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Part Number: ISOW7841
I have a circuit using three ISOW7841 to provide independently isolated interfaces. One of the circuits is shown below (the other two are identical as far as the power rails go).
If the +5V_C rail starts cleanly then the ISOW7841 regulates nicely at +5.0V. However if +5V_C oscillates during startup (due to current limiting the input supply), then the ISOW7841 can end up regulating at +7.0V, even after the input has become stable. This could be the breakdown voltage of the 5V TVS on the output, or the ISOW7841 regulating at this voltage, but it is a very well regulated rail with almost no noise or ripple.
The only way to get the output voltage back down to +5.0V is to power cycle the circuit. The ISOW7841 is undamaged, and operates correctly after providing a clean startup.
I am working on some modifications to ensure that the input voltage is stable during the startup of the ISOW7841, but I would also like to know if there is anything else I can do to prevent the ISOW7841 from regulating at such a high voltage?
Thanks in advance,
In reply to Abhi Aarey:
Thanks for the comprehensive reply.
I removed the 5V TVS (number 3) and did see some slight differences in performance. Below are all with the TVS removed.
1) Below is a screen shot of the startup under severe current limited condition. Dark blue is the input voltage that you can see is getting high enough to start the three parallel ISOW7841, but the rail drops as they all sink current, hit the UVLO level and recovers, causing the oscillation. The ISOW7841 is doing what I would expect in this situation until the last 10ms of this scan, where the voltage spikes to 7V rather than the 5V from earlier.
As the current limit is increased, we then see the input voltage rise enough that we aren't hitting the UVLO any more. So the ISOW7841 is now trying to regulate at 7V as shown below:
Once the input voltage and input current were allowed to increase enough that the 5V rail could stabilize properly, the following output was observed. Note that the scale of the output had to increase as we are now regulating at exactly 9.0V!
Is it suspicious that the device only regulates at exact odd voltages?
2) I can't explain the lack of ripple either, like in the last picture. When operating normally, the ripple is quite obvious (although small) at this same scale.
4) Floated pin 10 and the device regulates at 3.3V correctly when given enough current. It does seem better behaved, as I haven't yet seen the voltage go over 3.3V, unlike in 5V mode where I can consistently get higher voltages.
5) Shorting out the 47uH inductor doesn't change the behavior. I also removed the supply from the other two ISOW7841 devices, and the device under test was more stable, but I can still get it to fault. I believe it was harder to reach the fault point because the upstream devices weren't going into current limiting mode as easily with only one device pulling current.
In reply to Hayden Gray:
1) Since this is happening during startup, and I have test code in the microprocessors, there is no comms taking place. Therefore the load is only a CAN transceiver in bus recessive mode (TJA1051: 2.5 - 10mA), an I2C based power monitoring chip (ISL28023: <1.5mA), an I2C isolator (ISO1541: 1.9 - 3.5mA) and whatever the ISOW7841 is using on the secondary side, so approximately 6 - 15mA.
I initially had an LED on the secondary side drawing about 3mA in case I needed to add any additional load for stability, but this is currently not fitted.
The caps are very close to the pins (the closest 0603 is 0.5mm away). We also use an 8-layer board with 0.1mm spacing between power and ground planes, so we get a huge inter-plane capacitance (hundreds of pF at virtually no inductance).
In the meantime I have done a lot of additional experimentation. The end result is that if I put a 5.1V zener on the VISO rail (instead of the 5V TVS), the ISOW7841 doesn't get the opportunity to get out of regulation.
It never tries to get to 7V or 9V, but regulates nicely at 5V once the input has stabilized. In addition to this I have also improved the startup of the downstream regulator, so that the ISOW7841 should not see such a nasty input voltage to begin with.
Thank you for the clarifications on #1. Good to know about this and the Zener diode working well in clamping the output. If I get any additional inputs from our development team I will post it here.
Happy New Year to you as well, I am back after a nice 3 weeks away, I hope you were able to have a bit of a break from work too.
We are happy with the performance of the ISOW7841 when the zener diode is added to the output. We always use a TVS on every power rail (including internal isolated ones), so using a zener instead is not much of an issue.
The current limit for the VCC1 side is set to 750mA nominal (about 600mA minimum) by a TPS2114A with a 333R ILIM resistor, and all three ISOW are in parallel. There is not any individual current limiting. There is also an LDO on the rail that draws 115mA for a +3.3V rail for a micro and other logic and sensors.
I had tried a 1A nominal current limit during experiments, but it didn't appear to make much difference as then we start running into upstream current limits instead.
Thanks for the suggestion of the TCAN1051, the HV varient looks like it would work nicely. We didn't have many options a couple of years ago when choosing a new CAN driver, as we use +48V power in the same cable, so were looking for at least 70V for bus fault tolerance. Now there are lots of options.
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