JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 5:25 am

I found this thread viewtopic.php?f=91&t=290853 and more specifically this post viewtopic.php?f=91&t=290853&p=1758788&h ... e#p1758877 regarding under voltage detction for the the raspberry pi. This thread is specifically for the 4B and I am using a 3B.

In the above mentioned post this statement is made:

0-4.52 Will fire
4.52-4.65 Might fire
4.65-5.1+ Won't fire

Additionally there is this page https://www.raspberrypi.org/documentati ... /README.md indicates that the low voltage detection IC will trigger if the supply voltage drops below 4.63V (+/- 5%) which is a range of 4.3985V to 4.8615V. Based on these number I would expect that the following would be true instead.

0-4.3985V Will fire
4.3985V-4.8615V Might fire
4.8615V+ Won't fire

The above thread mentions that the pi is fine if the supply voltage is above 4.6V and also that the ranges are due too the PMIC. I'm not sure if the PMIC (assuming power management integrated circuit) is different than the low voltage detection circuit.

Finally I have seen threads where people say to never let the under voltage message show up as well as the thread above that says that if you use the official Pi power supply that you may get a low voltage warning but to ignore it because the official power supply is spec of the Pi but not in spec of the under voltage detection circuit.

So my question are:

Is there an error in the statements from jamesh or is there an error on the Pi Power Supply page?
Better yet, what is the part number for the Pi 3B PMIC (MxL7704?)?
Is the low voltage detection for the Pi 3B done by the PMIC or is there a second chip that provides this functionality (and if so what is it)?


What is the specified absolute minimum voltage for reliable Pi 3B operation?
What is the specified absolute maximum voltage for reliable Pi 3B operation?
Where is the best place to measure the voltage to monitor how close we are getting to the absolute maximum limit.

Thanks,

Jacob

P.S. The internet has told me numerous times that I'm fat, lazy, can't spell, I'm bad at grammar and generally stupid so there is no need to remind me.

User avatar
Imperf3kt
Posts: 4559
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia
Contact: Twitter

Re: Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 5:39 am

You should supply your Pi a stable 5 to 5.25 volts, it is recommended that the supply can handle a 2.5A load without the voltage sagging. It should be fast enough to respond to rapid current consumption changes.
This is the recommended range that falls within spec. (it actually goes down to 4.75v, but nobody recommends below 5v)

Out of spec, the safe range appears to be up to 5.5v, but nobody recommends this high a voltage.
The absolute maximum, not recommended voltage (as it may destroy usb devices or affect hdmi performance) is 6v at which point the you have reached the PMIC's specced limit.
55:55:44:44:4C
52:4C:52:42:41

Rose tinted glasses are difficult to see through.

User avatar
Milliways
Posts: 720
Joined: Fri Apr 25, 2014 12:18 am
Location: Sydney, Australia

Re: Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 6:30 am

I don't know what the authors of the documentation have been reading.


The APX803 data sheet says 4.56 4.63 4.70

MxL7704 Data Sheet (used in Pi3B+ and later) says
Voltage rising 4.59 4.63 4.7
Voltage falling 4.52 4.57 4.65

and Input voltage range: 4.0V to 5.5V

Testing I have conducted confirms these (although I didn't test to that precision).

I have tested and the Pi works down to ~4.0V (with no peripherals). Indeed there is nothing on the Pi boards that requires more than 3.3V (USB & HDMI power excepted)

drgeoff
Posts: 12055
Joined: Wed Jan 25, 2012 6:39 pm

Re: Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 8:27 am

JacobChristApium wrote:
Thu Jun 17, 2021 5:25 am
Is the low voltage detection for the Pi 3B done by the PMIC
Yes.
Quis custodiet ipsos custodes?

JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 3:45 pm

Milliways wrote:
Thu Jun 17, 2021 6:30 am
The APX803 data sheet says 4.56 4.63 4.70

MxL7704 Data Sheet (used in Pi3B+ and later) says
Voltage rising 4.59 4.63 4.7
Voltage falling 4.52 4.57 4.65
Ah, based on your reply my take away is that the Pi3B is using the APX803 and specifically the APX803-46 for under voltage detection and I see where the voltages come from in the datasheet. If this is the case this also tells me where to probe the board for the best indication of checking the power supply voltage, I just need to find the chip now (my money is on U4 based on the there being a cap on one side and a resistor on the other, but I need to breakout the microscope).

