A 12V/24V-to-1V DSD Power Converter with 56mV Droop and 0.9\mu \mathrm{S}$ 1% Settling Time for a 3A/20ns Load Transient
Jingyi Yuan, Zeguo Liu, Feng Wu, Lin Cheng
Abstract
12V/24V-to-1V power converters with high efficiency and fast transient responses are highly desirable in industrial and automotive applications. Double step-down (DSD, also known as series-capacitor) converters are among suitable candidates, as they provide 2× duty-cycle extension and 2× equivalent switching frequency with reduced voltage stress on power transistors by adding only one flying capacitor into a two-phase buck converter [1]. However, it is still challenging to robustly control a DSD converter to achieve fast transient responses. Hysteretic control is widely used in the buck converters for its simple structure and fast responses. However, it may not be feasible to apply hysteretic control to a DSD converter as the on times of the two phases must have 180° phase shift and cannot be overlapped. PWM control with fast Type-III compensation [2] is another attractive option, but it suffers from an inherent delay up to half of a switching period when responds to a load transient, as will be explained later. In [1], an adaptive on/off time (AO2T) control approach is proposed to improve the responses by extending the on time during the transient. However, the 10% settling time of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$8.2\mu\mathrm{s}$</tex> is longer than 16 switching periods for a load step of only 1A and a complicated phase mirroring circuit is also needed. Moreover, as the operation principle of the DSD converter prevents the two phases from being turned on simultaneously, the benefit of doubling the slew rate of the inductor current which is taken advantage of by the two-phase buck converter disappears.