Only 30 mA per pin? (P1 has 40 mA per pin)

Rayman

Just noticed this (but maybe I knew and forgot...)...

Was remembering how in a P1 system, I have it directly driving a communication type relay with coil impedance of 91 Ohms for 36 mA at 3.3 V. Figured this was OK as the datasheet says good for 40 mA.

Now, I'm planning on converting that system to P2. I see now that datasheet and hardware manual say the limit for P2 is 30 mA per pin.

Not a big deal, 30 mA is still a lot, but just have to keep that in mind.

Tubular

The P2 has significantly stronger drive than the P1, around 19 ohms from memory. The P1 was around 28 or 29 ohms (again from memory).

Also, the P2 only shares a VIO pin 4 ways, vs 8 or 16 on the P1 (and having to include core current too)

I suspect the figures on the datasheet perhaps come from the formal OnSemi process, or perhaps reflect a different load point strategy

evanh

You certainly wouldn't want every pin of the Prop1 doing that much, combined that's 1280 mA through Vdd pins. Compare that to the max rating of the core logic, which is about 100 mA.

cgracey

I think the reasoning behind the 30mA per pin was that four of them (which are driven from single-bonded GND and VIO pads) at 30mA each would create draws of 120mA per 1-mil GND/VIO bond wire. I believe the 1-mil bond wire was rated for around 120 mA. If you want to run 2 pins at 60mA each from a GND/VIO pair, that should be fine.

Mickster

Heck, not for me. I am buffering with the Micrel 2981. Just feels better

Craig

Rayman

@cgracey Thanks! Glad to hear I can stick with driving that relay directly from a P2 pin. Just have to be mindful of the total current in each quad...

Rayman

I don't really remember if I took the P1 internal impedance into account when using that 91 Ohm relay coil. If P1 source impedance really is 29 Ohms, then there'd only be 2.5 V on the coil that needs 2.6 V to be in spec...
Also, I guess the current would be less that I said above...

So, this will actually be much better with P2. Would get 2.7 V with 19 Ohm source impedance. I'll have to remember to check this. I'm sure I did with the P1, but it was a long time ago...
That would put the current right at 30 mA too.

cgracey

@Rayman said:
I don't really remember if I took the P1 internal impedance into account when using that 91 Ohm relay coil. If P1 source impedance really is 29 Ohms, then there'd only be 2.5 V on the coil that needs 2.6 V to be in spec...
Also, I guess the current would be less that I said above...

So, this will actually be much better with P2. Would get 2.7 V with 19 Ohm source impedance. I'll have to remember to check this. I'm sure I did with the P1, but it was a long time ago...
That would put the current right at 30 mA too.

And you can always gang TWO pins together to get lower impedance.