AMS1117 3.3v volt regulator draws power when bypassed

Hello,

I am creating a wireless meshing sensor node focusing on low power usage and using all SMD parts. I am powering the unit via USB or a CR2032 coin battery.

I am using a 3.3 volt regulator(AMS1117 3.3v) when my board is connected via USB to drop the voltage from 5v to 3.3v.
I also have a CR2032 3volt battery which bypasses the voltage regulator (connects to the regulator outputs). The battery is only connected when not powered by USB.

The problem I have is voltage regulator draws power when not used (4mAh), when in use I don't care about its power usage but when I power my board with a CR2032 battery it is drastically reducing my battery life. I have my board sleeping and waking only using about 0.4/0.7mAh when the power regulator is removed but when connected I am getting 4.4/4.7mAh.
Peak power consumption is only during radio RX/TX and is about 14mAh@3v (without regulator connected)

I don't want to add a switch due to size and usability reasons.

I have tried connecting a diode to the output that fixes the power usage problem but creates new problem of a voltage drop off of about 0.7volts when powered by 5volts so instead of getting 3.3volts I only get 2.5volts.
If I was do go down the diode path what is the best diode to use?

John posted how to use a diode without voltage drop over here http://digital-diy.com/forum/general-electronics/mini-tip-diode-without-a-voltage-drop-t2206.html#p13262

John did? I thought that was me that posted that :stuck_out_tongue:

Thanks be80be / majenko

The P-channel MOSFET does look a good idea but due to cost and size I might end up going with the diode options. I was hoping for something surface mount.
I was thinking about ether finding a diode that has a voltage drop off less then 0.3volts so I still have 3volts to power the device or replace the AMS1117-3.3 with a 4volt version and use the voltage drop to leave me with 3.3volts.
I think I have a few laying around that I have savaged so will give it a go anway but looking around I found that I can get SMD Switching Signal Diodes really cheap so I might have to explore that option more.

You can get SMD P-channel mosfets.

Do you have an Arduino UNO board? Component "T1" - not actually labeled on the board, but between the voltage regulatro and the "500Z" (or is it "Z005"?) fuse, is a p-channel mosfet used for switching between USB and external power.

Or - look at this image: http://www.siliconsidewalk.com/product_images/l/357/ArduinoUNO_Front__37376_zoom.jpg. On the left side of the board - top to bottom - USB, Fuse, P mosfet, Regulator, Power socket.

I guess you did post that But there was the same thing posted by Jon. That site you can't tell who did what it's changed to much. Still great idea majenko not new but Nice

The OP can use smd part it would be very small foot print.