Low drop discrete p-mosfet linear regulator

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Hello:

I need a circuit that can regulate a lithium batery voltage in the range

fdc638p

.-->|-. VCC d | | s Out o-----------o-+^+-o----o--------o----------o ||| | | === | .-. |g | | | | | | | R2 | | '-' | | | | >| | | |------o | /| | | | .-. o------' | | | | | R3 | '-' | | .-. | | | | | | R1 | '-' | | | | | | | o---------------o---------------o----------o GND (created by AACircuit v1.28.5 beta 02/06/05

Where R2 is chosen to drop 0.7V when the output is 3.6. Any comments/sugestions? I think i should use a zener, but couldn't figure out where to put it, without adding a lot of parts. I would like to use only p-mosfets or bipolar transistors.

Thanks in advance.

Hello: The faster one posts the more mistakes: There are to errors in my previous post. First, when the circuit is fed 4.2 volts the mosfet's internal diode will put 3.5V on the output, not good. In the paragraph below the circuit, where it says: "output is 3.6." should read "output is 3.3"

Hmm, Vout = 0.5 Vsupply. "The reference voltage is generated from a resistor divider of two internal 50kΩ resistors. This guarantees that VTT will track VDDQ / 2 precisely."

You have a clever way to get around this functionality?

Why not use an adjustable 3-terminal regulator, like the LM285 to control a pass transistor? A MOSFET is unacceptable, because one needs about 3V to drive it on; a DMOS or JFET would be better choices, and a bipolar might serve better.

That 400ma seems like a lot for Li battery application. You are wasting your time with this circuit- 3.3V is a standard LDO regulator:

I had my eye on lp3871 or lp8345, but wondered if it couldn't be done cheaper. The batery is 950mAh. 400mA is an estimated very worst case, tipical is around 60 to 230, but most of the time the circuit goes to low power sleep, less than 6mA.

That is something worth trying. The treshold for the mosfet i have is -1.5V, I have available lp2985-2.8

Thanks

You better focus on that worst-case estimate because the price you will pay is having to go with an LDO with relatively large quiescent current. By the time you make a full regulator in discrete, you could buy 10 LDOs.

-10 to +55

I have no ideia what the capacitance of the load is, currently there is a

That's bad.

I placed it that way to be able to control VGS more easily, but it creates a problem when the input is over 4V, so i can't use that way anyhow.

I was hoping a discrete solution for less than the cost of an lp3871, about

I was under the impression that a linear regulator using a p-mosfet would be relatively straight-forward, since i din't have a problem getting the mosfet, as i use it in other parts of the circuit, i was wrong.

I have available a lp2985-2.8, maybe i could use that and a pass transistor to "fake" a lp3871, so i can at least debug the thing during the weekend?

Other info: The batery is 950mAh. 400mA is an estimated very worst case, tipical is around 60 to 230, but most of the time the circuit goes to low power sleep, less than 60mA.

Thank you

Best Regards.

LP2996. Ignore what the datasheet calls it, look at what it does.

Regards Ian

Whooh! That should be fun. I think a good reference is already indicated. Do you, perhaps, already have one in the target system? Or is this the first use of one? If so, is there a higher supply avaliable?

capacitor is of that value because it's used in other parts

If you can guarantee a minimum capacitance, that is helpful. An upper bound would be nice and permit more latitude in the stabilization of the feedback loop. But it is not critical.

Forward biased Si junctions, acting alone, are not much good at being references. With a few more parts, you can use a pair of them to make a reference good enough to do your job.

problem when the input is over 4V, so i can't use

So, it's in frontwards now? I guess I would use it however worked best anyway, so my comment was in case you might build the thing.

That seems to make an op-amp solution reasonable. Is there any reason you have excluded that? Do you have any spares or singles that could become doubles? Are you sure there are no integrated low dropout regulators that do this?

relatively straight-forward, since i din't have a

wrong.

It will be, relatively, if there is a reference available. Otherwise, that will be one of the more fun parts. Have you looked into what a reference, PMOSFET, and cheap low power rail-to-rail op-amp solution would cost? Is that too much, considering your expected volume?

"fake" a lp3871, so i can at least debug the thing

Are you saying you mean to use parts you have laying around, or that you already have a 2.8V supply developed by that LDO? If the latter, then that supply may be a decent reference.

around 60 to 230, but most of the time the circuit goes

But it can stay at the 400 mA (worst case) level for a time period exceeding the thermal time constant of small parts, right?

Anytime.

Likewise,

Quite right, Win. The application I used this in had an available regulated voltage I could use for Vddq, so this was a very cheap, simple (low parts count) LDO regulator. For this application, the OP would also need to have a reference. That could be a say 2.5V fixed part. That plus a couple of resistors sets the output to 3.3V.

Regards Ian