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A New Inverse Charge Constant On-Time (IQCOT) Control for Transient Performance Improvement in Multi-Phase Operation

Fig. 1. Proposed IQCOT current mode control structure
As we all know, today's VRs need to supply high load current to multi-core CPUs with a large slew rate requirement. This creates a need for a multiphase operation with large phase numbers and a fast transient response. Currently, ripple based constant on-time (COT) current mode control is very widely used in VR controllers because of its excellent small signal property and light load efficiency. From the transient point of view, one issue with the ripple based COTCM is that, in the heavy load step up transient, the inductor current increment becomes limited by on time and minimum off time ratio in each cycle, which can create a large under-shoot at the output. Moreover, in the multiphase operation case, the limited pulse overlapping capability of different phases becomes an issue at heavy load step up transient. Some controllers use an external ramp to improve jittering for noise sensitivity at the output. However in that case, pulse overlapping in transient becomes even more difficult. Most of these cases, controllers use nonlinear controls to force pulse overlapping in load transient. The problem with these threshold based nonlinear controls are that they need to be optimized with the change of the cir-cuit parameters i.e. Vout. A new current mode COT control based on inverse charge control concept (IQCOT) is proposed in this paper to solve these limitations by allowing a natural and linear Ton extension and pulse overlapping in load step up transient without adding any nonlinear control in the system.

The proposed IQCOT structure (inside red box) with a 2-phase VR is presented in Fig. 1 where the difference between Vc and Isum*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 a pulse frequency fsw. When Vramp touches VTH, off time ends and a fixed on time (Ton) is started. This method is shown in the steady state part of the waveforms in Fig 2. 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 Ton time. In that way, the proposed IQCOT control can achieve the natural pulse overlapping feature between phases, (shown in Fig. 2) and improve the load step up transient performance. As in each phase, one Ton pulse can occur even before the previous Ton pulse finishes. They can also merge together to create a longer on time in each phase, using the proposed Ton generator. In that way, the proposed IQCOT structure enables the control to meet the high current with faster transient response requirement by achieving a smooth natural and linear pulse overlapping in a high phase count multi-phase operation along with natural and linear Ton extension at load step up transient. Test results are shown in Fig 3. With these two features together, the proposed control can reduce a significant amount of the output capacitor on the board causing a reduction in cost and space.


Fig. 2. Waveforms at load step up transient
Fig. 3. Test Result Waveforms
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