IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 49, NO. 11, NOVEMBER 2014

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A 0.6 V Input CCM/DCM Operating Digital Buck
Converter in 40 nm CMOSXin Zhang, Senior Member, IEEE, Po-Hung Chen, Member, IEEE, Yasuyuki Okuma, Koichi Ishida, Member, IEEE,
Yoshikatsu Ryu, Kazunori Watanabe, Takayasu Sakurai, Fellow, IEEE, and Makoto Takamiya, Senior Member, IEEE

Abstract—This paper presents a 0.6 V input, 0.3–0.55 V output
buck converter in 40 nm CMOS, for low-voltage low-power wireless sensor network systems. A low power CCM/DCM controllerof the buck converter enables automatic selection of DCM or CCM
operation depending on load situation, therefore improving thepower efficiency. A dual-mode-body-biased (DMBB) zero-crossing
detector with both forward body bias mode and zero body bias
mode is designed to enable DCM operation with both low supply
voltage and normal supply voltage. An ultra-low-power hysteresis
voltage detector is proposed for body bias modes selection. The
proposed buck converter achieves a peak efficiency of 94% withan output current range of 50 µA to 10 mA. Thanks to the DCM
operation, the efficiency at an output current of 10 µA is improvedby 20% and 9%, with an output voltage of 0.35 V and 0.5 V,
respectively.
Index Terms—Buck converter, DC-DC converter, forward bodybias, low voltage, voltage detector.

I. INTRODUCTIONT

HANKS to the mature fabrication technology, photovoltaic (PV) module becomes one of the most popular
power suppliers for wireless sensor nodes and wearable electronic devices [1]–[4]. Combining the PV module with thebattery-powered system can effectively extend the battery lifetime. Moreover, PV energy harvesting provides an attractive
solution for battery-less system. In these low-power applications, scaling the power supply of digital circuits down toManuscript received January 24, 2014; revised May 08, 2014; accepted June21, 2014. Date of publication August 06, 2014; date of current version October24, 2014. This paper was approved by Guest Editor Zhihua Wang. This work
was carried out as a part of the Extremely Low Power (ELP) project supportedby the Ministry of Economy, Trade and Industry (METI) and the New Energy
and Industrial Technology Development Organization (NEDO).
X. Zhang was with the University of Tokyo, Tokyo, Japan, and is now with
the IBM T. J. Watson Research Center, Yorktown Heights, NY 10598 USA.
P.-H. Chen was with the University of Tokyo, Tokyo, Japan, and is now with
National Chiao Tung University, Hsinchu 30010, Taiwan.
Y. Okuma was with the Semiconductor Technology Academic ResearchCenter (STARC), Yokohama, Japan, and is now with the Central ResearchLaboratory, Hitachi, Ltd., Kokubunji-shi, Tokyo 185-8601, Japan.
K. Ishida was with the University of Tokyo, Tokyo, Japan, and is now with
the Dresden University of Technology, Dresden 01069, Germany.
Y. Ryu was with the Semiconductor Technology Academic Research Center(STARC), Yokohama, Japan, and is now with the Sharp Corporation, Japan,
Osaka 545-0013, Japan.
K. Watanabe was with the Semiconductor Technology Academic ResearchCenter (STARC), Yokohama, Japan.
T. Sakurai and M. Takamiya are with the Institute of Industrial Science, University of Tokyo, Tokyo 153-8505, Japan.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSSC.2014.2339325

sub/near-threshold region is a promising technique to achievesignificant power reduction. In recent advanced CMOS techis down to 400 mV.
nology, the threshold voltage
Therefore, there is a strong demand for designing a high efficiency power converter to provide a regulated 0.3 V–0.55 V
with low output current ( 10 mA).
output voltage
Considering the maximum power point characteristics of aof the power
typical single-cell PV module, the targeted
converter should cover as low as 0.6 V.
Until now, various types of step-down voltage converters
have been developed for low-voltage operation. A digital
low-dropout regulator (LDO) is firstly reported in [5] and utilized for near/sub-threshold logic circuits [6]. By using digital
control circuits, the operating voltage can be reduced to 0.5 V
with low quiescent current. The conversion efficiency of theand, whichLDO, however, is limited by the ratio of
is not applicable for adaptive power supply voltage control
in digital circuits. A low-voltage buck converter with highconversion efficiency is realized in [7]. It uses a delay chain
based digital pulse-width modulation (DPWM) technique toreduce the operating voltage but the input voltage is fixed. In
addition, a large inductor of 47 mH is required to accommodate
low inductor current ripple for continuous conduction mode
(CCM) operation.
Even though small inductor is preferred for low-costsmall-area application, an inductor current ripple becomes aproblem. When the converter is operating under light loadcondition, the inductor current becomes negative for a certain
period which causes a significant loss. To reduce the inductor
size while maintaining high conversion efficiency, the discontinuous conduction mode (DCM) operation can be used. The
conventional DCM operation is realized by using diode-based
nonsynchronous buck converter [8]. However, a large voltage
drop cross the diode limits the conversion efficiency, especiallywhen output voltage is low and output current is high. Anothersolution is combining a current sensing circuit with a powertransistor to detect the inductor current. When the inductor current becomes negative, DCM controller keeps inductor current
in zero which eliminates an undesirable reverse current. Designing a current sensing circuit, however, is still a challenging
task in low-voltage low-power application.
In this paper, a low-voltage low-power buck converter ispresented with a new automatic CCM/DCM controller for PV
assisted energy-efficient LSI system [9]. The system block
diagram is shown in Fig. 1. To manage the power from either
the battery or the single-cell PV, the input voltage of the dc-dc

