Small efficient switch

[Scarr]

Hi,

I have a Atmega128 MCU and GPS being powered by the same 3.3v reg, I want to switch off the GPS to save juice but I don't want ot add another regulator with shutdown pin, can I use a simple power transistor (80ma max)

Ideally I want to simply cut the power line, insert the transistor and then be able to control it from the MCU whilst using as little current as possible when base is held low.


Thanks all

I think I know the asnwer but I want to have it confirmed but people who know

 

[akg]

could be done .but remember the drop acroos the transistor

 

[Nigel Goodwin]

could be done .but remember the drop acroos the transistor

Yes, you should use a PNP transistor, with positive into the emitter, and out from the collector - less voltage drop this way.

 

[akg]

Yes, you should use a PNP transistor, with positive into the emitter, and out from the collector - less voltage drop this way.

but if u take output from collector ..then it will act as a current source rather than a voltage source.. pushing a const: current to his module .

 

[Nigel Goodwin]

but if u take output from collector ..then it will act as a current source rather than a voltage source.. pushing a const: current to his module .

No, it will act as a switch - a constant current source isn't just a transistor, it's the rest of the circuit that makes it one. PNP transistors are commonly used for this purpose, it's FAR better than an NPN one (unless you're switching a negative rail of course?).

 

[Oznog]

Why are you talking about bipolars? He's talking a 3.3v rail here. The Vsat of around 0.3v is probably way too high for this.
You need a PMOS, but you need a special one with a very low threshold. Start by looking for "logic level MOSFET" and be aware that it probably won't be able to switch the full rated current of the device at vgs=-3.3v, so an oversized device may be necessary.

 

[Someone Electro]

Yes use a mosfet its volateg controled, and mosfets have resistance rather than volatge drop so this way les volatge gets lost in in transsitor.

 

[Roff]

Fairchild, for example, has PMOS transistors with Rds(on) as low as 14 milliohms at Vgs=2.5V (FDW258P). They have quite a few others that are specified down to Vgs=1.8V, as this one is.

 

[justDIY]

another thing to check is the datasheet for your GPS ... it probably already has a turn-off or sleep command (or a sleep/enable line), eliminating the need for an electronic switch. even my 90's era garmin has such a command, I'd hope more modern receivers would follow along the same lines.

 

[Papabravo]

One last thing to watch for is a sneak current path. If the Vdd of the GPS chip is close to ground and there is still a clock or a data pin which is high current may flow through that path to ground. The saving you get might not be what you expect. You might get more bang for the buck by stopping the clock or finding out if the GPS has a software controlled SLEEP mode.

Just an aside but what will be the effect on TTFF of turning off the Vdd to the GPS chip?