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
Reply by Anton Erasmus●October 28, 20042004-10-28
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
Reply by Peter Wallace●October 27, 20042004-10-27
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)
Peter Wallace
Reply by Anton Erasmus●October 26, 20042004-10-26
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
>"Anton Erasmus" <nobody@spam.prevent.net> wrote in message
>news:1098453045.wOJahZbp3BMeYBoWZSSY4w@teranews...
>> 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
>>
>>
>
Reply by Hul Tytus●October 23, 20042004-10-23
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
Anton Erasmus <nobody@spam.prevent.net> wrote:
> 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
>
>
Reply by Peter Wallace●October 23, 20042004-10-23
On Sat, 23 Oct 2004 04:43:25 -0700, Anton Erasmus wrote:
> On Fri, 22 Oct 2004 15:56:06 +0200, Tilmann Reh
> <tilmannreh@despammed.com> wrote:
>
>>Anton Erasmus schrieb:
>>
>>> 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)
>>
>>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.
>
> Can you give a specific part number ? I have looked at various
> regulators from Linear Technology, National, Motorola etc. And they seem
> to define low drop-out in the order of 1 to 1.5V.
>
Take a look at the TI TPS77XXXX series parts. These are very low dropout
LDOs (MOSFET pass transistor) in a wide range of fixed/adjustable/currents...
If you have a lot of current, as someone else suggested, use a switcher
Peter Wallace
Reply by Lewin A.R.W. Edwards●October 23, 20042004-10-23
> I have a product that takes a plug in module that uses 3.3V
> components. The supply to the plug in module is 5V. On a new
> updated product the whole system will run at 3.3V. I want to be
Different connector pinout perhaps?
i.e.:
on 5V appliance, pin1=+5V, pin2=NC
on 3.3V appliance, pin1=NC, pin2=3.3V
on peripheral, pin1->regulator input, pin2->regulator output?
Reply by dmm●October 23, 20042004-10-23
On Fri, 22 Oct 2004 15:54:37 +0200, Anton Erasmus <nobody@spam.prevent.net> wrote:
>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
>
There is no need for a low dropout linear regulator. Try a MAX1684/85 dc/dc converter..
2.7-14V dc in, up to 1A output. Up to 96% efficient, and it has an internal p-channel fet.
Connect an inductor, schottky diode, and a few caps and you're away.
Reply by Gary Pace●October 23, 20042004-10-23
It would be easy to build a bypass with a low Rds-on P-FET and a comparator
"Anton Erasmus" <nobody@spam.prevent.net> wrote in message
news:1098453045.wOJahZbp3BMeYBoWZSSY4w@teranews...
> 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
>
>
Reply by Anton Erasmus●October 23, 20042004-10-23
On Fri, 22 Oct 2004 15:56:06 +0200, Tilmann Reh
<tilmannreh@despammed.com> wrote:
>Anton Erasmus schrieb:
>
>> 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)
>
>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.
Can you give a specific part number ? I have looked at various
regulators from Linear Technology, National, Motorola etc. And
they seem to define low drop-out in the order of 1 to 1.5V.
>Maybe you're better of by regulating to a slightly smaller voltage,
>for example 3.0V.
I have a product that takes a plug in module that uses 3.3V
components. The supply to the plug in module is 5V. On a new
updated product the whole system will run at 3.3V. I want to be
able to use the same plug in module on the new as on the older
5V systems. Having the user move a jumper to switch the regulator
in or out seems to be asking for trouble.
Regards
Anton Erasmus