charge-borrowingdecap-noise - IEEE 2009 Custom Intergrated...

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Charge-Borrowing Decap: A Novel Circuit for Removal of Local Supply Noise Violations Xiongfei Meng, Resve Saleh, and Steve Wilton Dept. of Electrical and Computer Engineering, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada Abstract - We propose a novel circuit called charge-borrowing decap (CBD) as a drop-in replacement for passive decaps to reduce supply noise for removal of “hot-spot” IR- drop problems found late in the design process. Measurement results on a 90nm test chip show that a noise reduction improvement between 42%- 55% at 100MHz-1.5GHz over its passive counterpart. I. INTRODUCTION The increase in clock frequency and on-chip current demand makes power grid design a challenging task. Decoupling capacitors (decaps) are generally used to reduce IR drop and Ldi/dt effects, and hence keep the power supply relatively constant. Starting from 90nm, simply placing passive decaps in the available open areas of the chip may not be sufficient [1]. Large power supply noise levels in localized regions (called “hot spot” IR -drop violations) may unexpectedly be present in high-speed applications. These unresolved hot spots cause timing closure problems or result in functional failures in extreme cases. To remove them, active decaps [2][3][4] have been proposed for use as a drop-in replacement of the passive decaps. The use of active decaps saves time and effort near the tapeout deadline, and therefore provides an attractive solution. This paper proposes a novel charge-borrowing decap (CBD). The CBD design provides significantly more charge than a passive decap to reduce the local supply noise level, with only a minimum power loss during charge transfer from a clean supply node. As a result, the new circuit provides better noise reduction performance than passive decaps and even active decaps. With a relatively simple and robust design, the CBD only requires a small area overhead. II. CHARGE-BORROWING DECAP CONCEPT AND DESIGN A. Charge-Borrowing Decap Concept Fig. 1. CBD concept shown in (b), compared to a passive decap in (a). The main purpose of a decap is to provide charge to stabilize the supply voltage locally. Unlike a passive decap, which provides charge from itself, our new circuit “borrows” charge from a clean supply node to reduce local noise level. This charge borrowing is accomplished using the effect of capacitive feedthrough, as illustrated in Fig. 1. For a fixed decoupling capacitance of C decap , we assume certain supply noise k V DD is present on the supply, where k is a fractional number and normally in the range of 0.05 to 0.2. A passive decap, as shown in Fig. 1(a), provides a charge of k C decap V DD to the supply. In a CBD, as shown in Fig. 1(b), however, the charge provided by the CBD circuit in one clock cycle can ideally be up to C decap Δ V clk = C decap V DD , where Δ V clk is the clock swing. Clearly, over one clock cycle, the CBD provides 5X-20X more charge than a same-area passive decap, depending on the actual noise level. From another perspective,
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charge-borrowingdecap-noise - IEEE 2009 Custom Intergrated...

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