and 3 micros. Boosting for the micros and sensors is no problem but the motors are almost impossible. I have tried running 3 boost blocks in parallel with no luck. Thanks for the replies.

-Rich

Reply to
newsrichie

How about other energy storage devices? Are they allowed? For example, 1000uF Caps and 100uH Coils. Sound like your professor is forcing you to build the booster, not buying one.

Reply to
linnix

TPS61025: 500mA @ 3.3Vout

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A single LiIon cell, however, will pack more juice, weigh less, give you many more converter options, and possibly eliminate the need for a converter completely.

Cheers,

James Arthur

Reply to
dagmargoodboat

The catch is that below 1.4V input, it switches at 50%. Max. switching current is 1.5A. So, 1.2V at 0.75A will give you 0.9W. Still marginal.

His professor won't allow it. He is forcing students to design one without the limitations.

Reply to
linnix

This doesn't look so bad.

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Using the MAX1797 (typ. Iout = 0.55A), try this - *temporarily* put

*two* 1.2V batteries in *parallel*. Will you get 3.3V out then?
Reply to
onehappymadman

Powering sensors, micros *and* motors on 1 watt? Wow. Maybe the professor had some power-management in mind... run only one motor at a time?

What's the mAh raing on your NiMH, anyway? Energizer has AA's up to

2500 mAh now. My house is full of them.

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A shame that their Cs and Ds don't have more mAh...

Reply to
onehappymadman

The professor checked out all the commerical stuffs, to make sure they cannot be used. Students need to build one without the safety limitations of commerical chips. I.e. burning silcons on the go.

I am sure the battery is carefully selected and spec'd.

Reply to
linnix

That's the soft-start -- it only applies until the output rises above

1.4v.

Not for the reason you cited, but yes, marginal after all. Thanks for making me look more carefully.

Looking at the datasheet instead of the selection guide, the part will barely grunt out 250mA out at 3.3v with 0.9v in, increasing to about 400mA at 1.3v input. Possibly good enough, but marginal.

"My design requires" doesn't clarify whether he chose this, or whether it was dictated to him. Also, we know he's using a NiMH cell, but he did not say the single cell *had* to be NiMH. If allowed, LiIon has the advantages I mentioned, at the expense of necessitating a more complicated charger.

Regards, James Arthur

Reply to
dagmargoodboat

You might try first boosting to about 3 volts, and use a supercapacitor of maybe 5 or 10 farads. You could then add another booster from 3 volts to 6 volts, and then use efficient PWM techniques to drive your motors at vasriable voltage until the capacitor runs down. Even if your primary cell runs down to 1 volt or so, you could still probably get enough voltage to work with. Also, you should be able to slow down the servos and reduce your current draw. For the same amount of energy, you can reduce your power and increase the time for the activity you require.

Paul E. Schoen

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Reply to
Paul E. Schoen

Only if supercaps or coils are allowed. In that case, I would switch the voltage as high as possible, before regulating it down. Normally, I hate doing homeworks for students, but this is an interesting one.

We need more spec and reqt from the OP.

Reply to
linnix

Not a bad try, but the max-current-out vs. input voltage graph on page 6 of the spec sheet shows only 300mA at 3.3v out with 1.25volts input. Its 0.3 ohm n-FET and 0.4 ohm p-FET are just not quite hefty enough, same as the TI part I suggested.

The manufacturers like to spec large current output capability and low voltage capability, but the datasheets indicate you can't get both at the same time!

I can imagine paralleling chips & other methods to achieve the goal, but if the prof's purpose is to assign a converter challenge, I'd think it bad manners to interfere.

Not likely to make much difference -- NiMH impedances are _low_.

Best, James Arthur

Reply to
dagmargoodboat

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