Raspberry Pi MXL7704

Raspberry Pi MXL7704
Type	Circuit
Device(s)	Raspberry Pi
Difficulty	◉◉◌◌ Medium

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)

The MxL7704-AQB, MxL7704-XQB, MxL7704-R3 and MxL7704-P4 might be replaceable by the one-time programmable (OTP) successor MxL7704-BQB

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

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:

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 access the I²C interface:

Model	SDA	SCL
Raspberry Pi 3 (A+ and B+)	PP60	PP59
Raspberry Pi 4	The relevant traces MXL7704 location on Raspberry Pi 4

Compute Module 4 (CM4)

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 cannot start - so group0 cannot start. With it all the buck converters stay disabled.

A 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

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