Multiloop control for Fast Transient DC-DC Converter
Abstract : A novel ac coupled feedback (ACCF) is proposed to alternatively realize fast transient response while inherently controlling the start-up in-rush current of a dc–dc switching converter. The proposed ACCF is modified from a conventional capacitor multiplier and connected between the outputs of the converter and the transconductance. With this supplemental feedback, the transient response has been significantly improved due to the gain-boosting effect around the compensator’s midband. Moreover, the ACCF circuit assists to manage the ramping speed of the output voltage during power-up, thereby eliminating the bulky soft-start circuit. The new controller is very simple to implement and occupies a tiny footprint on-chip. A buck converter with the proposed scheme has been fabricated using the 0.18-µm standard CMOS process with an active silicon area of 0.573 mm2. Measurement results show that the output voltage rises linearly for a soft-start period of 1.05 ms according to the designed slope. Excellent load transient responses are achieved under different load current steps; the output voltage overshoot/undershoot of 60 mV settles down within 10 µs for a load variation from 50 µA to 1 A in 1 µs. Moreover, the proposed converter maintains both excellent load and line regulations of 0.018 mV/mA and 0.0056 mV/mV, respectively.
? There is a massive load current change when the system switches from an idle mode to an active mode. Ideally, the regulator should maintain a voltage level that is almost constant, which means that there should be a negligible output voltage overshoot /undershoot and rapid response time.
? To accomplish these stringent requirements, various research works on fast transient dc–dc converters have been proposed.
? The pole and zero are moved back immediately after the transient event to stabilize the system.
? However, a careful design targeting system stability during the whole process needs to be taken into consideration once the bandwidth is tentatively changed.
? Difficult to achieve fast load transient performance.
? Current boosting modules in existing system require more power, which degrades the overall efficiency.
? High electromagnetic interface (EMI) noise.
• This paper utilizes a current-mode controller to improve the response speed aspect by increasing the system bandwidth in order to enhance the load transient response.
• A novel ACCF is paralleled around the compensator to boost the response strength at the mid-band.
• With the help of the ACCF circuit, in this case, the fast-rising voltage appearing at Vout caused by the inrush start-up current is coupled through the ACCF and induces the ac current from Vc into the ACCF itself.
• This particular inherent function of the proposed ACCF eliminates the need for a dedicated soft-start circuit in the conventional designs.
? The demand for fast load transient performance has grown significantly, affecting the power supplies of modern high-speed processors especially processors targeting to achieve a fast transition from the low-power idle mode to the high-speed active mode.
? The load transient performance is improved on account of the gain boosting effect around the compensator’s mid-band.
? A variable resistor was connected at the output of the converter in series with a switch to measure the load transient performance.
? Moreover, current-boosting modules introduce more power consumption, which degrades the overall efficiency.
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