T
tehtehteh
I'm designing an in-car circuit with 2 step down converters (lm2596), to put them into standby the on/off pin must read high
I'm using a picaxe microcontroller powered with a linear regulator to control them, the linear regulator wastes too much power to run off the battery alone, so when the car is off the picaxe will also be off, this left me with a bit of a challenge in that I had to let the on/off pins default to high and have the picaxe come on and set them low (does this make sense?)
I have come up with the attached circuit
my issues with it is the on/off pin has to see potentially up to 2v for high with a max of 25v, so for protection I added R2 to make that into a voltage divider so the 12v can basically go up to 48v without doing any damage, but it wastes a lot of power (yenka says less than 0.7ua without R2 and 130ua with)
the converters use up to 250uA each in standby mode, so with 2 of these on/off circuits added I'm looking at 0.75mA standby current, which is too much
I'm also wondering does the on/off pin draw any current and can it cause current to flow from the transistor base out through the collector, but my knowledge isn't good enough to really understand what's going on there
so, thoughts and suggestions please?
thanks for reading
I'm using a picaxe microcontroller powered with a linear regulator to control them, the linear regulator wastes too much power to run off the battery alone, so when the car is off the picaxe will also be off, this left me with a bit of a challenge in that I had to let the on/off pins default to high and have the picaxe come on and set them low (does this make sense?)
I have come up with the attached circuit
my issues with it is the on/off pin has to see potentially up to 2v for high with a max of 25v, so for protection I added R2 to make that into a voltage divider so the 12v can basically go up to 48v without doing any damage, but it wastes a lot of power (yenka says less than 0.7ua without R2 and 130ua with)
the converters use up to 250uA each in standby mode, so with 2 of these on/off circuits added I'm looking at 0.75mA standby current, which is too much
I'm also wondering does the on/off pin draw any current and can it cause current to flow from the transistor base out through the collector, but my knowledge isn't good enough to really understand what's going on there
so, thoughts and suggestions please?
thanks for reading