MXL7704 (Raspberry Pi)

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The MXL7704[1] 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)


The MxL7704-AQB, MxL7704-XQB, MxL7704-R3 and MxL7704-P4 might be replacable[2] by the one time programmable (OTP) successor MxL7704-BQB[3]

The new model may be (one time) programmable to match the specific raspberry pi model default values.

Common cause of failure[edit | edit source]

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[edit | edit source]

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

Version Address

AQB and XQB versions

0x2D

R3 version (Raspberry Pi 3)

0x1D

P4 version (Raspberry Pi 4)

0x1B

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:

Model Default configuration

Raspberry Pi 3 (A+ and B+)

pi@raspberrypi:~ $ i2cdump -y 1 0x1d
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: a0 01 e4 27 43 57 27 00 00 00 02 00 XX XX XX XX    ???'CW'...?.XXXX
10: a5 a5 5a c0 c0 e4 1f 48 10 a9 7f 67 00 00 XX XX    ??Z????H???g..XX

Raspberry Pi 4

pi@raspberrypi:~ $ i2cdump -y 1 0x1b
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: a0 01 e4 27 43 57 27 00 00 00 02 00 XX XX XX XX    ???'CW'...?.XXXX
10: a5 a5 5a b0 a5 e4 de 48 10 e9 5e 66 00 27 XX XX    ??Z????H??^f.'XX

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

Address Setting Raspberry Pi 3A+ / 3B+ Raspberry Pi 4

Value

Meaning

Value

Meaning

0x02

Phase interleaving

0xe4

1 → 2 → 3 → 4

0xe4

1 → 2 → 3 → 4

0x10

V_out LDO

0xa5

3.3V

0xa5

3.3V

0x11

V_out buck 1

0xa5

3.3V

0xa5

3.3V

0x12

V_out buck 2

0x5a

1.8V

0x5a

1.8V

0x13

V_out buck 3

0xc0

1.2V

0xb0

1.1V

0x14

V_out buck 4

0xc0

1.2V

0xa5

1.03V

0x15

Buck Sequence Group Assignment

0xe4

  • 1: group 0
  • 2: group 1
  • 3: group 2
  • 4: group 3

0xe4

  • 1: group 0
  • 2: group 1
  • 3: group 2
  • 4: group 3

0x16

  • LDO Sequence Group Assignment
  • Channel Enables

0x1f

  • group 0
  • all enabled

0xde

  • group 3
  • bucks enabled, LDO disabled

0x17

  • SEQ EN Assign
  • PG1 Routing

0x48

  • SEQ EN: group 1
  • PG1 Routing: buck 4

0x48

  • SEQ EN: group 1
  • PG1 Routing: buck 4

0x18

PG2 Routing

0x10

buck 4

0x10

buck 4

0x19

  • Fault Actions
  • Down Sequencing
  • Frequency

0xa9

  • Reset whole chip if any channel faults
  • Soft power off disabled
  • 78 Ω discharge enabled
  • 1.81 MHz

0xe9

  • Reset whole chip if any channel faults
  • Soft power off enabled
  • 78 Ω discharge enabled
  • 1.81 MHz

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

Model SDA SCL

Raspberry Pi 3 (A+ and B+)

PP60

PP59

Raspberry Pi 4

The relevant traces

Compute Module 4 (CM4)[edit | edit source]

Since the LDO VIN is shorted to ground by CM4's layout design, the MxL7704-P4 (where the LDO is disabled by default programming) has to be used.

When using the MxL7704-R3 variant (which is the one available in aliexpress), the CM4 will not start since the LDO is assigned to group0 and can not start - so group0 can not start. With it all the buck converters stay disabled.

An tested alternative for using MxL7704-R3 for repairing CM4 is to disconnect Pin8 (LDO) and Pin7 (VIN) from the board and connect it (Pin7) permanently with +5V instead.

References[edit | edit source]

  1. MXL7704 datasheet, MaxLinear, Inc., 2018
  2. MXL7704 Product Discontinuation Notice, MaxLinear, Inc., 2021
  3. MXL7704-BQB datasheet, MaxLinear, Inc., 2022