Raspberry Pi 4 moves to USB-C power

Raspberry Pi 4 moves to USB-C power
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The Raspberry Pi 4 has moved to USB-C for power, sourcing a new power supply but retaining a power management chip from MaxLinear.
By Nick Flaherty

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Good, low-cost USB-C power supplies and cables are surprisingly hard to find, say the engineers at the Raspberry Pi Foundation, so they worked with Ktec to develop a suitable 5V/3A power supply for the Raspberry Pi 4 that provides  an extra 500mA of current, ensuring  a full 1.2A for downstream USB devices, even under heavy CPU load.. The supply is priced at $8, and is available in UK (type G), European (type C), North American (type A) and Australian (type I) plug formats.

The 4pin Power-over-Ethernet (PoE) connector remains in the same location, so the Raspberry Pi 4 remains compatible with the current PoE Hardware Attached on Top (HAT). This is a single-sided board that sits within the footprint of the Raspberry Pi and fits inside an official Raspberry Pi case. A small 25mm fan is pre-installed on the board to help with colling, and this is temperature-controlled over I2C via a small Atmel processor.

On the main board, MaxLinear’s MxL7704 Universal PMIC is used to power the $35 Raspberry Pi 4, linking directly to Broadcom’s BCM2711, a 1.5GHz quad-core 64-bit ARM Cortex-A72 CPU that provides three times the performance of the previous version.

It provides all key power rails required by the Raspberry Pi 4, including the low noise voltage rail used for audio circuitry. It also handles the unique power sequencing requirements of each rail with ease due to its convenient I2C programmability. The MxL7704’s I2C interface communicates with the computer’s SoC for dynamic voltage scaling, status monitoring, sequencing control and PGOOD routing. These features enable the Raspberry Pi 4 to save power by dynamically reducing the voltage to the SoC when the system is idle and boosting it when the processor is running at maximum speed.


The MxL7704 includes four synchronous step-down buck regulators that provide system, memory, I/O and core power from 1.5A to 4A. An on-board 100mA LDO provides clean 1.5V to 3.6V power for analog sub-systems. It also features an integrated 8bit ADC with two external inputs and temperature sensor that provide die temperature monitoring, telemetry and additional flexibility. The Raspberry Pi 4 also uses the MxL7704’s on-board ADC to determine if there is a high current delivery USB power supply. All these features are packed into a small 5x5mm 32-pin QFN package, which helps the Raspberry Pi 4 retain its small form factor.

“The MxL7704 provides five rails pre-optimized for ease of use in single-board computer systems,” said James Lougheed, Vice President of Marketing for MaxLinear’s High Performance Analog Products. “The PMIC includes a host of features that allow monitoring, telemetry and additional flexibility. These unique features provide the Raspberry Pi 4 with knowledge and control of power status and efficiency to ensure peak performance during various operating conditions.”

“After using the MxL7704 very successfully on the Raspberry Pi 3 Model B+, we were very pleased to select it again to power our next generation Raspberry Pi 4 Model B computer,” said James Adams, Chief Operating Officer for Raspberry Pi. “The combination of highly efficient, high current buck supplies in a very cost-effective package which also included I2C control, dynamic voltage scaling, programmable sequencing as well as a low noise LDO and ADC has been key to making sure we have met our design and cost targets for Raspberry Pi 4.”

The change to the connectors for USB-C has also meant a change to the case, so Kinneir Dufort and manufacturers T-Zero have developed an all-new two-part case, priced at $5. This takes into account the two USB 3.0 and two USB 2.0 ports, as well as a full throughput Gigabit Ethernet interface that has moved to the top right of the board, from the bottom right, greatly simplifying PCB routing. All three connectors on the right-hand side of the board overhang the edge by an additional millimetre, with the aim of simplifying case design. In all other respects, the connector and mounting hole layout remains the same, ensuring compatibility with existing HATs and other accessories.

The dual monitor support runs at resolutions up to 4K with VideoCore VI graphics, supporting OpenGL ES 3.x, 4Kp60 hardware decode of HEVC video and compatibility with earlier Raspberry Pi products. To accommodate the dual display output within the existing board footprint, the type-A (full-size) HDMI connector has been replaced with a pair of type-D (micro) HDMI connectors.

The 1GB version of the Raspberry Pi 4 costs $35, with the 2GB at $45 and 4GB for $55. This is the first time that the Raspberry Pi Foundation has offered different memory option.

www.maxlinear.com/mxl7704

www.raspberrypi.org

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