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'[EE]: Can you put multiple voltage regulators in p'
2002\01\26@014312 by Rick Mann

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I was going to use a voltage regulator from Toko America, what Digi-Key
listed as a 260 ma 5.5 volt regulator (datasheet:
http://207.208.84.9/semiconductors/pdf/tk112xxb.pdf).

However, when it arrived and I started looking at the data sheet, it seemed
to be only a 150 ma regulator. What I'd like to know is, can I safely put
two of these together, both outputs tied to the same positive (circuit)
supply rail? Will I then be able to get 300 ma out of the two regulators?

If I can't tie them to the same supply rail, can I run some of my components
off a separate rail? For example, can I have one regulator power the LCD in
my circuit, and the other regulator power the MCU, but still have output
pins of the MCU connected to input pins of the LCD, and everyone sharing a
common ground?

As a side question, does anyone know of a 250 - 300 ma 5.5 volt
surface-mount regulator?

TIA,

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2002\01\26@020942 by David VanHorn

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>
>If I can't tie them to the same supply rail, can I run some of my
>components off a separate rail? For example, can I have one regulator
>power the LCD in my circuit, and the other regulator power the MCU, but
>still have output pins of the MCU connected to input pins of the LCD, and
>everyone sharing a common ground?

This will definitely work.

As to putting active regs in parallel, probably not.
If their ref voltages aren't identical, then one ends up hogging the load,
and overheating I think.

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2002\01\26@025322 by 859-1?Q?Alexandre_Guimar=E3es?=

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> As a side question, does anyone know of a 250 - 300 ma 5.5 volt
> surface-mount regulator?

   What is the input voltage ? That will be the basis to determine how much
heat you will generate on the linear regulator. If it is too high you will
have to give up on the smd part. If it is low enough take a look at linear
technology, they have some low drop regulators that should fit your needs.

Best regards,
Alexandre Guimaraes

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2002\01\26@085831 by Byron A Jeff

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On Fri, Jan 25, 2002 at 09:46:08PM -0800, Rick Mann wrote:
> I was going to use a voltage regulator from Toko America, what Digi-Key
> listed as a 260 ma 5.5 volt regulator (datasheet:
> http://207.208.84.9/semiconductors/pdf/tk112xxb.pdf).
>
> However, when it arrived and I started looking at the data sheet, it seemed
> to be only a 150 ma regulator. What I'd like to know is, can I safely put
> two of these together, both outputs tied to the same positive (circuit)
> supply rail? Will I then be able to get 300 ma out of the two regulators?

Probably not a good idea. Linear bipolar regulators generally have a "feature"
called thermal runaway. Simply put the hotter it gets, the more current it
draws, making it even hotter. When you parallel once one becomes hotter than
the other, taking all of the current.

>
> If I can't tie them to the same supply rail, can I run some of my components
> off a separate rail? For example, can I have one regulator power the LCD in
> my circuit, and the other regulator power the MCU, but still have output
> pins of the MCU connected to input pins of the LCD, and everyone sharing a
> common ground?

Yes. That should be fine.

>
> As a side question, does anyone know of a 250 - 300 ma 5.5 volt
> surface-mount regulator?

Of course there is the third option: use a pass transistor in parallel with
the regulator. Then when the regulator starts passing too much current the
pass transistor kicks in and delivers the rest. A surface mount low sat
PNP or PMOS power transistor should be able to easily deliver a 1/2 amp.

Hope this gives you some ideas.

BAJ

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2002\01\26@090440 by info

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You can always put a diode in series with each regulators output. This
stops them seeing each other and one hogging. Make the diode a low
voltage drop type for best efficiency and to preserve as much of the
original regulated voltages output level as possible
Alan



{Original Message removed}

2002\01\26@095735 by Dave Dilatush

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Rick Mann wrote...

>I was going to use a voltage regulator from Toko America, what Digi-Key
>listed as a 260 ma 5.5 volt regulator (datasheet:
>http://207.208.84.9/semiconductors/pdf/tk112xxb.pdf).
>
>However, when it arrived and I started looking at the data sheet, it seemed
>to be only a 150 ma regulator. What I'd like to know is, can I safely put
>two of these together, both outputs tied to the same positive (circuit)
>supply rail? Will I then be able to get 300 ma out of the two regulators?

Looking at the data sheet for the TK11255BM (I'm assuming that's the
one), it would appear you might be able to do this.

If you do, be aware that the two regulators aren't going to share the
load current equally; in fact, one of them will end up supplying most of
the load current while the other one supplies only a small part of it,
or none at all.

The reason for this is that there's always some variation in the output
voltage setpoint from device to device; and if you have two devices in
parallel, one of them will be trying to set the output voltage just a
little bit higher than the other.  The one that wants the higher output
voltage (even if it's only higher by a few hundred microvolts) will end
up sourcing all the current, while the regulator that wants to set a
slightly lower voltage will simply shut off.

