Power supply sequencing for low power camera module

Question

Could you please suggest a simple method how to make power on/off sequence of three different input voltages?

I'm making a board camera using this image sensor. The sensor requires three different input voltages: 3.3v, 1.8v and 1.2v. Also around the page 101 it says that there exists a power on/off sequence, which means the 1.2v should be applied first and 1.8v, 3.3v later. And the order should be reversed when the image sensor is turned off. But I have no idea how to make it.

Before taking account of the sequence, I planed to use TPS62243(3.3v->1.8v converter) and TPS62242(3.3v->1.2v converter) so that I have 3 different input voltages. (This is their datasheet) Could I use these voltage regulators for making the sequence? The data sheet says you could use EN pin for this. But I couldn't understand clearly how it works.

Or could you please suggest a way to make the power on/off sequence by adding a simple circuit? Because my application requires very small power consumption and price, I'm afraid of using a complex system for it. It would be very helpful, if you could give me suggestions.

Answer 1

So, I got to the PDF link, and it is still loading after 10 minutes, so I'm not going to answer about your specific chip, since I'm currently in time that could be paid for. Pro tip for next time: If the PDF is slow, copy it to your own Dropbox/Google Drive/MS Cloud DriveThing and link from there.

You are feeding the sensor with 3.3V you say, so you cannot de-sequence without either an EN pin, or an energy buffer and power-fail detection on your camera board. The latter will make your camera board bigger, since I don't know the size or current consumption of your sensor, I cannot say how much it will influence your design. But most commonly you don't want that for a Sensor Board, since most people want them as small as possible.

So, you will need an EN pin anyway, since if you just turn off 3.3V to the board, the 3.3V will fall first: Bang, Wrong de-sequencing.

Which then makes me say, if you have one EN pin you need to handle correctly from the uC anyway, you might as well have 3. But, the easiest way would be either with sequence chips as such:

^{simulate this circuit – Schematic created using CircuitLab}

Then you can also make a simple power-fail detection, if you want:

^{simulate this circuit}

That's the least number of components I could think of, without using a Chip, since I'm out of time to also find you a chip, but there are Power Path and Power Failure chips out there, even very simple "Brownout Detectors" might work fine for you. But you will always need something like a diode in there, so you will need to really look very closely at your device's current usage profile and the diode drop that causes, to see if you can stay within the margins of the allowable power supply for 3.3V. A normal Schottky may work well enough if your supply is allowed to be within 3.1V to 3.5V, since I cannot think of a Schottky I recently used that had a larger forward-drop differential over a 2 decade current range. Or any diode I used, come to think of it, but it will depend on your main current set-point.

Again, all this would have been much more aimed had I been able to obtain the PDF in time.

Getting the power right, with a power-fail detection, is a lot easier if you use 5V in and buck-regulate all three voltages.