Voltage Regulator with built in bypass switch ?

On Wed, 27 Oct 2004 10:09:26 -0700, Peter Wallace <pcw@karpy.com>
wrote:

>On Tue, 26 Oct 2004 12:46:26 -0700, Anton Erasmus wrote:
>
>> On Sat, 23 Oct 2004 14:04:38 GMT, "Gary Pace" <xxx@yyy.com> wrote:
>> 
>>>It would be easy to build a bypass with a low Rds-on P-FET and a
>>>comparator
>> 
>> I was hoping that I could find a regulator with such a circuit built in.
>> 
>> Regards
>>    Anton Erasmus
>
>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)

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

On Thu, 28 Oct 2004 13:19:22 -0700, Anton Erasmus wrote:

> On Wed, 27 Oct 2004 10:09:26 -0700, Peter Wallace <pcw@karpy.com> wrote:
> 
>>On Tue, 26 Oct 2004 12:46:26 -0700, Anton Erasmus wrote:
>>
>>> On Sat, 23 Oct 2004 14:04:38 GMT, "Gary Pace" <xxx@yyy.com> wrote:
>>> 
>>>>It would be easy to build a bypass with a low Rds-on P-FET and a
>>>>comparator
>>> 
>>> I was hoping that I could find a regulator with such a circuit built
>>> in.
>>> 
>>> Regards
>>>    Anton Erasmus
>>
>>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)
> 
> 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
 
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