0018-9200 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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Fig. 1. System block diagram of PV assisted energy-efficient LSI system.

converter should cover from 0.6 V to 1.1 V. The targeted
output voltage provided to near/sub-threshold digital circuits
is between 0.3 V and 0.55 V for low-power operation. To
accommodate the voltage regulation under such a low voltage,a D-flip-flop (DFF) based digital pulse-width modulation(PWM) controller is designed. An automatic CCM/DCM
controller adaptively selects the operation mode under different load current to extend the output current range. To
detect the inductor current, a dual-mode-body-biased (DMBB)
zero-crossing detector is also proposed to enable low-voltage
DCM operation for wide operating voltage. The proposed buckconverter achieves a peak efficiency of 94% with an output
range of 50 A to 10 mA. Comparing with the conventional
diode-mode DCM operation, the conversion efficiency can be
improved effectively by 15%, with an output voltage of 0.35
V. Comparing with CCM operation, by applying DMBB for
CCM/DCM operation, the conversion efficiency at 100 A
output current is improved by 20% and 9%, with an output
voltage of 0.35 V and 0.5 V, respectively.
This paper is organized as follows. The top-level architecture and design considerations of the proposed buck converter
are described in Section II. Section III shows the detailed circuit implementation of key building blocks. The experimental
results and comparison with the state-of-the-art are shown in
Section IV. Finally, a conclusion is given in Section V.
II. SYSTEM ARCHITECTURE AND DESIGN CONSIDERATIONS
The top-level block diagram of the proposed buck converter
is shown in Fig. 2. The proposed buck converter consists ofa clock generator, a digital PWM controller, an automatic
CCM/DCM controller, and a power stage.
The power stage consists of gate drive buffers, power MOS(
and), and a LC filter. Sizes ofandare optimized at a center output current of 2 mA, with the trade-off
between conduction loss and gate driving loss. In designing a
power stage of a buck converter, three important design parameters have to be determined: output capacitor (C), output inductor (L), and switching frequency. These parameters
can be theoretically calculated from basic specifications of buckconverter, i.e., input voltage, output voltage,
maximum output current, output ripple (, current ripple of inductor, estimated efficiency, etc. In

IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 49, NO. 11, NOVEMBER 2014

this buck converter, digital control block is used for low power
applications with less than 10 mA output current, in order toachieve a high efficiency, we would expect a low switching frequency to minimize the switching loss and the digital control
loss. Therefore to accommodate low current in the inductor withwide clock period, the inductor size should be relatively large.
On the other hand, to minimize the cost of an off-chip inductor
and the board size, the package of the inductor should be small.
A trade off is made by selecting the largest inductor value available in a 0805 surface-mount device (SMD). Therefore, L of
220 H is selected.
The output capacitor C is normally calculated from,
and[10]. In this design, digital controller is used for
PWM modulation. Therefore to reduce power consumption of
the controller,
is set to a low frequency of 100 kHz. Thenthe minimum capacitance required can be calculated by the following equation:(1)
is estimated as 40% of the output current (10 mA),
where
and
is worst case output ripple of 30 mV Keeping in
mind that some ripple introduced by the ESR of the output capacitor is not included in the above equation, and the demand
to push down the cutoff frequency
to much lower than
, a larger capacitance of 1 F is chosen for output capacitor
C. Then the cutoff frequency of LC filter can be calculated asfollows:(2)
The ratio ofoveris around 9.3, which is large
enough for the LC filter to filter out the switching noise in apractical converter.
A clock generator is employed to generate different frequencies for sub-blocks in the digital PWM controller. The system
clock (CK1) frequency is 6.4 MHz, in order to have 100 kHz
of switching frequency at power stage. The clock generator issimply DFF based power-of-2 frequency divider, generating
frequencies of 1/64 and 1/1024 the rate of the main clock. CK1of 6.4 MHz is used by DFFs. CK2 of 100 kHz is used by theclocked comparator. CK3 is used by the bi-directional shift
register (SR). The switching frequency of power stage is set by
the digital PWM controller at 100 kHz.
The digital PWM controller is employed to perform closed
loop regulation of. A digital architecture is more attractive than an analog one for low supply voltage (0.6 V) and
low output current. It utilizes three clock signals CK1, CK2,and CK3 generated by the clock generator, compareswith, and then enables a pulse width modulated signal
CK_buck for the power stage. More details will be discussed in
Section III.
The CCM/DCM controller is employed to automatically generate the required gate signals (CKP and CKN) forandeither in CCM or DCM operation. It will be explained in detailin Section III.

