Task #438

verify BSS84 and BSS138 can be driven with 1.8V gate voltage

Added by Andrew Kohlsmith about 6 years ago. Updated about 6 years ago.

Status:In ProgressStart date:09/16/2013
Priority:NormalDue date:
Assignee:Andrew Kohlsmith% Done:

20%

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Target version:-
Duration:

Description

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.

History

#1 Updated by Andrew Kohlsmith about 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:

  1. 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.
  2. find a FET with even smaller gate drive requirements
  3. use transistors instead of FETs which are current controlled devices (with smaller base voltage requirements)
  4. don’t worry about the LEDs on the UART pins.

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