Help with a constant-current buck-boost regulator I designed... - Page 1
Forum section: electronics / projects

author: SAR_mango
author_rank: Newbie
timestamp: June 28, 2021, 08:07:35 pm
content: Hello everyone,First post! I've been watching Dave for years, though.I designed a low-mid power (up to 200mA out) laser driver that has a very wide input and output voltage range. I wasn't too happy with the limitations of the drivers people usually buy. Also, I needed a low battery indicator.Here's a little explanation of my schematic, although it doesn't differ much from other constant-current buck-boost drivers:- I used the LTC3129IMSE for its wide Vin and Vout ranges.- PWM pin is pulled low for efficiency at low output currents.- Power good pin goes low when it detects an undervoltage (batteries too low), which allows the LED to turn on. The pin can't sink much current so the LED has a series resistor to VCC. I tested this and it works fine.- Output is AC-coupled to the feedback pin to reduce ripple. Removing that 10pF capacitor had basically zero effect on output waveforms.- Output flows through a 1Ω shunt (a little high for a shunt, but still very little power loss if max output current is 200mA)- Shunt voltage is amplified by the MAX9938W. See the datasheet for that chip and look at my schematic. I actually set it up for adjustable gain even though it is meant to provide only a fixed gain. Surprisingly, this also worked when I tested it.- Gain ranges properly calculated and set to match the output current range I specified in the schematic. I measured this as well; it seems like it's working. I measured it by sending a constant current through the shunt, turning the potentiometer while measuring the output of the amplifier.- Output goes to LTC3129's feedback pin, completing the loop.- Filter caps everywhere! 22µF should definitely be enough.So the problem is with the output. It doesn't regulate constant current very well. There is a LOT of low-frequency (audible range, around 13kHz) output ripple throughout the current range. I have no idea what's wrong because the feedback is working as expected. Maybe it is because I pulled the PWM pin low, but that doesn't explain the poor constant current regulation. Anyone have any ideas regarding what I am doing wrong? Any help would be greatly appreciated. I also posted this on the Laser Pointer Forums.datasheets.maximintegrated.com/en/ds/MAX9938.pdfanalog.com/media/en/technical-documentation/data-sheets/3129fc.pdfEdit: fixed list

author: Zero999
author_rank: Super Contributor
timestamp: June 28, 2021, 08:15:02 pm
content: Why not use a constant current LED driver IC?

author: SAR_mango
author_rank: Newbie
timestamp: June 28, 2021, 08:25:11 pm
content: I didn't really look at those... they might actually be a better option. I used the LTC3129 because I looked at the schematic of one popular commercially-available driver and they just used a regular buck-boost chip as well. I'll probably switch to an LED driver chip, but it would still be good to know what might be wrong with my current schematic.

author: SAR_mango
author_rank: Newbie
timestamp: June 28, 2021, 08:31:28 pm
content: Also, min Vin of those chips seems to be 2.5V; that's a bit high for me. I wanted a maximum of around 2.2V since I'd be using this with AAA batteries.