Voltage Regulator with built in bypass switch ?

Anton Erasmus schrieb:

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.

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

Ant> Hi,

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

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