Biomedical Engineering Reference
In-Depth Information
Fig. 4 Size reduction trend of
integrated wireless sensor
network (WSN) nodes over
years
the node. Figure 4 summarizes commercially available integrated WSN nodes over
the past decade. We can see there is a clear trend of size reduction for the WSN
nodes [ 9 ]. In the late 1990s, WSN nodes were composed of multiple components
on a printed circuit board (PCB) as shown in Fig. 4 . These nodes consistently use
commercial-off-the-shelf (COTS) components, and as a result, their sizes are in the
tens of cubic centimeters. Higher levels of integration and better system-level node
design are pushing this to the mm scale. Nonetheless, there are few bottlenecks in
cubic-mm WSN system integration that need to be solved to achieve long lifetimes
and small volumes.
mm-Scale Energy Storage: Microbatteries
Rechargeable batteries are being manufactured today at the mm scale, termed micro-
batteries, by using solid-state lithium technology. These batteries are manufactured
on wafers with a planar process and with connections on the top side, similar to inte-
grated circuits (ICs), and they can be diced with sub-mm dimensions. Also like ICs,
no special packaging is required once the batteries are fabricated; therefore, they are
amenable to die stacking and integrating with other silicon ICs in a stack. However,
these microbatteries have limited capacity and small peak current due to the direct
relationship between capacity and volume [ 10 - 17 ]. For a 1.38
0.15 mm
custom lithium-ion (Li-ion) battery from Cymbet Corporation, the capacity is only 1
μ
×
0.85
×
A[ 11 ]. This presents
direct challenges on the radio circuits, which typically consume > 100
Ah (Fig. 5 ) and the maximum discharge current is only 10
μ
μ
W when
active. As a result, for a node to survive longer, it must be duty-cycled heavily and
energy must be harvested from other sources to supplement battery power. From a
circuit design point of view, energy usage must be reduced by clever circuit tech-
niques. The radio and oscillators easily dominate system energy usage if they are
operated continuously to maintain synchronization for WSN nodes to communicate
at the same frequency band [ 18 ]. A phase-locked loop (PLL) with a crystal reference
Search WWH ::




Custom Search