What is the driver output current when unpowered, IO(off), and the driver output current when disabled?
I could only find the receiver output current when disabled in the data sheet.
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What is the driver output current when unpowered, IO(off), and the driver output current when disabled?
I could only find the receiver output current when disabled in the data sheet.
Hi Erich,
The specification you are looking for is Ib - or the bus input current. Its listed in the receiver section - but this still applies to your question.
Ib is measured with VCC = 0v to 5.5V with DE = 0V - so the device is off or on and the driver is disabled - since A and B act as driver and reception pins that is the spec that will be closest. We measure at -7V and 12V (and for this part also -15 and 15V) in these cases - but this spec is more or less linear (or can pretty safely assumed to be) so you can take a linear interpolation to get an estimate at an input voltage. When I run a linear interpolation of the data in teh datasheet I get two equations:
1. Linear Appx. Typical values based on input voltage: Ib = 6u * Vi - 30u with an R^2 value = 0.9941
2. Linear Appx. Max/Min Values based on input voltage Ib = 10u * Vi - 30u with R^2 value = 0.9923
While these equations are not perfect and don't fully capture every potential value - that should guide you on the input/output current when DE = 0 and VCC = 0V - as a note the bus will approach 0 when it is idle or off - so there should be very little current.
That being said you can just simplify the whole thing and use the minimum input impedance (which is #nodes supported * 375 - its not perfect but it is a super common approximation for RS-485 devices) - so for a 256 node device - that is 96K input impedance - as any current in/out of the A/B pins when DE is off and VCC is off or on is going to be dependent on that implied impedance (most applications will see > 96K - but that should be close to the minimum value).
There really isn't direct leakage spec'd for many RS-485 drivers as the input bias current includes that in its specification and you really can't separate the two - as if DE is active than the impedance is very low as its actively driving and you want low output impedance.
Please let me know if you have any other questions and I will see what I can do!
Best,
Parker Dodson
Thank you so much for the help. The bus input current will work perfectly for my design goals. Thank you for performing that regression analysis! I will probably use that more complex model for my design.
For the alternative model that you propose based on minimum input impedance, where did you find the definition
#nodes supported * 375
?
This appears to come from the unit load characteristics of the TIA-485 specification
HI Erich,
Sorry for the delay!
Completely understand wanting to create a more complex model for a more accurate estimation.
Yes the 12K comes from eh TIA-485 specification - standard RS-485 defines a unit load as shown below:
A 12K load line will be compliant over the entire range (-7V to 12V) - in reality its a bit more complicated as this impedance can behave slightly non-linearly so there is some margin around the 12K on some of the devices (unless they explicitly state a minimum input impedance) - however most devices using the 12K as the standard is a very common short-cut that is generally a good design value to work with.
Best,
Parker Dodson