I have to generate a signal for optionally disabling a circuit subsection. The subsection has two power lines, at 12 V (unregulated) and 3.3 V (regulated), with a common ground. Those lines are also used in other circuit sections, so disabling the regulators is not an option. The signal must be in the 3.3 V logic domain.
The total power requested by the subsection is about 1 W, mostly on the 12 V line.
I was thinking about using an analog switch on the ground line, but I don't know if it is the best option I have. And, moreover, what if digital lines in the low state in the subsection have protection diodes able to conduct current to ground? Maybe I have to block all 3 lines (12 V, 3.3 V and ground)?
Thanks in advance.
Most digital I/O pins have protection diodes to both VCC and GND, so unless the devices in the subsection are actually designed to allow having signals applied in the powered-off state, you also have to disconnect signal lines.
Analog switches are designed for low-current signals; their resistance is usually too high for power supply lines. You probably want to use load switches, e.g., TPS22919 and TPS22810.
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In reply to Clemens Ladisch:
thanks for your reply.
So I could use TPS22810 for the 12 V line and TPS22919 for the 3.3 V line.
Do you think this would be enough if I also manually set all digital lines to their low state? Or would it be safer to use analog switches in order to separate them from the rest of my circuit? And what about the GND line?
In reply to Stefano Infante:
If all voltages are at the same level, no current can flow. You have a problem only if you disconnect the power and some data line pulls a device's VCC high, or if you disconnect GND and some data line pulls a device's GND low.
But how can I be assured that all voltages are at the very same level?
If I leave ground connected, and a single digital line has, say, a 0.3 V low level, there can be current flowing between that line and ground.
Hence I would say I need to block every single line, but it would be a problem.
By the way, as all digital lines are managed by the very same device (a microcontroller), I think I could try to leave them connected and only block power lines.
And this leads to the following problem: How could I block the ground line?
The TPS22810 and TPS22919 are load switches designed to turn on and off power rails is a system. The input of the load switches will be the voltages from your power lines,12V and 3.3V respectively, and they can be controlled with your digital signal through the enable/ON-pin of the load switch.
If you manually set the digital signal to a low state, the load switch will close its conduction channel turning off power to downstream circuitry connected to the respective power rail. With power turned off there should be no need to disconnect your GND line. To open the conduction channel of the load switches you would have to input a logic high, 3.3V logic domain is perfect, to the enable/ON-pin of the load switches.
Let me know if you have any questions.
In reply to Andy Robles:
Hi Andy, thanks for your reply.
I was only thinking about protection diodes on the digital pins in the disabled area, as I wrote to Clemens before.
If you think there's no need to worry about them, my life will surely be easier :)
A voltage of 0.3 V will not be able to forward bias a diode; it might begin to become dangerous at about 0.5 V. And even then the device's VCC would be one diode drop below the input voltage, so you'd need a signal of above 1 V for anything to happen.
Well, I think protection diodes can be biased by 0.3 V (maybe something more, say 0.35). Many devices have a -0.3 V absolute maximum rating on their digital pins, and I think that is the maximum voltage the protection diodes can withstand without being forward biased.
But I think you are right, probably I don't have to worry about that. I will add the two load switches and that's all.
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