Low drop discrete p-mosfet linear regulator

[Steve Sousa]

Hello:

I need a circuit that can regulate a lithium batery voltage in the range
3.5~4.2 down to 3.3V, max 400mA.
I came up with the following circuit:

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 www.tech-chat.de)

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.

 

[Steve Sousa]

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"

 

[Larry Brasfield]

Hello: Hi.
I need a circuit that can regulate a lithium batery voltage in the range 3.5~4.2 down to 3.3V, max 400mA.

It would help if you were to state the acceptable
range of output voltages and the temperature
range over which you require that accuracy.

The capacitive load that will exist should also
be given to enable stability analysis and help
narrow the viable design approaches. For
the same reason, if transient loads will not
be absorbed by the load capacitance, they
should be specified as well.

I came up with the following circuit:

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 www.tech-chat.de)

Where R2 is chosen to drop 0.7V when the output is 3.6.
Any comments/sugestions?

The circuit is going to exhibit a TC of about
13 mV per degree C, within a few mV. If
the output is to power logic requiring a 5%
supply, that drift alone would constrain the
operating temperature to stay within a 12
degree C band. But the initial tolerance
is going to be poorer than that, due partly
to variation in BJT beta and partly to the
varying threshold voltage of the MOSFET.

The MOSFET is backwards.

I think i should use a zener, but couldn't figure out where to put it, without adding a lot of parts.

It would also help if you were to indicate the
cost constraints applicable to the solution.

I would like to use only p-mosfets or bipolar transistors.

I guess a zener is also a candidate, (although
low voltage zeners are very soft references.)

Before designing improvements, it would
be smart to nail down the requirements
and separate them from the wishes.

You may see a number of circuits offered
here as an "improvement". They may not
satisfy your actual requirements unless
they are stated ahead of the time those
circuits are concocted. But if you have
stated your requirements, the circuits
offered will be scrutinized against them,
at least by some diligent participants.

Thanks in advance.

You're most welcome.

 

[Robert Baer]

Hello:

I need a circuit that can regulate a lithium batery voltage in the range
3.5~4.2 down to 3.3V, max 400mA.
I came up with the following circuit:

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 www.tech-chat.de)

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.

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.

 

[Fred Bloggs]

Hello:

I need a circuit that can regulate a lithium batery voltage in the range
3.5~4.2 down to 3.3V, max 400mA.

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:
http://www.national.com/pf//LP/LP3872.html

 

[Steve Sousa]

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:
http://www.national.com/pf//LP/LP3872.html

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.

 

[Steve Sousa]

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

Thanks

 

[Fred Bloggs]

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.

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.

 

[Steve Sousa]

It would help if you were to state the acceptable
range of output voltages and the temperature
range over which you require that accuracy.

3.3+-5%
-10 to +55

The capacitive load that will exist should also
be given to enable stability analysis and help
narrow the viable design approaches. For
the same reason, if transient loads will not
be absorbed by the load capacitance, they
should be specified as well.

I have no ideia what the capacitance of the load is, currently there is a
470uF FK series panasonic on board, the loads are a couple of ATMega, a gps
receiver (ublox) and a bluethoot module. That capacitor is of that value
because it's used in other parts of the circuit, it can be changed/removed.

the output is to power logic requiring a 5%
supply, that drift alone would constrain the
operating temperature to stay within a 12
degree C band. But the initial tolerance

That's bad.

The MOSFET is backwards.

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.

It would also help if you were to indicate the
cost constraints applicable to the solution.

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

I guess a zener is also a candidate, (although
low voltage zeners are very soft references.)

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.

 

[Winfield Hill]

Fred Bloggs wrote...

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: http://www.national.com/pf//LP/LP3872.html

LP3872ET-3.3 Wait, there's more!
You also get free 24-hour samples!

 

[Ian]

Hello:

I need a circuit that can regulate a lithium batery voltage in the range
3.5~4.2 down to 3.3V, max 400mA.
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.

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

http://www.national.com/pf/LP/LP2996.html#Datasheet

Regards
Ian

 

[Winfield Hill]

Ian wrote...

Steve Sousa wrote ...

LP2996. Ignore what the datasheet calls it, look at what it does.
http://www.national.com/pf/LP/LP2996.html#Datasheet

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?

 

[Larry Brasfield]

3.3+-5%
-10 to +55

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?

I have no ideia what the capacitance of the load is, currently there is a 470uF FK series panasonic on board, the loads are a
couple of ATMega, a gps receiver (ublox) and a bluethoot module. That capacitor is of that value because it's used in other parts
of the circuit, it can be changed/removed.

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.

That's bad.

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.

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.

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.

I was hoping a discrete solution for less than the cost of an lp3871, about 1.36 Eur.

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?

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.

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?

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?

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.

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.

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

Thank you Anytime.
Best Regards.

Likewise,

 

[Ian]

Ian wrote...

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?

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