3
\$\begingroup\$

This isn't my usual job, I'm not an EE, but I want to give it shot...

This question concerns developing an interface board to connect a device (DUT) to a test system in a manufacturing scenario.

The signals I have access to are a bunch of 1.8V cmos logic lines from the GPIO pins of an embedded CPU, a 5V power rail, and GND (see diagram below). Everything is in a fairly tight space, I have control over all power/ground etc.

schematic

simulate this circuit – Schematic created using CircuitLab

The 1.8V GPIO signals (s1,s2,s3...) eventually get read by a opto-isolated digital input device (happens to be a NI-6525). The opto-isolated digital inputs need at least 3.2V to sense a HIGH so I must level shift these, moreover, the current input per channel is given as "3 mA MAX"-- too much for 1.8V cmos I think?

To perform level shifting, I was thinking about the MAX3000E because it is designed for things like cellphone cradles, smart card readers, etc. It has ESD protection and it can handle 8 signals and I can use it to level-shift from 1.8 to 5.0, all I have to do is provide voltage rails for 1.8 and 5 from my test system. No problem, I thought...

enter image description here

BUT looking at the datasheet for the 3000E it seems that the output current I can provide from the 5V (Vcc) side is only 10uA max! That isn't enough to drive the diodes in the digital input device.

enter image description here

So now, I am thinking about adding mosfets to each output of the level converter so that I can deliver enough current to the opto-isolators.

Questions:

  1. Am I even using the right level converter for such an application? I get the feeling I am doing this wrong. The test system uses opto-isolators but it is a good pick for other reasons.

  2. I thought about ditching the level translator entirely and just using mosfets to drive the optoisolators from the 1.8V signals. But then, there's the problem of ESD protection, losing the nice "enable" pin, plus I don't feel confident with with just naked mosfets. Is this easy to do with mosfets in this scenario where DUT's are going to be constantly plugged and unplugged into the system? Should I invest the time in trying to do this with just mosfets? Will it be reliable in a factory system?


Looks like the moral of the story is that "self-configuring" bidirectional logic translators can only deliver a very small amount of current on the output side (~uA's). In my scenario, I am using the output side to drive opto-isolator inputs on the system side which require ~mA's of current. This requires using an additional buffer of some kind, or, selecting a different logic level translator. Since I don't need to change direction in-situ (never be tx'ing and rx'ing on the same line), selecting a translator that has fixed or settable direction solves the problem because these don't seem to have the extremely small max current spec on the output.

\$\endgroup\$
  • 1
    \$\begingroup\$ uA current is all it needs but it can drive xx mA per port just like 1.8V logic. Can it be inverting BJT level shifter? More robust. \$\endgroup\$ – Sunnyskyguy EE75 Jul 20 '18 at 16:14
  • \$\begingroup\$ @TonyEErocketscientist, thanks! I guess I don't understand some key things in the level translator datasheet. One would think that it would just specify how much current (or even "fanout") it can source from the outputs? The datasheet for the level translator doesn't say anything about how much current it can source on each output. If I can really drive the the optoisolator from the level-translator directly that would be great. \$\endgroup\$ – Spurius Jul 20 '18 at 16:40
  • \$\begingroup\$ Rohm makes these in IC’s with many per package for different needs . This is the theory electronics.stackexchange.com/questions/260995/… \$\endgroup\$ – Sunnyskyguy EE75 Jul 20 '18 at 16:40
  • \$\begingroup\$ I believe the NI unit is TTL threshold (1.5 V) low current so yes pullup out to any V+ limits current. The CMOS translators and all CMOS ports are low R switches that draw uA with no load and Vol/Iol=R \$\endgroup\$ – Sunnyskyguy EE75 Jul 20 '18 at 16:42
  • \$\begingroup\$ Is thus high speed logic or bidirectional? That’s the Max3000 advantage \$\endgroup\$ – Sunnyskyguy EE75 Jul 20 '18 at 16:49
1
\$\begingroup\$

Your NI-6525 device needs your driver to be able to source up to 3 mA current. However, the MAX3000E is specified only for 20 µA (see VOHx); a bidirectional level shifter with automatic direction sensing must have weak outputs to allow another device to override the voltage level.

To get stronger outputs, use a unidirectional or direction-controlled level shifter, such as the (SN)74LVC2T45.

\$\endgroup\$
  • \$\begingroup\$ thanks! I see that Tony EE pointed out in his comment that the 20uA figure was not the source capability limit of the outputs. I misunderstood the datasheet. The 20uA is merely the max of what each pin draws in H or L state under no load. For whatever reason the datasheet doesn't indicate how much current each pin can deliver. Perhaps because it is intended to just go into another CMOS input or output? \$\endgroup\$ – Spurius Jul 20 '18 at 17:59
  • \$\begingroup\$ But yeah, looking at a direction-controlled level shifter, I see that it is possible to get 32mA from each output! ti.com/lit/ds/symlink/sn74lvc8t245-ep.pdf \$\endgroup\$ – Spurius Jul 20 '18 at 18:18
  • \$\begingroup\$ The power usage of the MAX itself (not counting the outputs) is at most 10 µA. The outputs are specified for 20 µA (If you try to draw more than 20 µA, the voltage drop will become larger than VOHx). \$\endgroup\$ – CL. Jul 20 '18 at 18:29
  • \$\begingroup\$ I see, I am going to ditch the MAX3000E and go with a converter that has fixed or settable direction (SN74LVC8T245). That will work, I think for driving upto 3mA into an opto-isolator. I guess these auto-bidirectional translators are intended for situations where they're connected only to other cmos logic inputs. \$\endgroup\$ – Spurius Jul 21 '18 at 14:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.