Jacob

JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 9:06 pm

Imperf3kt wrote:
Thu Jun 17, 2021 5:39 am
You should supply your Pi a stable 5 to 5.25 volts, it is recommended that the supply can handle a 2.5A load without the voltage sagging. It should be fast enough to respond to rapid current consumption changes.
This is the recommended range that falls within spec. (it actually goes down to 4.75v, but nobody recommends below 5v)

Out of spec, the safe range appears to be up to 5.5v, but nobody recommends this high a voltage.
The absolute maximum, not recommended voltage (as it may destroy usb devices or affect hdmi performance) is 6v at which point the you have reached the PMIC's specced limit.
Where are you finding the specified voltage range of 4.75V to 5.25V for the Pi3B?
Its not this page: https://www.raspberrypi.org/documentati ... ower-specs

You say that the PMIC spec limit is 6v, which PMIC specifically?

Does the Pi3B have a PMIC or just the 3B+ on?

User avatar
Imperf3kt
Posts: 4559
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia
Contact: Twitter

Re: Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 10:03 pm

Sorry, the 6v was for the polyfuse fitted to a pi3b+ and earlier, not the PMIC. That was my mistake.

The specs I quoted come directly from a mixture of the many documentation sources about powering the Pi, forum posts by moderators, engineers and general users, as well as a few other sources.
55:55:44:44:4C
52:4C:52:42:41

Rose tinted glasses are difficult to see through.

User avatar
HawaiianPi
Posts: 6377
Joined: Mon Apr 08, 2013 4:53 am
Location: Aloha, Oregon USA

Re: Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 10:31 pm

JacobChristApium wrote:
Thu Jun 17, 2021 9:06 pm
Where are you finding the specified voltage range of 4.75V to 5.25V for the Pi3B?
The USB voltage specification for high power devices is 5V +/- 5% (4.75V to 5.25V). So it may not be in any Pi specific documentation, but you should be able to find that in the USB 2.0 power spec.

Pi computers tend to work better when sticking to the upper end of that range (5 to 5.25V). The official micro-USB PSU is rated 5.1V/2.5A and uses 18 AWG power wires. A non-official PSU that ran my Pi3B(+) computers for years output a bit over 5.3V under load and did no harm.

As I understand it (both from what I've read and my own experience), the low voltage warning triggers when voltage drops below 4.63V even briefly. This is why it may trigger when your DVM doesn't register a low voltage (typical volt meters won't show brief transient dips or spikes).
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?

User avatar
Imperf3kt
Posts: 4559
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia
Contact: Twitter

Re: Raspberry Pi 3B Under Voltage Warning

Thu Jun 17, 2021 11:30 pm

HawaiianPi wrote:
Thu Jun 17, 2021 10:31 pm
JacobChristApium wrote:
Thu Jun 17, 2021 9:06 pm
Where are you finding the specified voltage range of 4.75V to 5.25V for the Pi3B?
The USB voltage specification for high power devices is 5V +/- 5% (4.75V to 5.25V). So it may not be in any Pi specific documentation, but you should be able to find that in the USB 2.0 power spec.
I believe, though not (now) in the online documentation, the information is in the printed material that come with each new Pi.
I cannot confirm, but I would swear I've read it in some official documentation over the years...
55:55:44:44:4C
52:4C:52:42:41

Rose tinted glasses are difficult to see through.

User avatar
Milliways
Posts: 720
Joined: Fri Apr 25, 2014 12:18 am
Location: Sydney, Australia

Re: Raspberry Pi 3B Under Voltage Warning

Fri Jun 18, 2021 12:17 am

drgeoff wrote:
Thu Jun 17, 2021 8:27 am
JacobChristApium wrote:
Thu Jun 17, 2021 5:25 am
Is the low voltage detection for the Pi 3B done by the PMIC
Yes.
The Pi3 has 2 regulators (not PMIC - although this is semantics) neither of which is responsible for low voltage detection, which is done by a separate chip, all of which is clearly shown on the schematics.

JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Fri Jun 18, 2021 9:02 pm

HawaiianPi wrote:
Thu Jun 17, 2021 10:31 pm
JacobChristApium wrote:
Thu Jun 17, 2021 9:06 pm
Where are you finding the specified voltage range of 4.75V to 5.25V for the Pi3B?
The USB voltage specification for high power devices is 5V +/- 5% (4.75V to 5.25V). So it may not be in any Pi specific documentation, but you should be able to find that in the USB 2.0 power spec.
I can't say I know the USB Spec inside out but I did find Section 7.2.2 Voltage Drop Budget a few days ago. It say that that the voltage supplies from a high-powered hub port is between 4.75 V to 5.25 V but it also says you can have a maximum of 350 mV drop from the plug to the output. This implies that the connected device should operate between 4.4 V to 4.9V. If further goes on to say that a device drawing more than one unit load (100 mA) should expect to operate with 4.75 V minimum input voltage at the connector end of the upstream cable, which means that the the device should be expect to operate at 4.4 V with cable drops. Further, a USB device is not supposed to draw more than 100mA without requesting the additional power. My Pi3B happily gobbles more than 500 mA from a bench supply that doesn't speak USB.

