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alan
Joined: 12 Nov 2012
Posts: 357
Location: South Africa
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| Powering AVdd from separate regulator |
 Posted: Thu Oct 23, 2014 5:08 am |
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Hi All
I want to power the AVdd from a precision regulator on a dsPIC33EP128GM604.
However the AVdd and Vdd does not power up simultaneously so I get a latch-up condition where AVdd and Vdd differ by more than 0.3V blowing my internal regulator.
How can I address this issue short of just bypassing the extra regulator?
Regards
CCS 5.025 |
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RF_Developer
Joined: 07 Feb 2011
Posts: 839
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| Re: Powering AVdd from seperate regulator |
| Posted: Thu Oct 23, 2014 5:31 am |
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| alan wrote: |
However the AVdd and Vdd does not power up simultaneously so I get a latch-up condition where AVdd and Vdd differ by more than 0.3V blowing my internal regulator.
How can I address this issue short of just bypassing the extra regulator? |
You've pretty much answered your own question. You can't, without risking blowing things up.
Why do you feel you need "better" Avdd? If you want to improve analogue conversion accuracy, the answer is to use an external precision reference rather than the supply. If its something else, then what? |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19552
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| Posted: Thu Oct 23, 2014 7:19 am |
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The point about Avdd, is to try to reduce processor noise.
The recommendation from Microchip, is to feed it from Vdd, but with a small series resistor/inductor, and a low ESR capacitor really close to the pin.
Every regulator that may find it's output rail above it's input potentially always needs a reverse biased diode across it. This is true on the trusty old 78xx, and missing it often results in damage if there is significant capacitance on the output. In most cases the voltage and duration are small enough to not destroy the regulator, but two separate regulators feeding different paths into the same chip, is asking for trouble....
For accuracy, you want to use a separate Vref. A bandgap reference, not just a precision regulator.
Also, look at the data sheet. What voltage is allowed to be on AVdd?. It is specified to not be below 3v, or Vdd-0.3 (whichever is the higher). So 'no wonder' the circuit doesn't like it if the voltage differs by more than 0.3v.... |
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alan
Joined: 12 Nov 2012
Posts: 357
Location: South Africa
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| Posted: Fri Oct 24, 2014 1:12 am |
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Thanks for the answers.
The problem with using the series inductor although it works is that every board needs then to be calibrated, you have typical 0.1 - 0.2V difference.
To me it is actually silly to bring out the AVdd pin and then not be able to supply it with a precision voltage.
Will just bypassed it for now.
Regards |
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RF_Developer
Joined: 07 Feb 2011
Posts: 839
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| Posted: Fri Oct 24, 2014 1:30 am |
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| alan wrote: |
The problem with using the series inductor although it works is that every board needs then to be calibrated, you have typical 0.1 - 0.2V difference.
To me it is actually silly to bring out the AVdd pin and then not be able to supply it with a precision voltage.
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No, as both Ttelmah and I wrote, don't try to make the supply "precision" - that's "silly": use an external precision reference instead. The Avdd is there so that you can have a "clean" analogue supply, not a different voltage. Though personally none of my attempts at cleaning up the analogue supply have been particularly successful.
Using the supply as the ADC reference is only suitable for simple, low precision measurements or ratiometric measurements wrt the supply. For anything else, pretty much everything requiring more than five or six bits of resolution (one bit at six bits is about 1.5% of full scale and few +5V supplies are anywhere near even that good - typical is 5V, +/-.15V, or +/-3%) a better reference, whether the one built in to many newer PICs, or an IC intended for that purpose is a much better bet. There are many to choose from - Microchip do some low cost ones such as the MCP1541 and its friends.
Last edited by RF_Developer on Fri Oct 24, 2014 1:48 am; edited 3 times in total |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19552
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| Posted: Fri Oct 24, 2014 1:30 am |
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The point is that nobody wanting precision uses the Vdd as the reference.
Bringing it out, is done so you can reduce the noise introduced on this pin, not so it is used as a reference.
Basic rule of thumb. Vdd==reference > Low accuracy. High accuracy > separate reference.
The loads on the Vdd line change too much for accuracies above perhaps 1% to be practical.
This is especially true, when using the external Vref, allows you to scale the actual ADC range to suit your input.
You should not be getting 0.1v to 0.2v drop across an inductor of any sensible DC resistance. Something more like 0.002v would be typical, with something like a Murata BLM18PG181SN1D, feeding a 10uF ceramic capacitor....
I see RF_Developer posted at the same time.  |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Sun Oct 26, 2014 5:56 am |
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A quick look at the data sheet shows that chip has an internal voltage reference. You should probably be using that instead of AVdd for a precision voltage.
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