Inverse Charge Constant On-Time Control With Ultrafast Transient Performance

At the present time, current-mode constant on-time (CMCOT) control is widely used in voltage regulators (VRs) for higher light-load efficiency, higher bandwidth, and a simpler compensation network. In today’s VRs, a high load step with a very high slew rate is a very common requirement in microprocessor or memory applications. One issue of this ripple-based CMCOT control is the transient response limitation at load step-up and step-down, because of its fixed
$T_{\mathrm{\scriptscriptstyle ON}}$
time operation. In the heavy load step-up transient, the inductor current increment becomes limited by the on-time (
$T_{\mathrm{\scriptscriptstyle ON}}$
) and minimum off-time (
$T_{\mathrm{\scriptscriptstyle OFF}\_{}{\mathrm{ MIN}}}$
) ratio in each cycle, which can create large undershoot at the output. On the other hand, in the load step-down case, if a load change occurs at the beginning of the
$T_{\mathrm{\scriptscriptstyle ON}}$
, then the decrement of the inductor current will be delayed by the fixed
$T_{\mathrm{\scriptscriptstyle ON}}$
width, which can create a large overshoot at output. For the multiphase operation case, this ripple-based CMCOT control fails to operate at the ripple cancellation point where the inductor current ripple becomes zero. The state-of-the-art controllers add various nonlinear controls to the system to solve these issues; however, they are essentially difficult to optimize for different steady-state and transient conditions. In this article, a new concept called ‘inverse charge constant on-time’ (IQCOT) control is proposed, to improve the transient performance in CMCOT, by naturally increasing or decreasing the
$T_{\mathrm{\scriptscriptstyle ON}}$
time in the load step-up or step-down transient without adding any nonlinear control. As this
$T_{\mathrm{\scriptscriptstyle ON}}$
change is adaptive to the transient requirement, an optimized response is inherently provided, and hence, does not require any external adjustment. In a multiphase operation, IQCOT control can operate seamlessly at the ripple cancellation point. The proposed IQCOT control’s fast and optimized transient response, with higher light-load efficiency, makes it a perfect control candidate for today’s VR applications. Simulation and test results with a VR platform are also presented in this article to verify the benefits of the proposed control.