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lew247

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Joined: 23/12/2015
Location: United Kingdom
Posts: 1676

Posted: 05:08am 20 May 2016

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Anyone any good with Power supply design?
The 3.3V circuit is attached below, but it's giving problems.
Unless an EXTREMELY clean 5V supply is connected it is corrupting the Micromite when the power is disconnected.
So badly that it has to be reflashed with Micromite Basic and the complete program typed in again.

Any ideas/thoughts/suggestions on why this is, or how to fix it?

I suspect I might have "too much capacitance" and it's holding a charge when the power is disconnected and draining slowly corrupting the micromite.
By "slowly" I mean slow in computer terms... miliseconds rather than microseconds.

I want to keep the 3.3V regulator the same pinout but I don't mind changing the external circuitry.

OR is it simply a case that the 5V input has to be Extremely clean?

Edited by lew247 2016-05-21

redrok

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Joined: 15/09/2014
Location: United States
Posts: 209

Posted: 09:21am 20 May 2016

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Hi lew247;

Not enough information.

Show us the 5V circuitry.

No, to much capacitance is not the problem. Generally the more the better.

I suspect the 5V is not staying higher than 3.0V plus the regulators dropout voltage.

What regulator are you using?

redrok

lew247

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Posts: 1676

Posted: 09:24am 20 May 2016

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Sorry forgot to put the regulator name in
I'm using two
MIC2940A-3.3BU or
LM3940IS3.3

The 5V psu is a show bought 5v 1A psu - nothing special or fancy

It's only corrupting the Micromite when the 5V psu is removed
Edited by lew247 2016-05-21

redrok

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Posted: 10:08am 20 May 2016

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Hi lew247;

What is the minimum current drawn from the MIC2940A.
The minimum in the spec is 5ma. See the spec:

Quote The MIC2940A will remain stable and in regulation with
load currents ranging from 5mA on up to the full 1.25A
rating. The external resistors of the MIC2941A version
may be scaled to draw this minimum load current.

If you are using the LM3940IS3.3 you don't have enough output capacitance. See the spec:

Quote 8.2.2 Detailed Design Procedure
8.2.2.1 External Capacitors
The output capacitor is critical to maintaining regulator stability, and must meet the required conditions for both
equivalent series resistance (ESR) and minimum amount of capacitance.
8.2.2.1.1 Minimum Capacitance
The minimum output capacitance required to maintain stability is 33 μF (this value may be increased without
limit). Larger values of output capacitance will give improved transient response.
8.2.2.1.2 ESR Limits
The ESR of the output capacitor will cause loop instability if it is too high or too low. The acceptable range of
ESR plotted versus load current is shown in Figure 12. It is essential that the output capacitor meet these
requirements, or oscillations can result.

Both of these regulators are high performance types and the specifications for capacitance is critical.

redrok

redrok

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Posted: 11:01am 20 May 2016

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Hi lew247;

lew247 said It's only corrupting the Micromite when the 5V psu is removed

How are you "Removing" the 5V PSU?
Are you unplugging a "Wall Wort" type from the power line?
This can cause havoc due to sparking.

Simply switching the 5V off would not cause problems.

redrok

lew247

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Location: United Kingdom
Posts: 1676

Posted: 11:03am 20 May 2016

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The MIC2940A-3.3BU I wonder if I added 2 X 1K resistors in parallel on the putput to the chip which "should" give 6.6mA load if my calculations are correct) would help it.

The LM3940IS3.3 I forgot to mention has a 47uF capacitor on the output, not the 10uF, I wonder if that would help the MIC2940A-3.3BU as well?

redrok

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Posts: 209

Posted: 11:33am 20 May 2016

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Hi lew247;

lew247 said The MIC2940A-3.3BU I wonder if I added 2 X 1K resistors in parallel on the output to the chip which "should" give 6.6mA load if my calculations are correct) would help it.

You don't need the 5mA load on the MIC2940A-3.3BU.

But you do need it on the LM3940IS3.3.
Two 1K resistors will work fine.

Quote The LM3940IS3.3 I forgot to mention has a 47uF capacitor on the output, not the 10uF, I wonder if that would help the MIC2940A-3.3BU as well?

The 47uF is acceptable.

However, I don't think this is causing your problem. It sounds like the problem is in how the 5V PSU is being turned off. Use a switch between the 5V PSU and your 3.3V regulator.

redrok

disco4now

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Joined: 18/12/2014
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Posts: 844

Posted: 01:57pm 20 May 2016

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Are you using atleast MMbasic 5.0. There was an issue way back when disconnecting power. (4.7 I think)
Gerry

Latest F4 Latest H7

lew247

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Location: United Kingdom
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Posted: 12:27am 21 May 2016

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Thanks - on/off switch
simplest things are always forgotten.

WhiteWizzard
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Joined: 05/04/2013
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Posts: 2794

Posted: 04:16am 21 May 2016

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I am helping build this system so can answer some points here:

@disco4now
MMBasic v5.1+

@redrok
MIC29300 3v3 is being used with 10uF caps.
The 5V PSU when removed from the mains socket does not cause issue. And when the USB lead is disconnected at the PSU end there is no problem (NOTE: USB lead from PSU goes to 'external 5v input on PCB via header pins).

BUT - I had a 'loose/worn' jumper pin from the USB PSU to PCB causing the +5v power line to 'bounce' - it was this resulting consistently in wiped firmware.

Whenever I went to pack away the setup, I would move this loose lead, resulting in a wiped chip. BUT whenever I switched off the 5v external PSU first (before moving the loose connection) everything worked as it should.

For those involved in the early days of Beta testing - this was a real headache for Geoff - but a design was worked on that prevented this 'wiping'. The only thing I can think off is that Pin 34 (USB detect) is still getting power, and with a 'bounced' 3v3 PIC power, perhaps the 'internals' of the PIC are causing the 'wipe'.

As mentioned here - a switch will resolve the issue - I am just surprised that with caps on the 3v3 line that this situation arises.

I have emailed Geoff and will await his valuable input . . . .

WW

For everything Micromite visit micromite.org

Direct Email: whitewizzard@micromite.o

bigmik

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Joined: 20/06/2011
Location: Australia
Posts: 2870

Posted: 01:46pm 21 May 2016

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GDay Lew, WhiteWizard,

My suggestions are (in order) as follows:

1. USE a CLEAN 5V source There a a million different iPad chargers out there that will suffice.
2. Put a larger filer cap (around 1000uf) across the input and a 47uf across the output.
3. Knowing Phil he has probably used his 10uf VCap, Not wanting an international `fisty cuffs' but try a low ESR 47uf tantalum instead.

Kind Regards,

MickEdited by bigmik 2016-05-22

Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<<

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