IN5817 USB-backpower protection Surface Mount alternative
JellyTitan opened this issue · 2 comments
1N5817 is solid for add-in USB-backpower protection diode purposes, but could be swapped /optioned with a lower forward voltage alternative if a SOD123 surface mount pad option was added. The surface mount package reduces power consumption/heat from a reduced forward voltage current response. The MBR120VLSF diode already used for D1 on the Pico is pretty ideal and in the same SOD123 package as the 1n4148, so anyone who's open to surface mount shouldn't be intimidated, so making it the option for D31 would be nice.
Originally posted by @TheWerle in #11 (comment)
Having a surface mount alternative for the USB-backpower protection diode would be nice!
@JellyTitan
I'm starting board bring-up and integration. When I initially put the module on the LEDS wouldn't light because there was no "VCC"/'VDC" system voltage present, leading me to a shocking revelation....
The black USB-C Pico clones you link (Tenstar Robot / YD-2040) DO have a diode onboard already, its just a SOT23 dual-diode package. They've swapped the pins on the module, its not truly RPI Pico compliant. Pin40 "VOUT" is schematically equivalent to "VSYS" on the true RPI Pico schematic, hence the pin39 "VIN" label where with the second diode rectifies the secondary power input. The external diode isn't needed, by using it you're effectively serial chaining diodes and burning extra power. You should jumper D31 or Pins 39-40 to connect VSYS to the intended net.
As a downside, I think this breaks the ability to use the black Tenstars along with MOSFET dual-power protection circuit, as on the keyboard PCB VBUS and VSYS will both always be present and it would trigger the gate-switch. I bought 2 of the black Tenstars and 2 of an alternative vendor that more closely emulated the original Pico, I'll try swapping between all of them and will update with any downstream consequences.
Great work! I'm looking forward to your findings. I'd really love to simplify things for the end user. I know a bunch of clones drift from spec - maybe we should consider pulling the patch bay, considering people can wire things into the opposite side pico header pins if they really want to tinker. Plus that would free up some boards space for I2C pull ups and the overvoltage protection circuits?