Digitally Controlled Current-Mode DC-DC Converter IC

The main focus of this paper is the implementation of mixed-signal peak current mode control in low-power dc-dc converters for portable applications. A DAC is used to link the digital voltage loop compensator to the analog peak current mode loop. Conventional DAC architectures, such as flash or Delta Sigma are not suitable due to excessive power consumption and limited bandwidth of the reconstruction filter, respectively. The charge-pump based DAC (CP-DAC) used in this work has relatively poor linearity compared to more expensive DAC topologies; however, this can be tolerated since the linearity has a minor effect on the converter dynamics as long as the limit-cycle conditions are met. The CP-DAC has a guaranteed monotonic behavior fromthe digital current command to the peak inductor current, which is essential for maintaining stability. A buck converter IC, which was fabricated in a 0.18 mu m CMOS process with 5 V compatible transistors, achieves a response time of 4 mu s at f(s) = 3 MHz and V-out = 1V, for a 200 mA load-step. The active area of the controller is only 0.077 mm(2), and the total controller current-draw, which is heavily dominated by the on-chip senseFET current-sensor, is below 250 mu A for a load current of I-out = 50 mA.