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I'm using the driver DS26LV31T and need to know what is the maximum output current for the output pins (P,N, for example pins 2,3).

I'm confused because I'm not sure where to take that information from.

On the one hand, this is from the Absolute Maximum Ratings:

enter image description here

While I don't like looking at the Absolute Maximum Ratings, to me it seems that each output pin can handle an output current of + - 150mA.

On the other hand, when I look at the Electrical Characteristics, I don't see anything like what I'm looking for, but when looking at the test conditions, I do see this:

enter image description here

I am looking at the Io >= 20mA. I suppose the answer is no, but does this mean the maximum output current is 20mA?

Maybe I'm looking at the wrong places for this information?

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  • \$\begingroup\$ What load are you trying to drive? \$\endgroup\$
    – Andy aka
    Jan 28, 2020 at 13:00
  • \$\begingroup\$ Opto-couplers, and I need to know how many I can drive. But, for this question, does it matter what the load is? \$\endgroup\$
    – nettek
    Jan 28, 2020 at 13:02
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    \$\begingroup\$ The device is intended to drive a 100 ohm load and guarantees 2 volts can be delivered but it might be as high as 2.6 volts (26 mA) from a power rail of 3.3 volts. \$\endgroup\$
    – Andy aka
    Jan 28, 2020 at 13:10
  • \$\begingroup\$ Why are you using this particular part to drive optocouplers? A MOSFET would do as well in a standard configuration; you don't have to drive them differentially. \$\endgroup\$ Jan 28, 2020 at 13:19
  • \$\begingroup\$ Or buffer logic if you've got lots of different signals. Something like CMOS 4050's or SN74ALS1004, etc. \$\endgroup\$
    – rdtsc
    Jan 28, 2020 at 13:21

2 Answers 2

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You are attempting to use the part outside of its specified range. The Voltage vs Current characteristic is a curve, but the manufacture has only given you 2 data points, what happens outside this range is not guaranteed.

If you are a hobbyist, just figure it out by trial and error. If you are building a product to be mass produced, you should avoid using parts outside their specified range.

enter image description here

Driver output: 2.0 V @ 20 mA

Optocoupler input: 1.8 V max drop @ 10 mA

The differential is only 0.2 V, not good.

The optocoupler normal current input is 10 mA

R = 0.2 V / 0.01 A = 20 ohms

Two is the most you can drive without making assumptions about what happens at higher loads. Each should have its own resistor.

These calculations are only for the minimum case, the maximum voltage/current may take you outside of the recommended optocoupler current. You are fighting an uphill battle, reconsider your design.

schematic

simulate this circuit – Schematic created using CircuitLab

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The values given under "Absolute Maximum Ratings" are those conditions that can cause permanent damage to the device. You should never approach those conditions in actual use.

The values given under "Electrical Characteristics" indicate how the device is expected to be used. This device is an RS-422 line driver so it is designed to produce a differential voltage of at least 2V when loaded with 100Ω. As I interpret the \$\Delta V_{OD}\$, this is the maximum difference in the differential voltage for a logic 1 and a logic 0.

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  • \$\begingroup\$ Hmmm...The 400 mV, Isn't that really the minimum absolute diff voltage (yeah, I know what the data sheet says)? This is a driver spec, and it needs to work with a comparable receiver that has a 200 mV (max) differential input threshold. Therefore doesn't the driver have to put out greater than 200 mV diff in order for the interface to be guaranteed to work? If the 400 mV was really the max output, that means that the device could put out 10 mV and be spec compliant. \$\endgroup\$
    – SteveSh
    Jan 28, 2020 at 15:26
  • \$\begingroup\$ That datasheet is kind of hard to figure out. I looked at another TI RS-422 part and the \$\Delta V_{OD}\$ parameter is defined as "Change in magnitude of steady-state differential output voltage between states" with a specified load of 54\$\Omega\$. ti.com/lit/ds/symlink/sn65hvd179.pdf \$\endgroup\$ Jan 28, 2020 at 16:15
  • \$\begingroup\$ I've got a 1999 NSC data sheet for the DS26LV31. For Vod it lists 3 values. One of them is for no load, and has typ=3.3V and a max of 4N (don't ask me how they arrive at that for a part that has a max Vcc of 3.6V). For RL=100 ohms, it gives a min of 2V and typ of 2.6V, but nothing for max. Finally, for RL=3900 ohms, it lists a typ of 3.2V and a max of 3.6V, which at least is consistent with the max Vcc. \$\endgroup\$
    – SteveSh
    Jan 28, 2020 at 16:31

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