A New Inverse Charge Constant On-Time (IQCOT) Control with Fast and Optimized Transient Response
Due to its excellent small-signal property and light-load efficiency, ripple-based constant on-time current-mode control (COTCM) has been widely used in voltage regulator (VR) controllers. One issue with this ripple-based COT control is that, in the heavy load step-up transient, the increment of inductor current becomes limited by the fixed on-time and the systems minimum off-time ratio in each cycle, which can create a large under-shoot at the output. On the other hand, in the case of load step-down, if the load change occurs at the beginning of fixed Ton, because of the fixed Ton, the inductor current continues to increase until the end of Ton, instead of decreasing. In that case, a large over-shoot can also occur at the output. Some controllers use nonlinear controls to increase or decrease the Ton at the load transient. The problem with these threshold-based nonlinear controls is that they need to be optimized with changes in circuit parameters, i.e., Vout, to avoid overcorrection or ring-back at the output. This optimization process makes the system more complex. A new COTCM control, based on the concept of inverse charge control concept is proposed to resolve these limitations by allowing natural and linear Ton extension in the load step-up transient, and truncating the Ton in the load step-down transient, without adding any nonlinear control in the system.
The proposed structure with conventional COTCM control, the IQCOT, is presented in Fig. 1. As shown, the difference between Vc and IL × Ri is converted into current by using a gm amplifier, and this current is used to charge a capacitor. Then, this capacitor voltage (Vramp) is compared with a fixed threshold voltage (VTH) to create pulse frequency fsw. When Vramp touches VTH, the off-time ends and a fixed on-time (Ton) is started. In case of a large load step-up transient, when Vc-IL × Ri becomes very large, fsw pulses can occur even before the end of the previous on-time. If these very close pulses are allowed to merge together to create a longer on-time (Fig. 2), significant under-shoot reduction can occur at the output. Another important feature is that, since the fsw pulse increment is proportional to Vc-IL × Ri (Fig. 2), the Ton extension is eventually linearly proportional to the Vout under-shoot. This will eliminate any chance of overcorrection or ring-back of Vout, which is a major problem in Ton extension method by nonlinear controls. Fig. 4 shows that the IQCOT not only reduces under-shoot by naturally increasing the Ton, but also its response is naturally optimized at different transient conditions. Fig. 3 shows that when an over-shoot is created in Vout at the load step-down, Vc decreases very quickly and crosses the IL × Ri. This can be used to create a logic (like Vos in Fig. 3), and can be used to immediately truncate the constant Ton, thus reducing the Vout over-shoot (as shown in Fig. 5).