Normally with an optocoupler it will transmit data across the isolation barrier when the logic is high and turn off the led when the logic is low, does the default output state of the digital isolator need to be high or low to duplicate the same results of the optocoupler?
Greetings,
It’s not clear what type of “optocoupler” you are referring to; they can come with several different types of transistor outputs, photovoltaic outputs, triac outputs, buffered logic outputs, and can be used in different ways so as to produce either a “high” or “low” signal at the output when an input signal is absent.
The ADuM340 is produced in both default-high and default-low output versions; referring to the state of the output when the power supply for the input side is not valid. Which one you would choose in order to mimic the behavior of a “normal” optocoupler depends on what exactly you mean by “normal,” which isn’t entirely clear from this side of the screen.
1 Like
Sorry let me provide a bit more context, I’m replacing G3VM-61VR MOS FET Solid State Relay SPST-NO with ADuM3400BRWZ because tON and tOFF were not fast enough. My novice understanding of opto couplers might have seeped into this conversation. In order to run G3VM-61VR it requires the signal to be high to turn on the LED, which connects pins 3 and 4 allowing current to flow, when the signal is low pins 3 and 4 disconnect. Therefore the output would be high when current is flowing through G3VM-61VR ? I don’t have a grasp on how to run the ADuM340 yet, I know the OOk architecture is in a continuous state of sampling and transmitting inputs across the barrier which bolds well with the variable status of the PWM signal entering on the input side that I’m providing. Does that mean signals can only cross the barrier if they are high? What happens when the signal is low? How does the default state come into play? Not sure if I’m confusing you?
Ah, that context helps.
The G3VM-61VR is a “solid state relay” type of device, which are characterized by slow switching times and comparatively high output current capabilities. They’re targeted toward replacement of mechanical relays as the name would suggest, and function in the same basic way as a normally-open mechanical relay; apply drive current and the output contacts close, remove it and they open. There’s no “high” or “low” per-se, because a person can wire a contact-closure output to generate either one.
Devices like the ADuM340 are geared toward data transmission, their output behaves more like the output of a logic gate, toggling between “high” and “low” values that are clearly defined in reference to the output-side power supply. These outputs are capable only of sinking/sourcing currents in the mA range, and only in relation to that same output-side power supply, but with potential to do so at very high speed.
If one looks at the Isolator product category, there are a number of sub-families there; the “digital isolators” are the ADuM sort, which use complicated means to transmit data fast. the “Gate driver” isolators are targeted toward isolated gate drive applications, e.g. turning FETs and such on & off. The plain 'ol “optoisolators” are of the more generic variety that have an LED on the input that shines on some sorta semiconductor output. If that output device is amplified & buffered to look like a logic output, it’s a “logic output” optoisolator. If it’s just a plain transistor or “solar cell” output, it’s in the “Transistor, Photovoltaic Output” group, and likewise for the “Triac, SCR Output” group.
There is some overlap and crossover between devices found among these and in the Solid State Relay product family, because not every manufacturer agrees on the precise dividing lines.
Depending on what exactly you’re trying to achieve, you may wel be able to find a different diode-input opto that’ll offer much better multi-source potential than an ADuM340, while still meeting your timing needs.
2 Likes
Thank you for the clarification, I would like to provide even more context / pick your brain a little more. Hopefully I’m not being repetitive/ causing confusion. The PWM signal I mentioned will be used in a circuit that controls fan speed, hence the PWM signal being variable ( I would need tOn and tOff to be faster than 444.4ns). I believe the engineer before me was using G3VM-61VR as a switch because the signal from G3VM-61VR heads towards an N-Channel MOSFET which requires the signal to be high in order to pull through the rest of the circuitry. I have to consider worst case scenario in terms of temperature (the performance cannot decline when faced with conditions from -40c to 125c) and power consumption (there is currently a 5V linear regulator for all circuitry on this board). This is what pushed me towards a digital isolator. Potentially I could run 4 fans max and having four channels seemed to make sense.
A picture (schematic) can be worth 1024 words…
People usually speed-control small DC fans either using a purpose-built control input or by brute-forcing the matter by hacking up the power supply, as described in greater detail here. What’s appropriate or optimal for your situation would vary depending on the precise circumstances and objectives.
A fancy digital isolator could certainly be made to work, but if a person can achieve the same goals using a product available in equivalent form from multiple suppliers, the prospects of supply disruption are dramatically reduced.
1 Like
Try dropping an LTV-356T-B or comparable part in there and see what that does for you. The part itself itself should be an order of magnitude faster and function more or less the same for the purposes illustrated, but at least some of the mushiness in the pulse response of the above circuit would stem from having two sequential stages where FETs are being driven through relatively high-value resistors.
A structure closer to that shown below would likely be much more responsive.
Thank you, I’ll try this out!!