I'm building a circuit powered from a 12V 55mAh (A23 battery). Needless to say that power consumption is a primary concern. The circuit is an ATtiny85 which is driving 13 LED's (in 5 groups). I initially used a voltage divider circuit to reduce the 12V down to 5.5V to supply the ATtiny85. This worked but I found the voltage divider was eating up too much current, 5mA, even when the microcontroller is put to sleep and all LED's are turned off. When active it consumes 5mA to 7mA. I replaced the voltage divider with two 1N4733A 5.1V Zener Diodes in series:
This circuit works beautifully for saving power. Power consumption with this circuit is in the micro-amp range when the ATtiny is sleeping (too low for me to measure) and consumes only 0.1 mA to 0.3 mA when the ATtiny is awake and pulsing the LED's. A FIFTY fold decrease in power consumption.
The trouble with the new circuit is that it is very sensitive to the supply voltage. At 12V input (using a bench power supply) the circuit fires up and works well. I get around 5.1V at Vcc. At 11.5V supply voltage the circuit will not start.
When I was using a voltage divider instead of the diodes the circuit worked well down to about 8V supply voltage. With the diode setup it needs 12V to start and will keep running down to about 11.5V before it shuts off.
Is there a way to modify this circuit so that it works over a wider range of supply voltages? Ideally down to around 6V (as the battery discharges). The datasheet's I've looked at for the Zener diodes don't mention anything about the expected voltage drop across the diode when it's conducting at the breakdown zener voltage. If I replace D1 and D2 with 3.3V Zener's should I get the same voltage drop? I'm not sure how to calculate the expected voltage drop at Vcc.