Drive Current Sufficiency

[gunayburak]

Hello Everyone ;

The question in my mind might be quite simple for some experienced users maybe . Here it is .. Since I read the datasheet of the 18f87k22 I want to use all the pins (except MCLR of course )of the mcu to drive relays .. So to do this instead transistors I'll use ULN2003A . But as far as I read the datasheet the source/sink currents for some ports supply less current (around 2 ma) then we are used to from other PIC MCUs classic 25 ma source current . So I checked the input current vs input voltage graphics of ULN2003a and it says max 1.25 ma input current is required to drive an input of the uln2003a .As seen it is quite close to the specified 2 ma max source current , so can I possibly encounter problems in the future ? I ask these questions because I don't have neither the mcu nor the uln2003 so I can't make an experiment on this matter .. Your valuable experiences are required .
Regards

[HenrikOlsson]

I'm pretty sure you'll be fine. You have a margin of 60% even under worst conditions. What collector current are you designing for and do you have a specific ULN2803 in mind (several manufacturers makes them, they may not all be equal).

[gunayburak]

Henrik Thanks for being my guardian savior angel again :) . The relays have 1028 ohm coil resistance so that returns a 11.67 ma collector current for per relay at 12V .ULN2803 is not an option for this project since we doesn't have a chance for the component stock .

[gunayburak]

Also do you think this MCU's weak current supplying ports can handle and succesfully drive the A/B/C/INH signal inputs of CD4051 and A/B/C/D/INH signal inputs of 4514 ?

[HenrikOlsson]

I think perhaps we've both misinterpreted the datasheet a little bit.
Look at table 6.5 in this Texas Instruments datasheet (http://www.ti.com/lit/ds/slrs049h/slrs049h.pdf). There we can see that indeed the maximum input current is 1.35mA at 3.85V but a little bit further down we can see that an input current of 250uA will still produce a collector current of 100mA while only dropping a maximum of 1.1V.

Input current on CMOS series logic chips seems to be in the uA range so no problems there.

[gunayburak]

Don't you think the current at 3.85 V proportionally should increase at 5V drive voltage..? Secondly do you have any idea with my other question ?




Also do you think this MCU's weak current supplying ports can handle and succesfully drive the A/B/C/INH signal inputs of CD4051 and A/B/C/D/INH signal inputs of 4514 ?

[gunayburak]

I'm sorry I've been a victim of a mobile phone browsing because I haven't seen the line with the CMOS sentence :)

[HenrikOlsson]

Don't you think the current at 3.85 V proportionally should increase at 5V drive voltage..? Secondly do you have any idea with my other question ?
It might, if it was allowed to but it can't since the output can only source 2mA. If you're worried, why not measure it? Add an external series resistor if you want to be sure that the circuit can't ever pull more 2mA.


Secondly do you have any idea with my other question ?
Please read my previous reply:

Input current on CMOS series logic chips seems to be in the uA range so no problems there.

[gunayburak]

As I said , I'll design the PCB according to the technical specs of all these ICs so measuring it will be possible only when the PCBs arrive .. But so far you've been very helpful to all my questions digging me ...

Thanks so much again :)