Re: Driving a line from an opto-isolator


John Popelish wrote:
ianmazz@xxxxxxxxxxxxxx wrote:
Hello,

I have a board with a 74HCT573 with 10K pull-ups on the inputs. This is
connected to a sensor with an opto-isolator output (4N25) via 2m of
cable. The resistor for the opto led is 10K on a 5V supply, so
according to data*** the CTR can be as low as 20% at that drive
level. So the transistor output could be pulling as little as 80uA. I
can see them asking me to increase my 10K pull-ups by a factor of >
6.25.

What is the limiting factor here? Surely you cannot keep reducing the
current and expect the cct to work. Even the 500uA seems low to me.

The minimum current that can drive the CMOS as a pull up
depends on the leakage current into the COS, and the off
state leakage current of the opto. But it is possible that
you cannot get very close to this limit, because there is a
speed penalty to pay when the currents are small. The node
capacitances still need to be charged up by the current.
And that takes time.

Thanks John,

that makes sense

Ian

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