Understanding and using PMBus™ data formats

The Power Management Bus (PMBus™) in an open standard protocol for power system management that was introduced in 2005. The reception has been enthusiastic with adoption by both power supply and semiconductor manufacturers. A key part of the specification, which took more than 250 person-hours to define, is the formats for sending and receiving data to and from a PMBus device. One format, called the literal format, exchanges data in user friendly units like volts and amperes. This format uses a floating-point like format with an eleven bit signed mantissa and a five bit signed exponent. The literal format minimizes the burden on the host or master device at the expense of greater complexity in the PMBus device. The direct format provides an alternative that minimizes or eliminates the computation in the PMBus device at the expense of more bus communication and being more computation intensive for the host or master device. This paper explores both formats, showing examples of how data is exchanged with PMBus devices. Particular attention is paid to the direct format, including a rigorous derivation of the equations needed to define the coefficients of the equation used by the host to encode and decode information exchanged with PMBus devices.