The only time you'll get both regulators conducting is if your load
current exceeds the short-circuit current limit of the regulator which
is carrying the lion's share of the load (or the regulator gets too hot,
activating its overtemperature protection), whereupon the other one will
kick in and start passing current.

>If I can't tie them to the same supply rail, can I run some of my components
>off a separate rail?  For example, can I have one regulator power the LCD in
>my circuit, and the other regulator power the MCU, but still have output
>pins of the MCU connected to input pins of the LCD, and everyone sharing a
>common ground?

Yes, you can do that.  The trick, however, is to somehow partition your
circuit and divide the load more or less evenly between the two
regulators.  I don't know how much current your LCD takes; if it's just
a character LCD module it probably doesn't draw more than a few
milliamps and, if so, putting it on a separate regulator wouldn't help
your overall situation much.

>As a side question, does anyone know of a 250 - 300 ma 5.5 volt
>surface-mount regulator?

No, though you could look around and see what National Semiconductor,
Maxim, and Linear Technology might offer.

Hope this helps...

Dave

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2002\01\26@101124 by Dave Dilatush
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Byron A Jeff wrote...

>Linear bipolar regulators generally have a "feature"
>called thermal runaway. Simply put the hotter it gets, the more current it
>draws, making it even hotter.

Any examples?

All of the IC voltage regulators I've ever used or investigated--and
that's a LOT of them--not only have internal current limiting and output
short-circuit protection, but thermal overload protection and, in some
cases, pass transistor SOA protection as well.  Some go even further,
adding protection against input overvoltage and even protection against
being inserted into a PC board backwards.

I've never seen any integrated circuit voltage regulator do what you've
described, or anything even remotely resembling it; not ever.  My
experience has been that most IC voltage regulators are very rugged.

If you know any that do have this thermal runaway "feature", I'd be
interested in getting the manufacturer and part numbers so I know what
not to buy in the future.

Dave

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2002\01\26@103649 by Byron A Jeff

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On Sat, Jan 26, 2002 at 03:10:04PM +0000, Dave Dilatush wrote:
{Quote hidden}

I explained myself poorly. Sorry. Of course they have current limiting. But
the net effect is that one of the regulators will always be maxed out
because it runs warmer than the others.

All of the dicussions I've seen on parallel regulators indicates that their
outputs should never be tied directly together.

If you have an example of where regulator outputs are tied diectly togther
and they equally share the load, I'd sure like to see it because that's a
part I would use.

BAJ

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2002\01\26@122300 by Dave Dilatush

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Byron A Jeff wrote...

>But the net effect is that one of the regulators will always be maxed out
>because it runs warmer than the others.

Yes, one of the regulators will end up sourcing most of the load
current; but it will do so because it has the higher of the two output
voltage setpoints, not specifically due to its being warmer.

However, you could also have some interesting thermal dynamics going on
with two regulators in parallel like this.  What would happen, for
instance, if you have two regulators connected thus, whose output
voltage setpoints were very close at room temperature but they have
slightly different reference voltage temperature coefficients and/or
error amplifier gain tempcos?  The regulators might share current
differently at different load levels or temperatures; or they could even
swap current back and forth, heating and cooling alternately as some
kind of funky "thermal oscillator"; or, if both regulators had rather
strong negative output voltage tempcos they could settle into an
arrangement where they actually would share the load more or less
evenly.  Not likely, but possible.

The bottom line, though, is what Russell McMahon said: provided the
voltage outputs of the two regulators aren't vastly different and they
are both current-limited, there shouldn't be any huge problems.

The only "should never" I'm aware of is, don't ever just connect two
regulators in this fashion and casually assume they'll somehow
automagically share the load equally; they won't.  
Also, there are a few possible complexities that must be taken into
account:

One possible "gotcha" involves low-dropout ("LDO") regulators; these
tend to be rather picky in their input/output filter capacitor
requirements, with respect to both capacitance and ESR.  With the wrong
capacitors these things can become unstable and end up oscillating.
Putting two of them in parallel could end up making the input/output
filtering requirements even more complex.

A second possible complication involves the particular overcurrent
protection mechanism that ends up limiting the current in the regulator
that initially supplies the bulk of the load current.  If its output
current is limited by the internal current-limit circuitry, all is
probably well and the second regulator will simply step in and start
supplying the remainder of the load current.  But if the first regulator
ends up tripping its overtemperature protection BEFORE it goes into
current limit (for instance, because the package size is small and
there's a large input-output voltage differential), funny things can
happen.  Some regulators may implement this overtemperature protection
by simply cranking back on the internal current limiter when the die
temperature goes too high; but others may operate by actually shutting
down the regulator entirely and waiting for it to cool off, whereupon it
turns on again.  Obviously such on/off/on/off behaviour wouldn't be too
helpful in this situation, so a careful study of the datasheet is in
order.