I had decided that the port with witch you power a Pi is non-compliant USB a few days ago. What I'm trying to understand is what the actual requirements are to build a Pi3B power supply that will give the best possible operation of the Pi within my budget. Its unfortunate because to solve the problem of the port being non-compliant I have to build a non-compliant power supply that could damage a compliant USB device. It is what it is and you get what you pay for.

Thanks, your replies have been helpful to me,

Jacob

JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Fri Jun 18, 2021 9:16 pm

Milliways wrote:
Fri Jun 18, 2021 12:17 am
drgeoff wrote:
Thu Jun 17, 2021 8:27 am
JacobChristApium wrote:
Thu Jun 17, 2021 5:25 am
Is the low voltage detection for the Pi 3B done by the PMIC
Yes.
The Pi3 has 2 regulators (not PMIC - although this is semantics) neither of which is responsible for low voltage detection, which is done by a separate chip, all of which is clearly shown on the schematics.
Amazing! I didn't realize there was a published schematic. This is very helpful.
https://www.raspberrypi.org/documentati ... chematics/

For what its work I guessed right, U4 is the APX803-46SAG.

Jacob

JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Fri Jun 18, 2021 9:47 pm

So with the schematic in hand for the Pi3B I was able to find the Absolute Maximum Voltage for several chips on the 5V net:

MF-MSMF250/16X Vmax 60V (schematic has /X not /16X so I'm not sure if this is the same part)
NC7WZ16 Vcc Max 7.0V (though not powered by 5V NET so non-issue).
PAM2306 Vcc Max +6.5V
RT8088A VIN Max 6.5V
AXP806 Vcc Max +6.0V
RT9741 Vcc Max 6V

Based on the schematic I would say that the Absolute Maximum input voltage for the Pi3B is 6 V. For the benefit of future readers, if you do not understand what is meant by Absolute Maximum from a component supplier datasheet then you should first better understand this before putting 6 V into the Pi3B.

And to keep the APX803-46SAG under voltage sense from triggering you need to stay above 4.70 V

Thanks everyone,

Jacob

cleverca22
Posts: 3905
Joined: Sat Aug 18, 2012 2:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Fri Jun 18, 2021 11:44 pm

JacobChristApium wrote:
Fri Jun 18, 2021 9:47 pm
So with the schematic in hand for the Pi3B I was able to find the Absolute Maximum Voltage for several chips on the 5V net:

MF-MSMF250/16X Vmax 60V (schematic has /X not /16X so I'm not sure if this is the same part)
NC7WZ16 Vcc Max 7.0V (though not powered by 5V NET so non-issue).
PAM2306 Vcc Max +6.5V
RT8088A VIN Max 6.5V
AXP806 Vcc Max +6.0V
RT9741 Vcc Max 6V

Based on the schematic I would say that the Absolute Maximum input voltage for the Pi3B is 6 V. For the benefit of future readers, if you do not understand what is meant by Absolute Maximum from a component supplier datasheet then you should first better understand this before putting 6 V into the Pi3B.

And to keep the APX803-46SAG under voltage sense from triggering you need to stay above 4.70 V

Thanks everyone,

Jacob
ive seen an rpi over-volted to 6.73v before: https://www.youtube.com/watch?v=aKgLqIW65oc
but just watching that makes me scream internally, lol

JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Sat Jun 19, 2021 3:00 am

cleverca22 wrote:
Fri Jun 18, 2021 11:44 pm
JacobChristApium wrote:
Fri Jun 18, 2021 9:47 pm
So with the schematic in hand for the Pi3B I was able to find the Absolute Maximum Voltage for several chips on the 5V net:

MF-MSMF250/16X Vmax 60V (schematic has /X not /16X so I'm not sure if this is the same part)
NC7WZ16 Vcc Max 7.0V (though not powered by 5V NET so non-issue).
PAM2306 Vcc Max +6.5V
RT8088A VIN Max 6.5V
AXP806 Vcc Max +6.0V
RT9741 Vcc Max 6V

Based on the schematic I would say that the Absolute Maximum input voltage for the Pi3B is 6 V. For the benefit of future readers, if you do not understand what is meant by Absolute Maximum from a component supplier datasheet then you should first better understand this before putting 6 V into the Pi3B.

