Hi everyone,
First post on LPF! This is for the more electronics-oriented people at least for now, although I will gladly release the design files if this thing works and might even sell finished boards.
I designed a low-mid power (up to 200mA) driver that has a very wide input and output voltage range. I wanted to be able to use two AAA OR two 10440 Li-ion cells in series. The Micro Flexdrive V5 only supports the former while the other buck-boost drivers on DTR's website only support the latter.
Also, since I love the Leadlight Pen Host for my builds, I wanted the driver to have a low battery indicator for the small hole on the host. I measured the switch PCB typically used with the host and designed my board with those dimensions, placing the LED where it would be under the small hole when installed. The board contains the switch as well, so no need for a switch PCB and a separate driver. I tested this and everything is aligned and fits.
Here's a little explanation of my schematic, although it doesn't differ much from other constant-current buck-boost drivers:
*Sorry, I can't post proper links because I don't have enough posts yet.
datasheets.maximintegrated.com/en/ds/MAX9938.pdf
analog.com/media/en/technical-documentation/data-sheets/3129fc.pdf
First post on LPF! This is for the more electronics-oriented people at least for now, although I will gladly release the design files if this thing works and might even sell finished boards.
I designed a low-mid power (up to 200mA) driver that has a very wide input and output voltage range. I wanted to be able to use two AAA OR two 10440 Li-ion cells in series. The Micro Flexdrive V5 only supports the former while the other buck-boost drivers on DTR's website only support the latter.
Also, since I love the Leadlight Pen Host for my builds, I wanted the driver to have a low battery indicator for the small hole on the host. I measured the switch PCB typically used with the host and designed my board with those dimensions, placing the LED where it would be under the small hole when installed. The board contains the switch as well, so no need for a switch PCB and a separate driver. I tested this and everything is aligned and fits.
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.
- The 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.
- The output is AC-coupled to the feedback pin to reduce ripple. Removing that 10pF capacitor had basically zero effect on output waveforms.
- The output flows through a 1Ω shunt (a little high for a shunt, but still very little power loss if max output current is 200mA)
- The 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.
*Sorry, I can't post proper links because I don't have enough posts yet.
datasheets.maximintegrated.com/en/ds/MAX9938.pdf
analog.com/media/en/technical-documentation/data-sheets/3129fc.pdf