I am seeing some quite strange things happen as we power down sections of the board.
To isolate the powered down sections, I have used a high-side switch, and then isolated all of the logic signals using 2 x SN74AVC8T245, 1 x SN74AVC4T245, and 1 x SN74AVC1T45 devices. The first three parts have been configured with their ‘A’ ports powered from the permanent supply, and their ‘B’ ports on the switched side. The final single gate part is configured the other way around (‘B’ port to permanent supply and ‘A’ port to switched side).
When I measure the current being drawn on the switched supply side, I see the system drawing around 36mA through the high-side switch, which represents a power draw of around 120mWatts, however, the system power draw increases by around 340mWatts.
There is nothing else in the system that I can identify that is drawing current, so I am left looking at the logic devices. Apart from two signals on the SN74AVC4T245 device, all of the signals on the permanently powered side of the logic gates are inputs, so any switching current I believe would be included in the current that I am measuring through the high-side switch. Also, there is only one signal that is switching, which is a 32.768MHz clock signal through the single gate part (again, input from the power side, with output on switched side).
Can you help me with a couple of questions, please?
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Are these devices suitable for use as digital isolators in partially powered systems? The datasheets seem to suggest that they are, but I thought I would check to make sure I haven’t completely missed the point.
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I have permanently connected the output enable lines for the SN74AVC8T245 and SN74AVC4T245 to ground. Could this be causing the devices to draw excessive current once their ‘B’ side power supply is applied? When I first designed the system, I did not notice the recommendation to keep the outputs disabled until both supplies are stable.
There is a document that I read the shows the current drawn as the one of the two supplies is ramped up. This test did have the output enable signal tied low, so I would be surprised if this is an issue. -
While reviewing this, I have noticed that the datasheet for the SN74AVC1T45 suggests that the ‘A’ port supply should be applied first, and the ‘B’ port second. This is the opposite way from how I have the device at the moment. Is this likely to cause the part to draw excessive current? It doesn’t seem to be getting hot, and is operating normally.
I have been able to do some more investigation into the strange power issue.
The part that is easiest to remove is the SN74AVC1T45, and when I took this off the board and then linked across the data connection, I found that the current drop is pretty close to what I would expect it to be.
This means that I don’t believe this particular IC is drawing more current than it should.
The other parts are more difficult for me to remove, but I really can’t see how they could be the parts that are consuming the extra current.
Sujeeva Wettasinghe
Avnet FAE