And to keep the APX803-46SAG under voltage sense from triggering you need to stay above 4.70 V

Thanks everyone,

Jacob
ive seen an rpi over-volted to 6.73v before: https://www.youtube.com/watch?v=aKgLqIW65oc
but just watching that makes me scream internally, lol
I don't know what Pi he is using in this video, but on the Pi3B there is also the ideal diode at the input. FET has an Rds of 50-100m Ohm Rd. The 6.73 V in the video is at 600mA which meas there is at least a 30-60mV drop there. As well he mentioned that there is a drop in his cable. And since Absolute Maximums are the levels that the manufactures guarantee that there chips will not be destroyed it doesn't necessarily mean that they will not work above these levels. When I worked at a chip maker in the past we would often add margin to the Absolute Maximums to make sure we didn't get support calls for customers running close to the limit. With that said, a lesson I learned early in my career is always operate within the manufactures recommended specifications. If something is going wrong most suppliers will look at an out of spec use of there components as your problem not theirs.

With all the drops I have on my board I need to get to about 5.5V on my bench supply before there is 5.0V on the Pi3B 5V net.

Jacob

JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Sat Jun 19, 2021 4:16 am

So I soldered some wires on to the APX803-46SAG Vcc and RESET pins an looked at what is going on on a scope (while the Pi3B was at steady state after boot). The first think I notices is that there is constant 280-320 mV of ripple that with an average voltage much closer to the Vmax than Vmin. This means that means you the average voltage of your supply needs to be darn near to 5.0 V to keep that low voltage warning from triggering.

When I looked at the 5V net after the ideal diode at startup the ripple was closer to 440 mV which would mean you need 5.1 V in to keep the APX803-46SAG under voltage detection IC from triggering.

Also, I had to get the 5 V input to less than 3.64 V before I started to see it affecting the 3.3 V supply. I didn't look at the 1.8 V supply or Vdd core.

Jacob

User avatar
davidcoton
Posts: 6335
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: Raspberry Pi 3B Under Voltage Warning

Sat Jun 19, 2021 9:07 am

JacobChristApium wrote:
Sat Jun 19, 2021 4:16 am
Also, I had to get the 5 V input to less than 3.64 V before I started to see it affecting the 3.3 V supply. I didn't look at the 1.8 V supply or Vdd core.
That agrees with previous anecdotal evidence. The on-board regulators are switching types, so they compensate for lower input voltage by drawing more current. The problem is, that if the voltage is sagging because of cable resistance or hitting the supply's current limit, drawing more current will drop the voltage further, leading to a spiral of death.

Lessons:
  1. Make sure the supply wires are thick and short (18 AWG or lower/thicker, not more than 1m long)
  2. Get rid of ripple by filtering the PSU output at source., mainly done by adding capacitors with an appropriate frequency response.
Location: 345th cell on the right of the 210th row of L2 cache

drmacro
Posts: 44
Joined: Tue Mar 08, 2016 4:26 pm

Re: Raspberry Pi 3B Under Voltage Warning

Sat Jun 19, 2021 3:48 pm

I have a 3B V1.2 that I hadn't used for a while. Fired it up yesterday with the supply that has been on it for a couple years with no issue...low voltage warning.

I have now tried multiple supplies and cables, known good.

Monitored with a scope and meter, no dropout or droop, 5.02V to 5.1 on the board, depending on supply...PI reports low voltage.

Any ideas?

JacobChristApium
Posts: 11
Joined: Mon Mar 01, 2021 11:33 pm

Re: Raspberry Pi 3B Under Voltage Warning

Sat Jun 19, 2021 4:00 pm

drmacro wrote:
Sat Jun 19, 2021 3:48 pm
I have a 3B V1.2 that I hadn't used for a while. Fired it up yesterday with the supply that has been on it for a couple years with no issue...low voltage warning.

I have now tried multiple supplies and cables, known good.

Monitored with a scope and meter, no dropout or droop, 5.02V to 5.1 on the board, depending on supply...PI reports low voltage.

Any ideas?
My findings indicated that there was as much as 400mV of ripple on at Vcc of the U4 (under voltage detection IC). 5.02 to 5.1 V input with 400mV ripple is enough to fire this IC on my board.

