A New Inverse Charge Constant On-time (IQCOT) Control with Ultrafast Load Transient Response
Ripple based current mode constant on-time (COT) control is currently very widely used in VR controllers for its excellent small signal property and light load efficiency. One issue concerning this ripple based COT control is that in the heavy load step up transient, the inductor current increment becomes limited by the fixed on time and system minimum off time ratio in each cycle, which can create a large undershoot at the output. On the other hand, in the load step down case, if a load change occurs at the beginning of fixed Ton, the inductor current keeps increas-ing until the end of Ton, instead of decreasing. In that case, a large overshoot can occur at the output as well. Some controllers use nonlinear controls to increase or decrease the Ton at load transient. The problem with the threshold based nonlinear controls are, they need to be optimized with the change of the circuit parameters i.e. Vout to avoid the overcorrection or ring back at the output. This optimization process makes the system more complex. A new current mode COT control based on inverse charge control concept, IQCOT is proposed to solve these limitations by allowing for a natural and linear Ton extension in the load step up transient and to trun-cate the Ton in load step down transient, without adding any nonlinear control in the system.
The proposed IQCOT structure is presented in the Fig. 1 where 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, 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 previous on time. Now if these very close pulses are allowed to merged together to create a longer on time (Fig. 2), a significant undershoot reduction can be done at the output. Fig. 4 and 5 show that the Vout undershoot can be reduced by naturally in-creasing the Ton using the pulse merging feature of the proposed control. Another important feature is, as the fsw pulse increment is proportional to Vc-IL*Ri (Fig. 2), the Ton extension is eventually linearly proportional to the Vout undershoot. This will eliminate any chance of over-correction or ring-back of Vout which is a major problem in a fixed Ton extension method by nonlinear controls. Fig 3 shows that when an overshoot is created in Vout at load step down, Vc goes down very quickly and cross the IL*Ri which can be used to create a logic (like Vos in Fig. 3) and use it to truncate the constant Ton immediately in order to reduce the Vout overshoot (as shown in Fig. 6).