Dynomotion

Group: DynoMotion Message: 9390 From: wgripp Date: 4/10/2014
Subject: Jp6 pwm pins output current
Hello, I'm planning to drive some optocouplers with the pwm outputs on Jp6, but I fail to find in the documentation the maximum allowable current. Can I have some advice for this ? Thanks a lot
Group: DynoMotion Message: 9391 From: Tom Kerekes Date: 4/10/2014
Subject: Re: Jp6 pwm pins output current
The 8 PWM signals are LVTTL Outputs with a minimum of 16ma of source and sink capability but have 150ohm termination resistance to GND which reduces their source drive significantly.

HTH
Regards
TK

Group: DynoMotion Message: 9392 From: wgripp Date: 4/10/2014
Subject: Re: Jp6 pwm pins output current
Hello,
thank you for your answer Tom.

Unfortunately, my shallow knowledge in electronics prevents me to make good use of the information. I am concerned about damaging the KFlop by drawing too much current from the pins: in my hypotetical design one PWM should be able to drive two optocouplers.

As far as I can see, with pulldown resistors, when the voltage at the pin is high, a 22mA current flows. I guess I can lower the current in the optocouplers as much as they still work, but still I have no idea on how low I should keep that. Or I should change my design.

I hope I've better explained my concerns and sorry if this question departs from being KFlop specific.

Thank you for any help.
Group: DynoMotion Message: 9393 From: Tom Kerekes Date: 4/10/2014
Subject: Re: Jp6 pwm pins output current
You would probably want to put a resistor in series with the LED of your Opto.  Maybe connect the Anode of your LED to +3.3V then through some series resistor to the KFLOP pin.

Something like a 120 ohms resistor and assuming the LED has a voltage drop of 1.4V would sink:

(3.3V - 1.4V) / 120 = 16ma

What type of opto are you considering?  How fast do you need it to switch?

Regards
TK

Group: DynoMotion Message: 9394 From: wgripp Date: 4/10/2014
Subject: Re: Jp6 pwm pins output current
Thanks a lot for the clarification !
The question really aimed to have a parameter to determine the series resistor, but I completely missed the fact that 16mA is the available minimum I should keep close to.
The optocoupler is still to be determined, but I've been told (by a smart and knowledgeable guy that is guiding me) that even ACPL844 should do. http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_general_purpose_phototransistor_optocoupler/acpl-844-000e/

Thanks again !