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Fig. 2. Top-level block diagram of the proposed buck converter.

Fig. 3. Circuit diagram of digital PWM controller.

III. CIRCUIT LEVEL IMPLEMENTATION
A. Digital PWM Controller
In order to fit with the low supply voltage (0.6 V), a digital
feedback architecture is more attractive than an analog one. The
circuit diagram and timing diagram are shown in Figs. 3 and 4,
respectively. A Set signal is generated from CK1 and CK2 by
DFF0 and two logic gates. A latch-type comparator clocked at
100 kHz is used to comparewith, thereafter control the shifting direction of the SR. SR has thermal-code-like
outputs, i.e.,, as shown in Table I, which has 63 output
states. By connecting
to a serial of exclusive OR gates,and 63 switches, each of SR output state enables one of theswitches. Only one of switches is turned on and one of DFFs’
outputs is connect to Reset. In this way, a variable delay “T”
is obtained to generate variable duty cycle for CK_buck. The
variable duty varies from 1/64 (1.5625%) to 63/64 (98.4375%),
with a step dutyof 1/64 clock cycle. The reason to choose
64 clock phases in the Digital PWM Controller is to obtain avariable output voltage of buck converter with a tunable output
voltage step of 10 mV. Increasing the number of phase can further improve the resolution of digital PWM and ripple, but it also
increases the leakage and switching power of DFFs in the digital
PWM controller. The tunable output voltage stepiscalculated as follows:

Fig. 4. Timing diagram of digital PWM controller.

TABLE IOUTPUT STATE OF BI-DIRECTIONAL SHIFT REGISTER (SR)(3)
SR is clocked by CK3 which determines the duty varying
frequency. CK3 is further reduced to 6.25 kHz to reduce thepower consumption of SR, and to improve the light load efficiency. The transient response time is relatively long comparing

with high frequency operation. However, the output of the buckconverter is provided to near/sub-threshold digital circuits for
low-power application, for instance, wireless sensor network,

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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 49, NO. 11, NOVEMBER 2014

where the operating speed is limited and the transient performance requirement is relatively low.
CK_buck carrying appropriate duty information is then usedby CCM/DCM controller to drive the power stage and to regulate
towards.
B. CCM/DCM Controller
To achieve high conversion efficiency, DCM is usuallyrequired in light load condition [11]. For a wide output current
range, both CCM and DCM operations are essential for highefficiency. Selection of CCM/DCM is normally enabled byzero voltage switching (ZVS) technique [12], [13]. In previously published buck converters, either an external mode
selection signal is required to change modes between CCM
and DCM [14], or a complex synchronous logic circuit including DFF is required which consumes more power and may
suffer from setup and hold errors at low[15]. While theproposed method offers a simple and low power automatic
mode switching by using the proposed automatic CCM/DCM
controller.
The circuit diagram of proposed automatic CCM/DCM controller is shown in Fig. 5. In DCM, a zero-cross switching isachieved to maximize the efficiency. The controller consistsof only digital standard cells and a proposed dual-mode-bodybiased (DMBB) zero-crossing detector. The zero-crossing detector is used to comparewith ground, therefore detect if
the voltage at
has crossed 0 V. The rest digital standard cells
generate the required gate driving signals (CKP and CKN), andautomatically switch between CCM and DCM operations. A
pulse converter is realized with inverters and AND gate, to convert a clock signal IN1 into a pulse signal OUT1. The pulse signals are used to set or reset the inputs of the SR-latch to generate
CKN. The timing diagram of proposed automatic CCM/DCM
controller is shown in Fig. 6. In CCM condition (Fig. 6(a)),
when load current is high, inductor current
is always larger
than zero. CKP is approximately equal to CK_buck. When CKP
goes high, meaning
is turned off, a Set1 signal is generated
by the pulse converter, and then CKN is set to high, so
isturned on accordingly. Whenis turned off,is always
less than 0 V due to the conduction voltage drop of, andthe output of zero-crossing detector (ZCD) and Reset_DCM isalways zero. CKN is then reset to zero by Reset_CCM. In contrast, in DCM condition (Fig. 6(b)), sometimes drops to zero.
CKN is set to high in the similar way as CCM. Then at thezero-crossing point (whenV andis turned off),a pulse is generated for Reset_DCM and then CKN is reset tozero. In this way, the zero-cross switching (is turned off
when becomes zero) is accomplished.
Non-overlap generation function is also implemented in
the proposed automatic CCM/DCM controller. In Fig. 5,
two buffers (buffer1 and buffer2) are used to adjust delaysof different signal paths. The timing diagram of non-overlap
clock signal is shown in Fig. 7.
delay is the delay from
CK_buck to CKP,
delay is the delay from CK_buckthrough Set1 to CKN, anddelay is the delay from
CK_buck through Reset_CCM to CKN. By adjusting thedelays, non-overlap time can be generated. In this design, theloss contribution during the non-overlap time is not significant

