Voltage Regulator with built in bypass switch ?

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Hi,

Does anyone know of a 5V to 3.3V regulator with a built in bypass
switch ? I.e. When I connect 5V to the regulator I want a regulated
output of 3.3V. If I connect a 3.3V supply directly, I want the
regulator effectively out of the circuit.  (300 to 500mA on the 3.3V)

Regards
   Anton Erasmus



Re: Voltage Regulator with built in bypass switch ?
Anton Erasmus schrieb:

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Most LDOs will do exactly that. A small difference remains, however
(maybe 50 to 200 mV at these currents). And take care that quescient
current of most LDOs increases significantly when reaching the dropout
region.
Maybe you're better of by regulating to a slightly smaller voltage,
for example 3.0V.
--
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
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Re: Voltage Regulator with built in bypass switch ?
Anton

possibly useful info:

The older positive regulators, ie 7805 style, used an npn device to pass
the current, while the more recent versions use a pnp transistor.

How the older negative devices worked is an interesting question...

Hul



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Re: Voltage Regulator with built in bypass switch ?

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I was hoping that I could find a regulator with such a circuit built
in.

Regards
   Anton Erasmus

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Re: Voltage Regulator with built in bypass switch ?

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I think what some people are trying to say is that it _is_ built in, just
select a MOSFET based LDO regulator (like the TPS777XXX series I mentioned)
and you will get the behavior you need (when the input voltage is less
than then regulators set voltage the pass transistor will be full-on)

Peter Wallace

Re: Voltage Regulator with built in bypass switch ?

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I have found the ultra low drop out regulators. They max out at a few
hundred mA where Vdo is in the region of 700mV which is in the region
of 20% of a 3.3V supply. I have built the comparitor based solution in
the past, and had hoped an integrated solution was available.
In any case it seems that the solution proposed by dmmilne  to
use a small buck switching converter is the way to go. The switcher he
suggested is even available at Digikey.

Regards
   Anton Erasmus

Re: Voltage Regulator with built in bypass switch ?

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Um, no...

For a little part, The TPS77801 for example has a typical 260 mv dropout at 750
mA
(around .3 ohms) so if you had 250 ma your drop would only be around 80
mV. In larger packages, TI makes Mosfet based LDOs up to at least 5A. For
example the
TPS75201 is a 2A one. It has a ~60 mV dropout (less than 2% of 3.3V) at 500 mA

Peter Wallace

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