A Novel Methodology of PWM/PFM Mode Transition for Inverting Buck-Boost and Boost Converter for AMOLED Display Applications

A novel methodology for a smooth mode transition between Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) modes has been proposed for an Inverting Buck-Boost (IBB) and Boost converter. A dual mode (PWM and PFM) of operation with auto-mode transition by load current-sensing ensures high efficiency across load currents and smooth mode transition in DC-DC converters. In a conventional load current-sensing, the mode transition threshold varies with DCDC input and output voltages. The problem aggravates for an AMOLED display driver due to its support for wide range of input and wide programmability on output. The proposed techniques can accurately fix the thresholds for mode transition by accurately sensing the load current from PWM/PFM timing information. A wide hysteresis of 30mA and 25mA is maintained between two thresholds for IBB and Boost respectively, to avoid false triggering between the modes across inputs and outputs. Proposed circuits are implemented and tested in BCD-180nm technology. Mode transition logic of IBB and Boost occupies very small area of around 190x190$\mu \mathrm{m}^{2}$ each and consumes maximum quiescent current of 22$\mu$A and 20$\mu$A respectively.