Mixed-signal hysteretic internal model control of buck converters for ultra-fast envelope tracking

Envelope tracking (ET) applications demand high tracking bandwidth and high efficiency using a DC-DC converter. Design based on small-signal models using fixed frequency digital pulse width modulator (DPWM) often results in limited closed-loop bandwidth. This paper proposes a mixed-signal hysteresis current control (MSHCC) in a DC-DC buck converter with an analog current-loop and a digital voltage controller Gc(z). This achieves robust stability and parameter insensitive current ripple by sampling the output voltage at the rising edge of high side gate signal. A fixed-gain power amplifier driven by a buck converter can be assumed to be a constant resistive load. Thus the load current information is directly obtained from the reference command, which further improves the tracking performance. The real-time tuning of Gc(z) using the proposed MSHCC achieves fast recovery with an inherent current limiting, and the use of the internal model control (IMC) further minimizes the tracking error. A fixed frequency operation can be achieved through a real-time band adaptation. The proposed controller is implemented using an FPGA device.