I’ve just glanced at them, yet, but I’m sure to look depper with time.
Some things that I found curious:
No date and neither a designer is to be found in the schematic. Very disappointing, looks really unprofessional to me
The schematics don’t seem to be complete. For instance the strings “AUDIO_POWER_KEY” or “SMC_STM_SWDIO” can only be found on one page, but logically they shouldn’t be open end, so there seems to be something missing.
The main powermanagement IC doesn’t have all it’s ground pins connected. In the schematic four of five of them are actually floating. This strikes me as really odd. On the x-rays, however, it seems that they connected the corner ground pins to the big cooling pad in the middle. But it’s almost just wishful thinking as I cannot see anything clearly there.
I haven’t found any switching transistor/IC with any sort of latch-up-protection. Is this common in handheld consumerproducts? It looks like bad practice to me. I’d have thought that you want to protect against ESD-transients?
It’s not a very handsome schematic. Doesn’t really look like somebody put a lot of love into it. And the added chinese parts are the cherry on top :\
@bluez thanks for your start in the collection of the datasheets of the parts used in the Librem 5.
Unfortunately i couldn’t get your link to the PMIC to work. Probably found the device on digikey but their datasheet link also seems to be dead. If you or someone else got the time could you please update it.
I think the point about the missing name might be a cultural thing, since it seems the schematic is designed in china.
I’ve found both your missing strings within the schematic by searching for them.
On the Picture of the PCB it seems like they are connected at least from my perspective.
From what I found all the connections to the outside even the Killswitches are protected from ESD-transients with TVS diodes and chokes which seems to be the norm for consumer products.
I agree with you the schematic could use some TLC and I think the Board was developed in China so those Chinese characters are to be expected.
I’ve adjusted the link. There was a superfluous whitespace.
Did you find BOTH ends? I only find each string once. So i cant see where the signal goes. Obviously they go to the processor which is on another schematic.
I really found very little protection. Just the suppressordiodes on the USB.
Each transistor has a diode connecting the gate with the output, but this is a built-in diode and the manufacturer strongly recommends using additonal protection. Also the proper protection would ensure a maximum gate-voltage, e.g. by a zenerdiode from the powerrail to the gate.
I’m also wondering about the charging curcuitry. Does anybody have an idea what the purpose of all those tiny resistances are? Are they supposed to be inductances? I’ve never seen such a weird symbol.
And why didnt they use any output capacity?
Also: Why did they choose to implement a seperate fuel gauge, if the charging IC they picked can do it as well?
The AUDIO_POWER_KEY seems to originate on page 10 of the schematic from the killswitch and end on page 20 in the LDO used to supply the Power to the audio chip.
And the SMC_STM_SWDIO starts on page 13 on top of the stm32l and ends on the debug connector on page 21
AFAIK it’s standard procedure to only place Protection at connections that can be directly touched by the user of an closed device so you Protect the USB Connector Audio which also has some TVS diodes connected to it.
Within a normal higher voltage system I’d agree with you but the FET you’ve linked got a maximum gate voltage of 8V where as the max voltage within the system seems to be 5V so in my opinion that’s a big enough margin. And don’t forget that most of those FETs are driven by ICs which normal have their internal Diode to their power-rail.
I’ve seen similar Parts in other schematics where they are used as requirements to the layouter of the Board to keep certain Traces within a maximum resistance.
I think we all want to be very sure that the battery does not explode, and measuring the charge of Lithium-ION cell under load can be very difficult, I understand their decision to use two chips. especial since the second one seems to only measure voltage and temperature.
What do you mean with output capacity? There are some capacitors in the third schematic block on page 12 directly connected to the battery voltage.
