Many many thanks for this quick reply again. We are also experimenting with stobing the LEDs very fast using digital out of the MCU's I/O to kind of PWM the LED to calibrate it properly for our application .I plan to use 3 x AA batteries. If I add a 4th battery, The solution has to be really cheap since there will probably be 4 constant current circuits -
The 74HC14 is cost effective, so it's a possible solution if the circuit works well. Yes - I need to be 100% sure that the LED brightness is the same for full battery to say 80% battery
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You could look at this TPS61043: Constant Current LED Driver
The TPS61043 is a high-frequency boost converter with constant current output that drives white LEDs or similar. The LED current is set with the external sense resistor (RS) and is directly regulated by the feedback pin (FB) that regulates the voltage across the sense resistor RS to 252 mV (typical).
The 74HC14 can be used to drive-light 4-6 LEDs. It can be directly connected to MCU.
The LEDs should be flashed at around 5 Khz. Then a constant current source is not required.
Why do you wish to use constant current. Is it for a steady light.
If the problem is dropping voltage. Another battery or voltage doubler is required
An voltage doubler may not be cost effective. Another battery may be a simpler solution.
By using a 74HC14 you can avoid the PNP transistors. This is an inverter, you can use a buffer chip too.
A series resistor with LED or 10Kz flash is to limit current.
The 74HC14 just replaces your PNP transistor saving 1/2 a Volt. The Microcontroller PWM can be used to control brightness. Then you can skip even the series resistor. You can even use a 555 chip for pwm.
Flashing must be above 1 KHz. The persistence of Vision is 20mS. Very high rate will make it dim, something optimum.
A better Idea is to use LM317 to get 3.3V regulated supply from 5-6V of batteries. Use this regulated supply to drive LEDs. This also needs to be tried practically. As voltage is constant the current in leds will be constant. for 20% drop.