What if the input voltage to a boost regulator is too high?

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Suppose I have a small boost regulator to bump up 2 AA cells to 5V, and I want to extend the battery life by going to 3 AA cells (4.8V with fresh batteries). This is outside the acceptable range of input voltage for such a regulator in the sense that the PWM control loop will be driven to one extreme or the other, right? But is this a bad thing? Even if it's out of regulation, if it just feeds the 4.8V through unregulated, that would be fine. Then as the batteries drain down, the regulator can start regulating normally.

BTW, going to larger-diameter batteries is not an option. Too bad they don't make longer AA cells with more capacity.

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As long as Vin < Vout, which is the case with 3 AA-cells, I don't see a problem. The effect will be that the switching element will be driven with just small PWM pulses. So its job gets easier :-)

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Depends a LOT on the device and the switching frequency. I don't think you can say anything, generally. I have been working with a small SOT-23 LT switcher. Initially, I was trying to get 5V out from 1.8 to 5V in. It got really unpleasant above 4.5V or so.

Since then, I have changed the input range to 2.7V to 24V and changed the topology to SEPIC and all is beautiful. 125ma max load or so.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:
Depends a LOT on the device and the switching frequency. I don't think you can say anything, generally. I have been working with a small SOT-23 LT switcher. Initially, I was trying to get 5V out from 1.8 to 5V in. It got really unpleasant above 4.5V or so.

That's what I didn't want to hear, but I can't say I'm surprised. Care to reveal a part number?

Quote:
Since then, I have changed the input range to 2.7V to 24V and changed the topology to SEPIC and all is beautiful. 125ma max load or so.

That's about the same current load I'm looking for, in bursts.

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Quote:
Too bad they don't make longer AA cells with more capacity.
Take a look at the Duracell Staycharged NiMH rechargables:
http://www.duracell.com/en-US/pr...

My 4year old son has a VTECH InnoTab2s and it used to go through 4 AA cells a week. I picked up a kit of a charger, 6AA cells and 2AAA cells and he now gets up to 10 days use before the lo battery indicator starts up. The AAA cells are rated at 800mAH and the AA cells are rated 2200mAH. I paid $25.00usd for the kit. Worth every penny. MAybe this can give you the runtime you need and the cost savings of not having to purchase new cells all the time.

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jgmdesign wrote:

My 4year old son has a VTECH InnoTab2s and it used to go through 4 AA cells a week. I picked up a kit of a charger, 6AA cells and 2AAA cells and he now gets up to 10 days use before the lo battery indicator starts up.

Many toys (and baby monitors) specifically say do not put in rechargeable batteries for safety reasons. More than once I have seen a toy motor stalled by amounts of hair and dust jamming a wheel or something. Regular AA batteries have short circuit current of about 2-3 amps the last time I checked, and with AA rechargeable, I'd say it is 10-30 amps. Enough to start a fire.

As a kid, I learnt this the hard way. Do not "repair" or "improve" toys when rechargeable batteries are installed. It's a pretty astonishing effect, after gotten used to regular batteries.

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I've seen the warnings before, but I did not know why.

I learned something new!

Thanks, Jepael

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Started with LT1613 - this is actually one of a pair, rated at different current limits.

Not sure what I am using now for the SEPIC. That is at home.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Well for a rediculous amount of money I can purchase the vetch rechargeable kit that received mixed reviews. I have been running these cells for close to two months and no heating issues.

The reason for my post was to give the OP an option to increase his/her runtime

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Sorry to take so long.

What I ended up with is SO8/DIP LT1372/LT1377 in SEPIC architecture designed to work from 2.7V to 24V input and 5V output with load from 1ma to 125ma.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
Depends a LOT on the device and the switching frequency. I don't think you can say anything, generally. I have been working with a small SOT-23 LT switcher. Initially, I was trying to get 5V out from 1.8 to 5V in. It got really unpleasant above 4.5V or so.

Since then, I have changed the input range to 2.7V to 24V and changed the topology to SEPIC and all is beautiful. 125ma max load or so.


Jim, your post is misleading and not on topic.
The question was about what happen if a boost regulator is powered with a higher voltage. Plons got it right. Even if the input voltage equal or exceeds the output voltage, the output voltage will follow closely the input one, unless some parameters exceeds the maximum ones.

Now, about your experience, your regulator at very low input voltage it may have not been able to handle the necessary current in order to deliver the output power you asked. This not means the boost regulators should be replaced with sepic ones. To compare the boost regulator at 1.8V with a sepic one at minimum 2.7 means nothing.

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Since a boost regulator usually has the input going to the coil to a diode to the output and a switch from the diode end of the coil to ground...

I think if you applied too high an input voltage, your output voltage would follow the input voltage.

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Torby wrote:
Since a boost regulator usually has the input going to the coil to a diode to the output and a switch from the diode end of the coil to ground...

I think if you applied too high an input voltage, your output voltage would follow the input voltage.


That was my first instinct as well, and such behavior would be fine. I'll just have to get some parts in and try it.

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My solution is about how to deal with the question. Now,KK6GM says that it is OK if the output rises. This is a late spec change.

So, SEPIC is not needed.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:
My solution is about how to deal with the question. Now,KK6GM says that it is OK if the output rises. This is a late spec change.

So, SEPIC is not needed.

Jim


The goal is to drive a 5V regulator with 3 alkaline cells, which means about 4.8V with fresh batteries. If the output of a boost regulator stays between 5V and 4.8V in the case with 4.8V input, (even if unregulated) that's fine. If the output goes bonkers, that's not OK and I have to do something else.

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It won't go bonkers. It will behave as Torby suggests.

With the input slightly above 5V, the feedback will sense that the output is too high and will shut off. Once the input comes down to around 5V, the boost will come back on, though possibly in bursts. This may result in more than anticipated ripple. The burst operation will go away depending on the load current and the input voltage.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Yea. I don't know what the regulator chip is going to do when it senses.. I think all it will do is run very short pulses 'cause it doesn't think it needs any boost.

I'd build one of these and experiment with it, perhaps a lot, before I used it to power an expensive gizmo.

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