SLVSC86A January 2014 – March 2014 UCD9244-EP
PRODUCTION DATA.
By using digital circuits to create the control function for a switch-mode power supply, additional features can be implemented. One of those features is the measurement of the open loop gain and stability margin of the power supply without the use of external test equipment. This capability is called automatic system identification or Auto-ID™. To identify the frequency response, the UCD9244 internally synthesizes a sine wave signal and injects it into the loop at the Vref DAC. This signal excites the system, and the closed-loop response to that excitation can be measured at another point in the loop. The UCD9244 measures the response to the excitation at the output of the digital compensator. From the closed-loop response, the open-loop transfer function is calculated. The open-loop transfer function may be calculated from the closed-loop response.
Note that since the compensator and DPWM are digital, their transfer functions are known exactly and can be divided out of the measured open-loop gain. In this way the UCD9244 can accurately measure the power stage/load plant transfer function in situ (in place), on the factory floor or in an end equipment application and send the measurement data back to a host through the PMBus interface without the need for external test equipment. Details of the Auto-ID™ PMBus measurement commands can be found in the UCD92xx PMBus Command Reference (SLUU337).