>If you have an example of where regulator outputs are tied diectly togther
>and they equally share the load, I'd sure like to see it because that's a
>part I would use.

Like I said, both here and in my earlier reply to the OP, this business
of equal load sharing simply isn't too likely to happen.  
Frankly, I think it's a whole lot easier--and cleaner--just to get a
heftier voltage regulator if you need more current.  
Probably be cheaper, too.

Dave

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2002\01\26@125429 by andy n1yew

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or you can use a pass transistor..

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2002\01\26@131053 by jim

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Why not use a small transistor as a pass element?
Connect the collector to the supply voltage, the base to the output of the
voltage regulator and the emitter to the circuit to be powered.  This will
be
~.7 volts lower than the output of the regulator, but this will get you to
about 5 volts
which I assume is good enough for the curcuit you are powering.

                                                           Regards,

                                                               Jim
{Original Message removed}

2002\01\26@134055 by Thomas C. Sefranek

face picon face
Use a power PNP WITH the regulator for a bulk current element and have NO loss
of voltage.
The usual way is base of the PNP to the regulator input, collecot r to the
output.
add a bias resistor (1 ohm) from emitter to base, connect the emitter to the raw
source.

jim wrote:

{Quote hidden}

> {Original Message removed}

2002\01\27@024717 by Roman Black

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Rick Mann wrote:
>
> I was going to use a voltage regulator from Toko America, what Digi-Key
> listed as a 260 ma 5.5 volt regulator (datasheet:
> http://207.208.84.9/semiconductors/pdf/tk112xxb.pdf).
>
> However, when it arrived and I started looking at the data sheet, it seemed
> to be only a 150 ma regulator. What I'd like to know is, can I safely put
> two of these together, both outputs tied to the same positive (circuit)
> supply rail? Will I then be able to get 300 ma out of the two regulators?
>
> If I can't tie them to the same supply rail, can I run some of my components
> off a separate rail? For example, can I have one regulator power the LCD in
> my circuit, and the other regulator power the MCU, but still have output
> pins of the MCU connected to input pins of the LCD, and everyone sharing a
> common ground?


Yes you can have 2 regulators, grounds connected, and
each supplying different chips/devices. There is enough
range in the logic in/out voltage thresholds that it
will work fine even if there is a 0.5v or more
difference in the two 5v rails. I have seen this done
in commercial devices. :o)

Another way is to parallel both the regulator inputs,
and grounds, and parallel the 5v outputs each through a
small resistor. Like the old emitter balance resistor
trick. At 150mA each try a 1.8ohm resistor, so you will
lose about 0.27v total from your 5v rail, but it will
happily supply the 300mA and they will balance equally.
Obviously your 5v rail regulation will vary from 5v to
4.73v from 0mA to 300mA.
-Roman

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2002\01\27@044103 by Alan Shinn

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Subject: [EE]: Can you put multiple voltage regulators in parallel for more
        power?
Many have answered already, here's my $.02.

Give your regulator a heat sink, you'll probably be OK. glue a penny
onto it. I wouldn't base a product on  it though.

I would *Guess* that you could tie two together but have not seen such
in the app notes (in fact they show some complicated opamp current
equality sensing circuit).With a simple parrallel hookup the regs would
not share current equally, but when one started to droop the other would
turn on and supply the difference. I wouldn't  base a product on  it.

Folks have mentioned using a booster transistor.  Is good but may
increase voltage drop.

Multiple regulators for different parts of the circuit would be fine
with the already mentioned caution to make sure the current is shared --
the regulators don't even have to have the same voltages. For instance,
you could have a 12V regulator for some relays and opamp and also a 5
volt regulator for a pic chip and a bunch of LEDs.

If the current draw has a known minimum value (like half), you can shunt
a resistor around the regulator to supply some of the current. The great
part about this approach is the cost and availability of resistors!  :-)

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>>>>>>>>>>>>>>>>>>>
>I was going to use a voltage regulator from Toko America, what Digi-Key
listed as a 260 ma 5.5 volt regulator (datasheet:
http://207.208.84.9/semiconductors/pdf/tk112xxb.pdf).

However, when it arrived and I started looking at the data sheet, it seemed
to be only a 150 ma regulator. What I'd like to know is, can I safely put
two of these together, both outputs tied to the same positive (circuit)
supply rail? Will I then be able to get 300 ma out of the two regulators?

If I can't tie them to the same supply rail, can I run some of my components
off a separate rail? For example, can I have one regulator power the LCD in
my circuit, and the other regulator power the MCU, but still have output
pins of the MCU connected to input pins of the LCD, and everyone sharing a
common ground?

As a side question, does anyone know of a 250 - 300 ma 5.5 volt
surface-mount regulator?

TIA,

--
Rick

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