What volts can I supply the VBAT pin with?

Hi,

This may be kinda dumb, but I'd rather ask a dumb question before I break my Pico :)

On the Pico page (http://www.espruino.com/Pico) it talks about the voltage regulator accepting between 3.5-16v and the board being happy to run on a 3.7v Lithium cell - can I use the VBAT pin for either or if I wanted to supply a higher voltage should I be supplying it via some other pin?

So far I've been running a regulated buck converter set on 5V through a USB plug I've wired up "just in case", but I'm wondering if I can just chuck a less well regulated supply straight at VBAT?

Cheers,
Dave

VBAT would be the pin to connect V+ (3.5-16v) indeed.

For some reason, your question got me uncertain - but I don't see it being any other way.

We are talking BAT_IN on schematic, which on the board is the pin next to B15 - marked Bat - and the pad on the back side - marked + - (where a battery connector can be mounted). The markngs on the back are not very conclusive / matching with the schematic... rather confusing... On the other hand, only savvy poeple look at the schematics, regulars just look at the markings... and schematic's BAT_IN is board's marked Bat and + sign - no ambiguity on the board itself.

According to the schematics / the board routing, USB power feeds the regulator over a diode and the BAT(tery)_IN - Bat pin next to B15 - over the P FET. The regulator feeding point is connected to the 0.1" and 0.05" (V)BAT pins - marked VCC - grouped with the 0.1" and 0.05" GND and 3.3V pins.

I do not know the behavior when connecting a battery to (V)Bat:

• I'm not savy about the PFET's behavior / what happens when you feed it on the (V)Bat / VCC / regulator feeding point.

• The Diode protects 'back'-powering the USB when (V)Bat / VCC higher than 4.7V. When (V)Bat / VCC though lower and board plugged into USB with power, USB power will charge the battery. This becomes a roblem for the PCB tracks when the charging (or 'short'-cut) current goes beyond... (I d not know how the tracks are reated. From looking at the board, the connection from/to 0.1" (V)Bat / VCC pin is much fatter than the one from/to 0.05". You can prevent charging by putting a diode between the battery and the (V)Bat / VCC pin, but that's not much different then using the BAT_IN / Bat pin over the P FET, but with less options.

@alexanderbrevig concludes that the (V)Bat is the place to connect a bat from 3.5..16V... and see no issues when not connected to USB power/USB connector. When connected to USB Power, it can become an issue when the (V)Bat conneted battery starts to draw current (Battery delivers less than 4.7V).

With other words, BAT_IN is safe for sure, and V(BAT) is safe as long there is no USB POWER connected. Therefore, when you do not plug it into a powered USB porte (kond of an oxymoron by the definition of the USB port...) you can use either.

Putting the FET/B0 solder jumper in, makes it a different ball game.

The PFET prevents the USB power from trying to charge the battery when its connected to BAT_IN, but not when it's on VBAT

Yes, as others have said - if you have a battery, connect it between Bat/BAT_IN and GND. Sorry the schematic/board don't quite agree, but I was trying to keep some kind of continuity between the Pico and the original Espruino board.

You're totally fine with 3.5-16v on there, but you should be aware that when you do that, VCC then becomes whatever the battery voltage is (not 5V). 3.3v is still a nice regulated 3.3v supply though.

Hi All, Thanks for all the answers.

I've had a go at following all of the above and also spent more time following traces, probing points on the board, etc, my understanding is now as follows:

• It's safe to power the Pico via the BAT_IN pin (aka BAT) from any DC 3.5-16V source
  
• The VBAT pin (aka VCC or 5V) will get power from a 5V regulator
  
• The 3.3V pin (aka VDO or 3.3) will get power through a 3.3V regulator (which is powered from the 5V regulator)
  
• USB will always run through a schottky diode to prevent BAT_IN from giving power to USB if it's > 5V
  
  

I think my original question was doubly confusing as I'd confused VBAT and BAT_IN (partly due to all the different names for the same pins in different places).

Another question comes up though if I need a few 3.3V devices - the regulators according to the power section appear to be MCP1703T-3302E-CB which is rated up to 250mA - is this the correct limit for the version on the board and are there any other limits that will cause a problem first?

Would it make sense to put as many devices on the 5V regulator (VBAT pin) as reasonable to increase efficiency / raise maximum Watts if I'm coming at all close to that limit?

I don't think I will come close to the limit for anything I'm currently trying to solve (but I will actually test mA on each device individually now), but I'd just like to know what the best approach would be (short of introducing another regulator / external voltage follower / etc).

Cheers,
Dave

The VBAT pin (aka VCC or 5V) will get power from a 5V regulator

It doesn't I'm afraid (there's no 5v regulator) - the VBAT pin is battery or USB volts, so you need to be careful there. There's actually the circuit diagram of the power supply here: http://www.espruino.com/Pico

Afaik the MCP1703T will handle 400mA peaks, but only 250mA constantly (even at that it's likely to get toasty regulating down from 16v though). I think it depends what you're doing - if you have devices that will run from whatever battery volts is then I'd run them off battery volts, but if you're running some very heavy duty 3.3v stuff then I'd go for an external regulator.

