There has been a lot of speculation about the power supply design for the production Raspberry Pi devices. The alpha boards use a pair of switch-mode power supplies to generate 5V and 3V3 rails from a 6-20V input on a coaxial jack, and LDOs to generate the low-current 2V5 and 1V8 rails for the analog TV DAC and various I/O functions. This is a flexible and power-efficient design, but suffers from two drawbacks:
- Expense. The two switch-mode parts and their accompanying inductors add roughly $2 to the cost of the device.
- No support for 5V input. To correctly generate the 5V rail, more than 5V has to be presented at the input.
After a lot of experiments and spreadsheet work, we finally settled on an LDO-only power supply design, with a fixed 5V input, and the 1V2 core voltage generated directly from the input using the internal switch-mode supply on the BCM2835 die. We have chosen a 5V micro-USB jack to supply power to the board, for two reasons:
- Ubiquity. Micro USB has been chosen as the GSMA’s Universal Charging Solution, so we expect AC adapters with this connector to be cheap and plentiful.
- Voltage assurance. Unlike coaxial jacks, we know that we’ll be receiving 5V over this connector, which we can pass on unregulated to HDMI and USB devices.
Model B owners using networking and high-current USB peripherals will require a supply which can source 700mA (many phone chargers meet this requirement). Model A owners with powered USB devices will be able to get away with a much lower current capacity (300mA feels like a reasonable safety margin).