Programmable Digital Controller for Multi-Output DC-DC Converters with a Time-Shared Inductor Amir Parayandeh, Andrija Stupar, Aleksandar Prodić Laboratory for Low-Power Management and Integrated SMPS University of Toronto, ECE Department Email: {amiran, prodic}@ele.utoronto.ca; a.stupar@utoronto.ca Abstract— This paper introduces a low power digital PFM controller for multi-output dc-dc converters suitable for integration in modern low-power management systems. It utilizes only one inductor to provide multiple output voltages and has very low power consumption. In addition, its reference voltages and switching frequency can be programmed dynamically. To achieve these characteristics two new key functional blocks are developed, namely Σ-∆ programmable delay-line based comparator utilizing natural filtering of delay cells and on-time control logic. The controller is implemented both on FPGA and a 0.18-µm CMOS application specific IC. Experimental results obtained with a 1 W, 9 V, four-output buck prototype and IC simulations successfully verify controller operation. I. INTRODUCTION Modern portable applications require multiple supply voltages for two notable reasons. Modern portable devices generally require multiple low-power supplies for their functional blocks. For example, in digital still cameras (DSC) and cell phones, different supply voltages are used for a LCD screen, handset lighting, and camera flash. Additionally, with increase in speed and circuit density, power consumption has become a critical issue in battery operated portable devices. It has been shown that using multiple supply voltages on chip appears as a viable solution for reducing power consumption [1]-[5]. As a result, the means of providing different supply voltages on chip, in low cost and small area solutions, are becoming increasingly important [6]-[11]. Most of the existing commercial solutions combine multiple switching converters to supply the different blocks. In these systems each power stage uses a separate inductor, power switches and controller resulting in relatively large part number and size of the power module. To minimize the number of components and simplify control, multiple output flyback converters are also commonly used. However, they suffer from poor output regulation due to cross-coupling [12]. In addition, switching transistor is exposed to excessive voltage stress [13]. The controller proposed in [6] uses only one feedback loop to provide multiple supply voltages with improved regulation. This solution still requires several inductors, which are among the largest obstacles in successful minimization of low-power switching converters. Reduction in inductor numbers can be achieved through designs proposed in [7-10]. Using time-multiplexing of a single inductor these designs provide multiple regulated outputs. These solutions use analog controllers operating in discontinuous conduction mode (DCM) to provide precise regulation of output voltages However, in these solutions, output voltages are fixed and implementation of advanced power management techniques through dynamic output voltages adjustment is not a simple task. Even though the flexibility of digital hardware is known, in portable applications, digital PFM controllers for multiple-output supplies have not been commonly used, mostly due to their high power consumption that often exceeds the power delivered to the loads. In this paper we introduce a new low power digital controller architecture that provides tight regulation of multiple programmable supply voltages for light loads and allows simple implementation of power management techniques in portable applications. As shown in Fig.1, similar to analog solutions [7-10], the controller utilizes the fact that in PFM the inductor current is discontinues hence it can be time-shared between multiple outputs. Besides output voltages, the on-times of power switchers can also be dynamically changed. Therefore the switching frequency, maximum currents stress, and output ripple can also be indirectly controlled. Dynamic adjustment of on-times also allows for minimizing the switching losses for a given converter topology [14]. In the following section we briefly explain operation of the time-shared multi-output digital PFM controller. Section III describes novel low-power architectures of basic functional blocks. IC implementation and experimental results obtained with a FPGA prototype utilizing the new architecture are shown in Section IV and Section V summarizes the main results. in V H1 H2 Q 0 L C1 C2 C3 C4 load load load load Vout1(t) V out3 (t) Vout 2(t) Vout4(t) H1Vout1(t) H2Vout2(t) H3Vout3(t) H4Vout4(t) Vref1(t) Vref2(t) Vref3(t) Vref4(t) tr1(t) c 0 (t) Programmable on-time logic H3 H4 DAC Filter tonLUT Q1 Q 4 Q 2 Q3 tr2(t) tr3(t) tr4(t) c 1 (t) c 2 (t) c 3 (t) c 4 (t) 3µH 6-bit 10-bit VDAC LUT Σ∆ D 1 D 2 D 3 D4 Fig. 1: Single inductor multi-output DC-DC converter based on a Digital PFM controller. This work of Laboratory for Low-Power Management and integrated SMPS is supported by Sipex, Milpitas, CA, USA.