Sensorless Digital Peak Current Controller for Low-Power DC-DC SMPS Based on a Bi-Directional Delay Line

Peak current mode control is challenging to implement in integrated dc-dc converters for sub 1 W applications due to the need for an integrated high-bandwidth, low-noise current sensor. The analog sensor must typically consumes less than a few hundred micro-amps while amplifying the current through the high-side switch, which has frequency components extending into the ten´s of MHz. Sensorless current mode control (SCM) eliminates the need for an explicit current sensor by reconstructing the inductor current from the system input/output voltages and the PWM signal pulse-width. In this work, a bidirectional delay line based digital SCM scheme is proposed. The inductor current is mapped onto the bi-directional delay line whose propagation delay tracks the inductor current slopes. An integrated digital current observer designed in a 0.18 mum CMOS process is compared to a benchmark analog current sensor operating at 2 MHz and fabricated in the same process. The digital current observer consumes 168 muA, or 20% less then the benchmark at I_out = 100 mA, while avoiding signal-to-noise degradation at light loads. A prototype of the digital current sensor operating at 1 MHz is demonstrated on a CPLD platform.