PMBus stands for Power
Management Bus, an abbreviation for a open-standards protocol
established over the last few years for digital
power managment in systems. The protocol uses the I2C
data bus (created by Philips
in the '80's for inter-chip communications).
Digital power management involves incorporating management and monitoring logic in all the power supplies on the circuit board to enable each to be individually monitored for temperature, output voltage & current, and be programmed for not only voltage and current but also turn-on sequence with the other supplies on the board and how fast and steep the "slope" of the power-up curve should be.