Full Boot from Bench Power
Once you have confirmed there are no shorts on the first-stage boot rails, you are safe to move on to checking the battery charger and VSYS circuit.
The very first place power enters the system is from the battery, through the BQ24193 which then sends out VSYS voltage on the output inductor to the rest of the system (which we injected 4.2V into on the last step).
In order for the Switch to fully boot to second-stage system, it will require the following ICs all working:
There may be other proprietary none-replaceable chips needed for boot however I have not personally come across any of those failing yet preventing boot.
The BQ24193 is located at the bottom right of the motherboard next to the battery.
The datasheet can be found in our Public Files or attached below.
Here are the important pins of the BQ chip on the Nintendo Switch.
The BQ expects a single cell LiPo battery on the BAT pin, which has a nominal voltage of 3.7V. A LiPo battery is totally flat at around 3.0V, low at 3.4V, average at 3.7V and fully charged at 4.2V.
The BQ charges this battery through the VBUS input pin. This pin can handle inputs from 3.9V to 17V.
The Switch will only allow negotiated power through the USB into the system to the BQ chip.
This means a dumb 5V USB cable without any data pins, or a dumb charger without any actual USB negotiation will not charge the Switch as it has to be correctly negotiated and a FET turned on by the MT92T36 which negotiates over USB-C protocol for power delivery.
The MT92T36 will talk to the USB power input device and negotiate power into the system. It will accept 5V or 15V only. If it gets successful negotiation then the MT92T36 opens a FET that passes this 5V or 15V USB power into the BQ chip VBUS pin.
NOTE: Unlike most popular information online, the P13USB has nothing at all to do with charging the battery, power negotiation or any link to the MT92T36 circuit at all other than a shared power pin (the M92T36 and P13USB share the same power rail). This means the P13USB can be completely removed from the Switch and you will still have a fully working Switch just without Dock support.
The only time the P13USB will interfere with interfere with the M92T36 is through a dead short bringing down the power rail of the M92T36.
As we are often working with a non-functional/faulty system, I do not like to use actual batteries to power or test the console, and in fact I exclusively use a bench power supply to power the Switch for all send-in repairs, no exceptions. Using a bench power supply to
The P13USB and MT92T36 are the most highly "swapped for no reason" chips on the Switch. It drives me nuts.
You can can completely ignore and even remove both chips if you like until later when we want to check USB-C charging and dock functionality. The Switch can be fully booted into second stage boot, with working LCD, backlight, touchscreen and all regulators and systems operational without them.
You will get an in-system boot error if you remove the MT92T36 as the CPU talks to it, so your system will show an error message on screen like this, but there is no point in "fixing" either of these chips until we get the system to boot up with the Nintendo Logo on screen. Save yourself the pennies and don't go down a rabbit hole trying to fix these 2 chips until we need to.
Now we have confirmed the Switch has no major shorts, we can go ahead and power the system entirely from the bench power supply. This is super powerful and allows us to take accurate current measurements as strong indicators to where problems lie in the system.
IMPORTANT: Set your bench power supply current limit now up from 1A to 2A. The Switch uses just over 1 amp during full second stage boot so if you have a 1A limit the console will get stuck mid-boot and sit drawing around 450mA (0.45A) after a few seconds.
With the battery still removed and the ground wire still on the USB shield, simply remove the wire from the inductor VSYS pad and instead solder a wire to the VBAT pad.
In order to trick the BQ chip into accepting our battery we must pass in a temperature voltage value so the BQ does not think our battery is overheating. Solder a 10k resistor (or close, 5k to 20k works) between the two test pads by the battery connector.
With the 10k resistor and 4.2V injected into the VBAT input of the BQ chip, we can now fully boot the Switch. If this was a working console and we powered on now, the entire system would fully boot.
Considering the whole point of all this is to fix non-working consoles, it is now important to figure out how we correctly identify what chips or parts of the circuit are failing and how to fix them.
To properly troubleshoot it is very important we boot the system in this way from a bench power supply so we can analyse the power draws which are incredibly useful for diagnostics. It also removes the potential issue of a bad battery, and it allows us to "see" the system booting without a working screen, identify if the eMMC is bad, bricked or missing, if a specific regulator is faulty and more, all from the simple current draw readings.
Next we will go over testing each chip in order for correct functionality.