However, the obvious 3.3v regulators have a ~4mA quiescent current (so drawn all the time), so if you're going after power consumption I'd avoid them. A few regulators have an enable pin though, which could be very useful - allowing you to actually turn off stuff that isn't needed.

@Gordon - according to the datasheet, the MCP1703T is not as good as you're suggesting... The limit is power dissipation from the SOT-23 package.

See pages 13 and 14, about the power dissipation calculations... Running off USB, taking 4.7v, bringing it down to 3.3 (ie, dropping 1.4v), well, let's do the example at 25C ambient...

SOT-23A (336.0°C/Watt = RθJA)
PD(MAX) = (125°C - 25°C) / 336°C/W
PD(MAX) = 297 milli-Watts (let's call it 300)

300mW / 1.4v = 213 mA

So even just running off USB, it's not rated for 250mA continuous...

Ahh - thanks. Missed that... I figured 250mA was very optimistic at 16v, but it's interesting it can't hit that even at under 5v.

following traces, probing points on the board

@DavidSchoen - visualization provides understanding... (and matching schematics to board). That's what I had done to understand what's going on.

1 Attachment

It doesn't I'm afraid (there's no 5v regulator) - the VBAT pin is battery or USB volts, so you need to be careful there. There's actually the circuit diagram of the power supply here: espruino.com/Pico

Given this is the only big thing you thought I got wrong, I had enough confidence to wire up 10V to the BAT_IN and check it with more than just USB coming in, you are of course right :)

I've had a look at that power diagram a couple of times, but without knowing more about how it worked I couldn't really get much out of it because the pins on it are VBAT, 5V and VDO - given they're not referenced like that anywhere else on that page it's not obvious (to me) that they were pins so I'd originally assumed this was somewhere in the middle of the circuit and hadn't realised I could easily probe any of those points.

Given the little business card that comes with the kit, the board itself and the schematics all have different names for these pins - do you think that a section could be added to that page clarifying which pins have changed names over time and what those various names are?

Thanks +allObjects, that's way more illustrative!

Do you know what the device between the GATE/SRC/DRN pads is? I had assumed that would be a regulator for 16V -> 5V (and hadn't yet been confident enough to supply > 3.7V to BAT_IN), but clearly it's something else and now I'm just curious :)

Yeah - we definitely need to unify the nomenclature for power pins... See also http://forum.espruino.com/conversations/­266857/ for more discussion of this with an eye towards the silkscreen on the second run Picos.

Yes, sorry about that, it's a bit of a mess - I didn't realise quite how bad! Bit busy today but I'll make sure I update the Pico page soon to clarify what's happening with the power. For now:

Do you know what the device between the GATE/SRC/DRN pads is

Yes, it's a PFET (Q1 in the circuit). Basically it turns itself on when USB is disconnected, which means you can then run off a battery without any voltage drop (which you'd get if you just used a diode).

1 Attachment

Awesome, thanks again +Gordon (especially for taking the time to humour me about figuring out what the PFET was) :)

No problem - there's a bunch of documentation, and often I only know that bits of it are inadequate when someone tells me :)

I know this is an old thread and I just had a quick question that is related. For a battery greater than 12 volts (less than 16 volts) my PICO disconnects from the Web IDE. Could not find this covered anywhere.

Is it shutting itself down because youre putting a load on it and it's oberheating?

I thought that so I unplugged it from the board so that it was completely solo and it still disconnected.

So even though it's just on it's own, it's no longer working?

How did you connect it to the battery? If you use VBAT then you should be fine. If you used the USB connector it's going to have broken one of the inputs - USB is only meant for 5v, and there's an IO pin connected to it so the Pico can tell if it's powered from USB or not. If it's happened there's still a way to get USB working, but it's not perfect.

The other option is might you have had something connected to an input that was also 12v or more? Like a PNP transistor connected to 12v?

While you can power the Pico from a high voltage, it's GPIOs will only handle 5v.

I am using a power supply and it is connected through the BAT_IN. Just tested another one. The LED starts blinking (it shouldn't be) when the power supply is connected.

Do you have your own software on the board, or is it a plain board that you're just applying power to?

Its a PICO with 1v85. Other than that nothing is on it when I tested this

I made this little video to show what I am talking about.
https://drive.google.com/file/d/1djESTRF­6I6BbyA2_IGST9CBFjgB1BcivfA/view?usp=sha­ring
(Sorry about my kid in the background)

Thanks for the video - that's really helpful.

So if you write:

setInterval(function() {
  digitalPulse(LED2,1,100);
}, 1000);

to the board, then type save() on the left hand side, what happens when you plug the board in to power supply without USB? Does the green LED keep blinking once a second?

I'm just wondering if this could be some kind of strange grounding issue (as maybe both the laptop and power supply are powered off the mains)... I've got a Pico doing plant watering that I left connected to a car battery (so 13v-ish) all summer, and I never had issues even when plugging in a laptop.

Other thing is - did you do the Pico FET tutorial? Any chance you left the solder jumper on the back soldered up? Even if not, if you have some contact cleaning fluid (isopropyl alcohol?) you could rub over the rear of the board, making sure the two solder jumpers are cleaned.

Potentially some salt from fingers could have got on it, and that might be causing problems for the Pico.