Single cell boost help.

[[email protected]]

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

 

[linnix]

Hey guys, I am haven't quite a bit of trouble boosting a single nimh
cell to 3.3V at 300mA.

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

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.

What is the input voltage?

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

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

 

[[email protected]]

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

 

[[email protected]]

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

 

[linnix]

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

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.

 

[[email protected]]

1.2V

 

[MK]

1.2V

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

www.mkesc.co.uk

 

[[email protected]]

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

 

[[email protected]]

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

-Rich

 

[linnix]

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

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

 

[[email protected]]

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

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

 

[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

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.

 

[[email protected]]

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

TPS61025: 500mA @ 3.3Vout

http://focus.ti.com/paramsearch/doc...T&sectionId=null&tabId=null&resetCompare=true

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

 

[linnix]

TPS61025: 500mA @ 3.3Vout

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.

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.

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

 

[[email protected]]

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

This doesn't look so bad.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2377

Using the MAX1797 (typ. Iout = 0.55A), try this - *temporarily* put
*two* 1.2V batteries in *parallel*. Will you get 3.3V out then?

 

[[email protected]]

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

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.

http://www.energizer.com/products/rechargeables/sizes.aspx

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

 

[linnix]

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?

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.

What's the mAh raing on your NiMH, anyway? Energizer has AA's up to
2500 mAh now. My house is full of them.

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

 

[[email protected]]

The catch is that below 1.4V input, it switches at 50%.

That's the soft-start -- it only applies until the output rises above
1.4v.

Max. switching current is 1.5A.
So, 1.2V at 0.75A will give you 0.9W.
Still marginal.

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.

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

I'm not sure. Re-reading the poster's comments, he said:

"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."

"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

 

[Paul E. Schoen]

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

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
www.pstech-inc.com

 

[linnix]

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.

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.