Driving Common Cathode display

[TechnoGilles]

Hi all,

Normally, when driving high current/voltage loads from an MCU, I simply insert an N-Channel MOSFET between my load and GND and that's it.

But now, I'm in a situation where I have to insert my MOSFET switch between my load and its VCC supply (my load is a common cathode display so I have to control it via its anodes). My MCU supply is at 3V3 and my load supply is at 7V5.

What type of MOSFET should I use in this case and in which configuration ?

Thank you !

 

[MikeMl]

Almost any PFet will work, as long as the max Vds is consistent with your supply voltage.

 

[TechnoGilles]

Ok. Great. I think I get it !

- We use a P-channel FET because we are working at the positive supply
- R2 is used to "pull up" the FET's gate at V1 level
- Q1 is used to bring the FET's gate down to GND to turn on the FET

I'm not sure of the role of R1 here. Couldn't I connect the MCU output directly to the base of Q1 ?

Thanks a lot !

 

[MrDEB]

ever consider using a ULN2803?
they solve lots of such problems.

 

[MikeMl]

Ok. Great. I think I get it !

- We use a P-channel FET because we are working at the positive supply
- R2 is used to "pull up" the FET's gate at V1 level
- Q1 is used to bring the FET's gate down to GND to turn on the FET

I'm not sure of the role of R1 here. Couldn't I connect the MCU output directly to the base of Q1 ?

Thanks a lot !

R1 is used to limit the base current of Q1. Imagine what would happen (without R1) if the Pic Pin is high. The Pic Pin is capable of sourcing ~25mA, but because the base-emitter junction of Q1 is a forward-biased diode, the Vbe of Q1 is at ~0.7V, which holds the Port Pin at 0.7V.

If you put R1 in, then the Pic Pin can rise all the way to near 3.3V, and the current sourced by the pin is ~(3.3-0.7)/R1 = 2.6/4700 = 550uA, which is more than enough to saturate Q1. So it comes down to a 1/2mA vs 25mA, wasted power, heating the PIC, heating Q1, etc.

 

[crutschow]

ever consider using a ULN2803?
they solve lots of such problems.

It doesn't solve this problem. The ULN2803 is a grounded emitter NPN Darlington, which is used for driving high-side loads. Here the op has a common cathode display which needs to be grounded, thus requiring a high side driver.

 

[TechnoGilles]

Make sense !
Thank you Mike !