Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
Multiple-output MEMS DC/DC converter
A. Chaehoi, M. Begbie, D. Weiland, S. Scherner.
Institute for System Level Integration, Heriot Watt University Research Park, Research Avenue North, EH14 4AP Edinburgh, Scotland,
www.isli.co.uk
Contact: Aboubacar Chaehoi, tel: +44 (0)131 510 0681, fax: +44 (0)131 449 3141, aboubacar.chaehoi@sli-institute.ac.uk
Abstract
DC/DC converters are widely used in consumer
electronic devices where usually a single power source is
available while the electronic board of the device requires
different voltage levels in order to power-up different block
functions. In this paper we present the design of a MEMS
single-input multi-output voltage level shifter. The lowvoltage
to high-voltage conversion is based on the
electrostatic transduction of variable capacitors built using
interdigitated comb fingers. A 1mm2 MEMS prototype has
been designed and fabricated using the SOIMUMPs process.
In this study we present the co-design and co-simulation of
the whole system (the MEMS device and its dedicated
charge-pump-circuit) in a single EDA environment through
MEMS+ [a Coventorware® tool that allows the cosimulation
of MEMS and electronics in the Cadence Analog
Design Environment]. We present analytical, FEM and
MEMS+ models of the multi-output DC-DC converter and
show that all our models converge towards the experimental
results.
Key words: MEMS, co-design, co-simulation, multi-output
DC/DC converter.
I. Introduction
Electronic devices usually require multi level voltage supplies
which must be derived from a single unique power source. For
instance common handheld products are powered using one
battery cell and at the same time different voltages level are
required for different functions ranging from the microprocessor
(a few volts) to the different the ASICs and memories to the
screen display (up to 40 volts). Two types of purely electronic
converter are dominant in the market: inductor-based DC/DC
converter where a bulky inductor is needed and switch-mode
DC/DC converters which suffer from switching losses and
switching noise. Applying these architectures to multi-output
systems inevitably leads to a large size system with the increase
of inductor and/or capacitor number. We propose in this study a
single-structure MEMS device that converts a single input DC
voltage into two different DC output voltages. The principal of
this MEMS device can easily be extended into multiple (more
than two) output DC/DC converter. Voltage conversion is based
on the electrostatic transduction of variable capacitors built
using interdigitated comb fingers. The multiple-outputs are
achieved by incorporating comb structures with different finger
gap spacings into the same single structure.
In a previous publication [1] the authors presented the design of
a 1mm2 MEMS prototype designed and fabricated using the
SOIMUMPs process [2]. The prototype exhibits 6.8V and 9V
outputs from a supply driving voltage of 5V, with an initial rise
time of 50ms to reach full output voltages. The development of
the DC-DC converter was performed by separate simulations of
the MEMS and electronics. In this study we propose and
demonstrate a new design approach to the DC/DC converter.
Designers usually develop their own models (VHDL, AHDL)
for co-simulation of MEMS with CMOS or more commonly
design and simulate MEMS and electronics separately, manually
passing results from one simulation domain to another. This can
lead to a number of problems including: incompatibility of
interfaces, non-standard operation conditions and dynamic
interaction between MEMS and electronics which cannot
properly be simulated. Moreover this approach typically does
not allow the IC designer any flexibility in the behaviour of the
MEMS device. As an example the resonant frequency of
vibrating structures is fixed by the mechanical design performed
by the MEMS designer.
In this new study we are able to co-simulate the whole system in
both domains through MEMS+, a Coventorware® tool that
allows the co-simulation of MEMS and electronics in the
Cadence Analog Design Environment [2]. We have thus been
able within the electronic design spaceto modify the mechanical
structure and behavior within limits defined in the MEMS
design space. The benefit we realize is the optimization of both
the MEMS and the IC at the same time. The accurate MEMS
behavioral model generated with MEMS+ is imported in
Cadence Virtuoso as a schematic bloc in which parameters such
as geometry and environmental variables can be changed, thus
allowing the co-simulation of the MEMS device and its
dedicated charge-pump-circuit in a single EDA environment. As
part of this work both elements have been optimized based on
their interaction (gap spacing, driving frequency) and the design
space is explored in more detail than previously possible. We
present analytical, FEM and MEMS+ models of the DC-DC
converter and show that all our models converge towards the
experimental results. We also analyse the influence of the
resonance frequency of the mechanical device on the whole
DC/DC converter efficiency.
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