MXL7704 (Raspberry Pi)

The MXL7704 is a programmable quadruple step-down DC/DC converter that is used in the following Raspberry Pi models:


 * Raspberry Pi 3 (A+ and B+) (MXL7704-R3 version)
 * Raspberry Pi 4 (all) (MXL7704-P4 version)

Common cause of failure
If the Pi's 5V pin is shorted to the neighboring 3.3V pin, the MXL7704 will try to clamp the voltage on the 3.3V line. This destroys the MXL7704 chip, leaving the 3.3V line permanently shorted to GND.

Configuring
The MXL7704 chip can be configured via an I2C interface. The I2C address depends on the version of the chip:

Despite having a different I2C address, the R3 version of the chip will be correctly recognized by the OS if it's installed in a Raspberry Pi 4.

Note that any changes made to the configuration are not permanently stored. Every time the chip is powered off, its default configuration is restored. Different versions of the chip have different default configurations:

Here's the meaning of the two default configurations (for more info, see the datasheet linked at the bottom of the page):

The following pads can be used to acces the I2C interface:

Repeated failures?
Currently it is suspected that in some cases, a failure of some other component is destroying the MXL7704.

On multiple Pi 3A+ boards, the resistance between the 5V and the 3.3V rails has been measured after soldering in a new chip (but before applying power to the board). It has been observed that supplying power to the board destroys the MXL7704 immediately if a resistance of 50k was measured. This happened every time, even when the chip was swapped multiple times in a row. On functioning boards, the resistance between the 5V and the 3.3V rail is above 100k.

More research is needed to determine if the resistance actually is linked to those failures, and to locate the component that is causing the issue.

Note that the I2C interface usually doesn't work if the chip is broken.