Forums

Voltage Regulator with built in bypass switch ?

Started by Anton Erasmus October 22, 2004
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


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. 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 Ma�. http://www.autometer.de
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
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 > >
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.
> 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?
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
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 > >
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 >> >> >
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