How to handle power on/off sequencing?

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Jepael
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Posted by Jepael: Thu. Sep 29, 2011 - 12:27 PM
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So I have this relatively difficult system to power, it needs three voltages to operate, 5V before anything else, then after it has been stable long enough, 3.3V and 1.2V can be applied.

But the voltage between 3.3V and 1.2V rails can never be larger than 2.5V or -1V, meaning that if 3.3V rail is still 0V, the 1.2V rail must be limited to 1.0V or smoke happens. Or if 1.2V rail is still 0V, applying more than 2.5V to the 3V3 rail means smoke again.

So okay there are tracking regulators, so I can make the 1V2 track the 3V3, and for the tracking to work fast enough I can use a 3V3 regulator with slow start so 3V3 ramps slowly enough. And I can use the microcontroller to control 3V3 regulator so 5V has enough time to stabilise.

But how to make sure on sudden loss of power situations the thing shuts down gracefully? I don't want to have the microcontroller to be responsible of monitoring the voltage or shutting down the regulator.

Any pro tips like voltage monitoring chips or regulators that have also slow shutdown capability?
I guess I could make the 3V3 track some input power supply perhaps? Some Schottky diodes so that 3V3 rail pulls down the 1V2 rail as well?

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Learning and Information, General Electronics
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fingar
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Posted by fingar: Thu. Sep 29, 2011 - 12:46 PM
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Why not power the 1.2V regulator from the 3.3V? They must track each other then.

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pixel2001n
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Posted by pixel2001n: Thu. Sep 29, 2011 - 12:58 PM
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What about a ~2.3V zener between the 1.2V and 3.3V lines? The forward voltage satisfies the -1V, and the zener satisfies the 2.5V. That'll keep the rails close enough during start up and shut down, and shouldn't interfere with normal operation. You can use a parallel schottky diode if the zener's forward voltage is too high, but it'll save you a part if you don't need it.

You'll have to look carefully at the zener specs and your application's current specifications to make sure it will work.

You'll have to find a power sequencer for the 5V and 3.3V/1.2V. Perhaps someone else can recommend a good one.

Jeff Nichols

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Jepael
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Posted by Jepael: Thu. Sep 29, 2011 - 01:17 PM
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fingar wrote:
Why not power the 1.2V regulator from the 3.3V? They must track each other then.

Good point. This might be possible if both the 3.3V and 1.2V regulators are properly selected.

I have not lately considered this, because most regulators have some kind of undervoltage lockout built into them, which keeps them shut down until input voltage crosses some sensible threshold. It means the 1.2V regulator gets turned on after there is almost 1.2V on input. This might certainly work.

However the 3.3V regulator cannot be of this undervoltage lockout disable type, as they jump from 0V to over 1V almost instantly when input voltage crosses the threshold.

Properly selecting the 1.2V regulator also means that the 1.2V output is taken down when 3.3V input drops. Some regulators just turn the internal FET off so VIN can drop to 0 while VOUT still is 1.2V, this is bad. Can be prevented with a diode between rails though.

It is just that most regulators either have enable input or undervoltage lockout, and the regulators that do not have them are rarely 1.2V regulators, with 1.8V I would not hesitate to put LM1117 there..

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Jepael
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Posted by Jepael: Thu. Sep 29, 2011 - 01:30 PM
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pixel2001n wrote:
What about a ~2.3V zener between the 1.2V and 3.3V lines? The forward voltage satisfies the -1V, and the zener satisfies the 2.5V. That'll keep the rails close enough during start up and shut down, and shouldn't interfere with normal operation. You can use a parallel schottky diode if the zener's forward voltage is too high, but it'll save you a part if you don't need it.

You'll have to look carefully at the zener specs and your application's current specifications to make sure it will work.

I'd go for a schottky to the other way around, but it is difficult to define a zener or several diodes in series there for the other way around. Both voltages consume around 400mA of current, and there are several 10uF and 100nF ceramic caps per voltage for bypassing, so the surge current can be huge.

And zeners are specified with some given test current like 5mA, and zeners around 2.4V have huge resistance at low Vz like 100 ohms. If I need 10mA current or 5mA more than specified, it raises voltage by 0.5V to 2.9V, kaboom.

pixel2001n wrote:

You'll have to find a power sequencer for the 5V and 3.3V/1.2V. Perhaps someone else can recommend a good one.

Well fortunately that is not so big problem. 5V can be there infinitely before the rest is turned on and I can use the MCU to turn the rest on. I think the only poweroff sequencing requirement is that the 5V does not go away before both 3.3V and 1.2V have been gracefully shut down.

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pixel2001n
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Posted by pixel2001n: Thu. Sep 29, 2011 - 01:56 PM
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Yes, at those current levels a zener will not do the job alone.

You could use the output of a zener (or other voltage detector) to drive a transistor connected between the two rails. I'm not much of a controls guy so I don't know if there are instability issues to look for in that sort of situation.

Jeff Nichols

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hugoboss
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Posted by hugoboss: Thu. Sep 29, 2011 - 04:38 PM
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http://www.national.com/pf/LM/LM...

Otherwise you could use some comparators:

5V goes online. When comparator A reaches stability (say 80%) switch on 3.3v. Stick a resistor divider on the output of the 3.3v reg with a barrier schottky into the 1.2V reg output, and when comparator B reaches stability on 3.3v switch on 1.2V reg.

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jayjay1974
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Posted by jayjay1974: Thu. Sep 29, 2011 - 10:32 PM
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What's the circuit and why is it so picky?

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Jepael
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Posted by Jepael: Fri. Sep 30, 2011 - 06:41 AM
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It is a chip for handling video signals and the datasheet says so. It has multiple different 3V3 pins like for IO buffers, and multiple different 1V2 pins for core etc, and most likely the internal ESD structures are the reason to keep voltages within the given range or it will blow up.

It is not so unheard of, as I have sometimes ran into DSPs and FPGAs that need 3V3 and 1V8 applied very carefully, but I have never designed this kind of power system myself and the 1V2 instead of 1V8 makes it extra hard.

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Bingo600
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Posted by Bingo600: Fri. Sep 30, 2011 - 07:23 AM
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@Jepael

Doesn't some of the FPGA's have same powersequencing req's ?
Though i'd expect 5v is a No-No there.

But TI makes a lot of FPGA PSU chips.

Maybe have a look

Edit:

TI is mentioned on the S3E starterboard.
http://www.digilentinc.com/Produ...
http://www.xilinx.com/support/do...

I think i have seen LT on another demoboard.

/Bingo

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