verify BSS84 and BSS138 can be driven with 1.8V gate voltage
|Status:||In Progress||Start date:||09/16/2013|
|Assignee:||Andrew Kohlsmith||% Done:|
The PIC can operate down to 1.8V. Determine Voh and verify that the FETs that drive the UART LEDs and GPIO can operate down to that level.
#1 Updated by Andrew Kohlsmith over 6 years ago
- Status changed from New to In Progress
- % Done changed from 0 to 20
The BSS84 VGS threshold is -1.7V typ and -2V max. The BSS138 VGS threshold is 1.3V typ and 1.5V max.
The PIC’s specifications are preliminary only, but VOH is VDD-0.7V with moderate loading (1mA at 1.8V VDD). VOL is 0.6V (1.8mA at 1.8V). With high-efficiency LEDs and some tweaking of the LED resistors we should be able to draw less than 1mA, but without a better answer on VOH and VOL we might be in trouble.
Several options if we’re not able to use the FETs:
- remove the FETs and drive the GPIO directly. This isn’t a bad idea, we already have current limit resistors in the design to help protect the FET, which would now protect the PIC.
- find a FET with even smaller gate drive requirements
- use transistors instead of FETs which are current controlled devices (with smaller base voltage requirements)
- don’t worry about the LEDs on the UART pins.