US Patent 10425089 Master/slave frequency locked loop

A master/slave configuration of a frequency locked Loop (FLL) decouples the process, target voltage, temperature (PVT) tracking goals of locking the loop from adapting the clock frequency in response to voltage droops in the supply. A master oscillator circuit receives a regulated supply voltage and supplies a master oscillator signal. A control circuit supplies a master frequency control signal to control a frequency of the master oscillator signal to a target frequency. A slave oscillator circuit is coupled to a regulated supply voltage and a droopy supply voltage and supplies a slave oscillator signal having a frequency responsive to a slave frequency control signal that is based on the master frequency control signal. The frequency of the second oscillator signal is further responsive to a voltage change of the droopy supply voltage.