Hey guys,
So I decided to use an IO-expander IC in a design, and I was not paying attention to the amount the GPIOs can source and sink. This thing can sink 20mA, and for some reason I assumed it could source close to 20mA as well.....turns out it can only source 0.1mA max on a GPIO! Because of this the MMBT3904 transistor cannot activate.
Datasheet for IO-expander:
https://www.nxp.com/docs/en/data-sheet/PCF8574_PCF8574A.pdf
Circuit:
Would a good solution be to use a darlington transistor instead, such as this one? (MMBTA13 or MMBTA14)
Datasheet:
http://www.mouser.com/ds/2/258/MMBTA13-MMBTA14(SOT-23)-349671.pdf
The old transistor had a gain of 30, while this one has a gain of 10,000, thus only requiring around 0.01 mA instead of 4mA.... if I am understanding this correctly that is. So if it only needs 0.01, then I should be able to drive this directly with my IO-expander IC I am thinking, and don't need to change anything else.
Is there a better way I should re-design this for the future? Or is the darlington transistor a good way of doing this?
Thanks and any help or advice is greatly appreciated!
So I decided to use an IO-expander IC in a design, and I was not paying attention to the amount the GPIOs can source and sink. This thing can sink 20mA, and for some reason I assumed it could source close to 20mA as well.....turns out it can only source 0.1mA max on a GPIO! Because of this the MMBT3904 transistor cannot activate.
Datasheet for IO-expander:
https://www.nxp.com/docs/en/data-sheet/PCF8574_PCF8574A.pdf
Circuit:
Would a good solution be to use a darlington transistor instead, such as this one? (MMBTA13 or MMBTA14)
Datasheet:
http://www.mouser.com/ds/2/258/MMBTA13-MMBTA14(SOT-23)-349671.pdf
The old transistor had a gain of 30, while this one has a gain of 10,000, thus only requiring around 0.01 mA instead of 4mA.... if I am understanding this correctly that is. So if it only needs 0.01, then I should be able to drive this directly with my IO-expander IC I am thinking, and don't need to change anything else.
Is there a better way I should re-design this for the future? Or is the darlington transistor a good way of doing this?
Thanks and any help or advice is greatly appreciated!