I have been using multiple Femtobuck drivers to run a few series of 6-10 LEDs. With either no control input or with PWM input, it works as expected. But with analog voltage from a PWM->Analog DAC on the Femtobuck control pin, I can’t get it to work properly.
I have a DAC circuit (Analog Devices LTC2644) that takes the PWM in and outputs analog voltage in a range from 0-2.5V. This works as expected. But no matter where I set it within that range, the light intensity never changes.
If I put pure PWM into the Femtobuck, 100% is full brightness, 50% duty cycle is half as bright. With the DAC, 2.5V should be 100% brightness, but it’s dim, and 1.2V should be about half, but is effectively the same as 2.5V.
Problem is, we need to feed the driver analog voltage for dimming because we’re using this as the light source for a camera, and with PWM we get dark lines in the image, from the off cycles in the PWM signal.
Analog dimming works when I use a benchtop variable power supply to change the input voltage. But why wouldn’t it work with the DAC, which is outputting voltage in the same range as the bench power supply?
Here’s the circuit for the DAC. PWM comes in on pin 9, 0-2.5V analog voltage out is on pin 2, into the femtobuck.
https://live.staticflickr.com/65535/515 … e85a_o.jpg