Jacob

User avatar
Imperf3kt
Posts: 4559
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia
Contact: Twitter

Re: Raspberry Pi 3B Under Voltage Warning

Sat Jun 19, 2021 11:12 pm

JacobChristApium wrote:
Sat Jun 19, 2021 3:00 am
Absolute Maximums are the levels that the manufactures guarantee that there chips will not be destroyed it doesn't necessarily mean that they will not work above these levels. When I worked at a chip maker in the past we would often add margin to the Absolute Maximums to make sure we didn't get support calls for customers running close to the limit
Thats true, except the Pi3B also has a polyfuse that is specified and intended to trip at 6V. If it doesn't then its faulty and you'd expect a lot of customer complaints...
Although, in the video The polyfuse is being bypassed because power is being supplied via the GPIO header instead.
55:55:44:44:4C
52:4C:52:42:41

Rose tinted glasses are difficult to see through.

User avatar
rpdom
Posts: 18855
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: Raspberry Pi 3B Under Voltage Warning

Sun Jun 20, 2021 5:46 am

Imperf3kt wrote:
Sat Jun 19, 2021 11:12 pm
Thats true, except the Pi3B also has a polyfuse that is specified and intended to trip at 6V. If it doesn't then its faulty and you'd expect a lot of customer complaints.
Fuses trip on current, not voltage. IIRC the polyfuse on the Pi 3B is rated at 2.6A.

However there is also a protection diode which will short circuit the power lines if the voltage exceeds 6V. If power is being supplied via the microUSB, that will cause the fuse to blow. If supplied via the GPIO it will possibly cause things to melt.
Unreadable squiggle

User avatar
Imperf3kt
Posts: 4559
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia
Contact: Twitter

Re: Raspberry Pi 3B Under Voltage Warning

Sun Jun 20, 2021 12:00 pm

rpdom wrote:
Sun Jun 20, 2021 5:46 am
Imperf3kt wrote:
Sat Jun 19, 2021 11:12 pm
Thats true, except the Pi3B also has a polyfuse that is specified and intended to trip at 6V. If it doesn't then its faulty and you'd expect a lot of customer complaints.
Fuses trip on current, not voltage. IIRC the polyfuse on the Pi 3B is rated at 2.6A.

However there is also a protection diode which will short circuit the power lines if the voltage exceeds 6V. If power is being supplied via the microUSB, that will cause the fuse to blow. If supplied via the GPIO it will possibly cause things to melt.
Yes, it's rated at 2.6A and the part (MF-MSMF250/X) has a rated vmax of 6 volts.

https://www.bourns.com/pdfs/mfmsmf.pdf
55:55:44:44:4C
52:4C:52:42:41

Rose tinted glasses are difficult to see through.

User avatar
rpdom
Posts: 18855
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: Raspberry Pi 3B Under Voltage Warning

Sun Jun 20, 2021 12:08 pm

Imperf3kt wrote:
Sun Jun 20, 2021 12:00 pm
rpdom wrote:
Sun Jun 20, 2021 5:46 am
Imperf3kt wrote:
Sat Jun 19, 2021 11:12 pm
Thats true, except the Pi3B also has a polyfuse that is specified and intended to trip at 6V. If it doesn't then its faulty and you'd expect a lot of customer complaints.
Fuses trip on current, not voltage. IIRC the polyfuse on the Pi 3B is rated at 2.6A.

However there is also a protection diode which will short circuit the power lines if the voltage exceeds 6V. If power is being supplied via the microUSB, that will cause the fuse to blow. If supplied via the GPIO it will possibly cause things to melt.
Yes, it's rated at 2.6A and the part (MF-MSMF250/X) has a rated vmax of 6 volts.

https://www.bourns.com/pdfs/mfmsmf.pdf
Which is irrelevant as the voltage across the fuse shouldn't exceed more than about 100mV. How does the supplied voltage affect the inline fuse which is between the 5V input and the 5V supply of the Pi?
Unreadable squiggle

User avatar
Imperf3kt
Posts: 4559
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia
Contact: Twitter

Re: Raspberry Pi 3B Under Voltage Warning

Sun Jun 20, 2021 12:13 pm

is that inline fuse not the polyfuse?
55:55:44:44:4C
52:4C:52:42:41

Rose tinted glasses are difficult to see through.

User avatar
rpdom
Posts: 18855
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: Raspberry Pi 3B Under Voltage Warning

Sun Jun 20, 2021 12:33 pm

Imperf3kt wrote:
Sun Jun 20, 2021 12:13 pm
is that inline fuse not the polyfuse?
The polyfuse is inline.
Unreadable squiggle

Return to “Beginners”