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A New Inverse Charge Constant On-time (IQCOT) Control with Ultrafast Load Transient Response
Year: 2016
Fig. 1. Proposed IQCOT current mode control structure
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).
Fig. 2. Waveforms at load step up transient
Fig. 3. Waveforms at load step down transient
Fig. 4. Conventional COT with no pulse merging
Fig. 5. Proposed IQCOT with pulse merging
Fig. 6. Overshoot reduction at load step up using proposed control
