Hi folks,
I got inspired by this Geiger counter kit and I'm now designing my own which will be similar. It will be powered by two AA NiMH cells which give only 2.4V when nearly empty, and is converted to 5V by a DC-DC step-up converter from Dealextreme. That kit mentioned draws current up to 50mA and I'm not expecting any less from mine, which will mean drawing more than 100mA by the step-up converter.
Instead of using mechanical power switch, I want to use high-side PNP transistor which is initially turned on by pressing the power button, and then the microcontroller I/O pin holds it up so the whole device can be shut down by software (see the attachment). I have tested this and it works okay with 5V, however with 2.4V input I'm having problems to get minimal voltage loss and base current for the transistor. Even with 2.2k base resistor there's 0.25V of Vce loss when having 80mA load, so I guess it's not fully saturated. By reducing the base resistor to 560 ohms I get thru 150 mA with Vce of 0.1V, however the base resistor alone consumes then 2.6mA, and with full batteries (2.8V) it's 3.5 mA. Smaller BC557 would have higher hFE so needing less base current, but it's rated max. 100 mA. With a 5V wall-wart I could happily use BD136 and ignore the loss, but with 2 AA's I have to pay attention to minimize current usage.
My question is, would any MOSFET do better in this case and how to connect it? My knowledge of MOSFETs is very limited, but I know they can have almost zero voltage loss and the gate doesn't consume current like BJT's base does. However, is 2.4V enough for any MOSFET to get saturated? I wanted to use 4 AAs instead of 2, but the only suitable enclosure with battery compartment I could find was for 2 AAs so I have to live with it.
I got inspired by this Geiger counter kit and I'm now designing my own which will be similar. It will be powered by two AA NiMH cells which give only 2.4V when nearly empty, and is converted to 5V by a DC-DC step-up converter from Dealextreme. That kit mentioned draws current up to 50mA and I'm not expecting any less from mine, which will mean drawing more than 100mA by the step-up converter.
Instead of using mechanical power switch, I want to use high-side PNP transistor which is initially turned on by pressing the power button, and then the microcontroller I/O pin holds it up so the whole device can be shut down by software (see the attachment). I have tested this and it works okay with 5V, however with 2.4V input I'm having problems to get minimal voltage loss and base current for the transistor. Even with 2.2k base resistor there's 0.25V of Vce loss when having 80mA load, so I guess it's not fully saturated. By reducing the base resistor to 560 ohms I get thru 150 mA with Vce of 0.1V, however the base resistor alone consumes then 2.6mA, and with full batteries (2.8V) it's 3.5 mA. Smaller BC557 would have higher hFE so needing less base current, but it's rated max. 100 mA. With a 5V wall-wart I could happily use BD136 and ignore the loss, but with 2 AA's I have to pay attention to minimize current usage.
My question is, would any MOSFET do better in this case and how to connect it? My knowledge of MOSFETs is very limited, but I know they can have almost zero voltage loss and the gate doesn't consume current like BJT's base does. However, is 2.4V enough for any MOSFET to get saturated? I wanted to use 4 AAs instead of 2, but the only suitable enclosure with battery compartment I could find was for 2 AAs so I have to live with it.
