XO 1.75 Power Distribution

This page describes the power distribution network of the XO-1.75 laptop, specifically the production C2 version.

Introduction

Inside a laptop, power has to be distributed from the battery to the different laptop components and efficiently transformed to suitable voltages. Low power is one of our highest design goals, and an efficient power distribution network is essential.

In the majority of places where we need to provide a particular voltage, a switching voltage regulator is used which efficiently transforms 85 - 90% of the power. In a few low power situations, small linear regulators are used --- these dissipate any excess voltage as waste heat.

Battery Technology

The battery technology used in the XO-1.75 is the same Lithium Ferro-Phosphate (LiFePO4) used in all XO laptops. It is similar to the more common Li-Ion battery, but safer. For power efficiency reasons, we use two cells in series. This provides a battery with an operating voltage (+VBAT in the diagram) of 5.4 to 7.4V.

Armada 610

In previous XO Laptop designs (XO-1,XO-1.5), the lowest power mode that the processor and companion chip were able to enter (the ACPI S2 state) still drew over a Watt of power. As a result, there was much effort expended in the electrical design of the motherboards to allow the system to quickly completely power down the processor and companion chip (S3), placing the main memory into self-refresh mode, while keeping the networking and display subsystems powered and running.

The Marvell Armada 610 was designed using a low-power technology which allows the power consumed by the processor and other circuitry to drop very close to zero when clocks are stopped. In XO-1.75, power is never removed from the processor when it is suspended. It does, however, use the normal Linux suspend/resume path when entering/exiting this "powered suspend", as reaching the lowest power states requires stopping much of the clock infrastructure in the Armada 610 used by integrated peripherals. It has two effects:

• All power supplies should be reasonably efficient from 1mA to full power, as they will remain on when the system is suspended.
• Unlike previous laptops, there are few power rails which are turned off during suspend.

Power Distribution

The following diagram attempts to illustrate the power distribution network: