AVR controlling subsystem via FET (minimum power usage)

Go To Last Post
5 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi,

My problem:
Mega324P running at 5V, wanting to control a subsystem running at 7.5V, upto about 500 mA. I need to control the high side of the subsystem (ground will remain connected). I also need minimal (if any) drain current in the unactivated state - the AVR will be 'sleeping' most of the time, also minimum part count / size would be good.

My understanding is something along this line (number 3) [from ka7ehk / Jim] -
https://www.avrfreaks.net/modules/PNphpBB2/files/drivers.gif

Will work - but I'm needing a bit more handholding / pointers to some application notes / examples for the part numbers / resistor values (or how I can calculate them). I'm looking for the FET equiv of the BC557 / BC546 type 'generic' components. Do I need JFET, MOSFET, IGFET... N or P.... I was originally thinking one P - but now am confused.

I've been digging and struggling to find the key information. I'm sure this isn't an uncommon basic requirement - someone must have writen it up!

Thanks in advance,

Carl

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

How fast transitions do You need ?

There are Pchannel mosfets available in SOT-something package (the small SMD package).

You will need two resistors and two mosfets. This if there are no special demands for fast switch times.

Attachment(s): 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi,

Thanks for that.... Your correct in the assumption - no special rise / fall demands - it is to be used more like a low power relay. Power up and wait for the subsystem to stabilise, use, shutdown.

Your schematic looks interesting - the 2N7002 seems like a common N-type FET which is easy to source locally.

The IRLS9422 number I'm struggling to read - can you confirm? And it is a P-type?

For my education, I'm working on the theory that the AVR sets the output high - that results in a voltage/current at gate of T1, causing it to pass current between V and ground which then feeds the G of T2 which provide power to the subsystem.

When the system is sleeping - does any current go through R2-T1-R1 ?

Thanks for your help,

Carl

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:

When the system is sleeping - does any current go through R2-T1-R1 ?

Only the T1 cutoff current - negligible - will flow through T1 and R2 (and T2 gate leakage current (even more negligible)). Do keep in mind the state of the port driving T1 under all conditions, eg. when the uC itself is powered down or initializing.

If you think education is expensive, try ignorance.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Sorry, there is a typing mistake in the type of T2.

The type I was thinking is IRF9622. It is a 3A TO220 P-Channel Mosfet. P-Channel mosfets are a tad harder to come by but still they are not that bad. This particular type may be obsolete but You can substitute it by almost any P-channel mosfet as long as the gate threshold voltage is suitably small and the current capacity is big enough.

Your supply voltage is so small that this is not going to be a problem. I also assume here that the 7 volt supply never goes beyond 18 volts. If it does that then the gate insulation on T2 will fry.