Fig. 5. Circuit diagram of proposed automatic CCM/DCM controller.

because of large clock cycle. Therefore, dynamic non-overlap
control is not implemented to save power.
C. Dual-Mode-Body-Biased (DMBB) Zero-Crossing Detector
It is of great importance to detect when the inductor current
becomes zero in a buck converter. There are several ways of
doing current sensing. Simply inserting a resistor in series with
the inductor will incur power loss in the resistor and therefore
reduces the efficiency. Using low-pass RC network to filter thevoltage across the inductor [16] can sense the current, but itneeds exact value of inductor and additional R, C components.
Using current mirror to sense the current in power MOSFET[17] needs OpAmp and therefore not suitable for low input
voltage design.
In this work, a zero-crossing detector is used to sense
node voltage to detect when inductor current hits zero. The
circuit diagram of proposed DMBB zero-crossing detector isshown in Fig. 8. It is designed to enable DCM operation withboth low supply voltage (0.6 V) and normal supply voltage. The
DMBB zero-crossing detector is based on a common-source
differential input amplifier.
andserve as the differential input pair,
provides bias current for the amplifier,andare the mirroring components. The matching between
andand, are important, because due
to process mismatch variation, there is an offset voltage in thezero-crossing detector. The offset may be positive or negative.
If the offset is too large, the power NMOS
may be turned
off early or late. Therefore, to reduce the offset voltage due toprocess variation, the transistor widths ofandare designed to be at least ten times larger than the minimum width, and a gate length of four times the minimum gate
length is used. Therefore, the offset of the zero-crossing detector is reduced and the loss caused by offset voltage variationis reduced.
At low, voltage headroom of the amplifier is quite limited. In order to have enough gm, either transistor size or bias
current needs to be enlarged, which are definitely not desired for
low-power design. A forward body bias is therefore beneficial
in this design to alleviate the lack of voltage headroom with low

ZHANG et al.: A 0.6 V INPUT CCM/DCM OPERATING DIGITAL BUCK CONVERTER IN 40 nm CMOS

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Fig. 6. Timing diagram of proposed automatic CCM/DCM controller with (a) CCM, (b) DCM operation.

Fig. 7. Non-overlap time of proposed automatic CCM/DCM controller.
Fig. 9. Measured
enabled.

of zero-crossing detector when forward body bias is

Fig. 8. Circuit diagram of proposed DMBB zero-crossing detector.

Fig. 10. Schematic of proposed ultra-low-power hysteresis voltage detector.

. The bodies ofare connected to ground to enable
the forward body bias, therefore reduce the threshold voltage ofthem.
However, such forward body bias is only applicable when
is low, otherwise the body diode of PMOS would be turned
on, and large forward current would flow through the diode. As
shown in Fig. 9,of the zero-crossing detector increases
exponentially with
. In order to avoid the large leak current from, forward body bias must be turned off whenis larger than 0.7 V. An ultra-low-power voltage detector[18] is therefore required to switch body connection with regarding to different.
should be set to around 0.7 V

in this design. Thus, when
is lower than 0.7 V, the DMBB
zero-crossing detector is forward body biased. In contrast, whenis higher than 0.7 V, the detector is zero body biased.
Therefore, the DMBB architecture achieves both a fast voltage
detection operation at low
and small body diode leak current at high.
D. Ultra-Low-Power Hysteresis Voltage Detector
To adaptively switch the body of the zero-crossing detector,
an ultra-low-power hysteresis voltage detector is added. As
shown in Fig. 10, the proposed circuit is constructed from a
PMOS-based voltage detector core [18], inverter buffers and2382

IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 49, NO. 11, NOVEMBER 2014

Fig. 11. Simulated dependence ofonin: (a) conventional voltage detector, (b) proposed voltage detector; and state analysis of proposed voltage
detector in: (c) falling transition, (b) rising transition.

a latch. The detector core consists of three cascode PMOS
transistors:, and.
are chosen to be
pMOS transistors because pMOS has less process variation
than nMOS. The upper side (and) are operating
under sub-threshold region while the lower sideisoperating under cut-off region.
changes rapidly from low
to high when the available drain current of upper side getslarger than off-current of. The inverter buffers are used toamplify the compared voltage signal. During the transition
state, whenchanges from low to high,is in the range
from 0 to. Therefore, the length of the PMOS transistor
in the first inverter
is designed with four times gate
length than that of NMOS transistor to correctly detect thetrigger voltage. Also after
changes from low to high,is still lower than, andcannot be fully turned off.
By applying four times larger gate length, the leakage current
after
goes high can be reduced.
The voltage detector in [18] provides only one trigger level.
In order to avoid miss-detection caused by noise or disturbancearound the trigger level, a hysteresis characteristic is required.
A cross-coupled CMOS latch is added after the inverter buffers
to realize hysteresis levels. The latch is composed of transistors, and, which use twice of the minimum channel length to reduce the leakage current. The sourceofis connected towhile source ofis connected toto shift the level of trigger voltage in rising transition. Because the hysteresis voltage detector does not need ahigh operating speed, the Vdd/2 does not need to provide large
current. Therefore it is generated by off-chip resistive voltage
divider, and consumes only a few nA current. Also the Vdd/2

does not have to be very accurate, which alleviates the PVT variation requirement. The simulated dependence ofonof conventional and proposed voltage detectors with risingand falling transition ofare shown in Fig. 11(a) and (b).
The DC operating points in each phase of Fig. 11(b) are shownin Fig. 11(c) and (d). Whenis decreasing (falling transition), there are two different operation phases: phase 1 and phase
2&3, as shown in Fig. 11(c). DC operation condition in phase
2 and phase 3 are the same. The trigger voltage of the proposed
scheme is equal to the conventional voltage detector. Whenchanges fromto 0 V,is pushed to 0 V ignoring thevoltage at, because
is in the middle between
toground. Thus
provides “low”.
On the other hand, whenis increasing (rising transition),
there are three operation phases, as shown in Fig. 11(d). Phase 2
and phase 3 in rising transition have different dc operation condition to generate the hysteresis voltage levels. During phase
2 operation,and, which means
andare both operating under sub-threshold region
and turned-on slightly. Since the current flowing throughandare the same,is determined byratio of transistors
and. Initially,
provides “low” because in
the end of phase 1,
is 0 V andis. Thenincreases asincreases. Whenreaches a critical value,decreases because of voltage division caused by transistor
and, and positive feedback in latch flips
fromlow to high. The critical value ofis determined by the size
ratio between NMOS and PMOS transistors in latch circuit. In
our design, the channel width ofandis three times
larger than that ofandto generate 20 mV hysteresis.

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Fig. 12. Layout and die photo.
Fig. 14. Measured waveforms of buck converter in CCM and DCM at
V and
V.

Fig. 13. Measured CK buck waveforms of digital PWM controller with dif.
ferentFig. 15. Simplified circuit diagram of diode mode buck converter.

IV. EXPERIMENTAL RESULTS
The proposed buck converter is fabricated with 40 nm CMOS
process. Fig. 12 shows the layout and the chip microphotograph.
The total area including test structure is 450 m by 1050 m.
The active area is 0.084 mm .
Fig. 13 shows the measured output signals of the digital
PWM controller, i.e., CK_buck, at different. It is observed that duty cycle of CK_buck changes proportionally aschanges meaning successful pulse width modulation of
the buck converter.
Fig. 14 shows the measured waveforms of the proposed buckconverter at=
V and
V withdifferent output current. At
of 3 mA, the buckconverter is in CCM operation, it can be seen from the fact thatCKN is in the same shape with CKP. At
from 2 mA to
0.5 mA, CKN transits fromto zero before CKP goes tozero, meaning
is turned off before
is turned on, andbothandare turned off whenis zero. Therefore it
is in DCM operation. The worst case output voltage ripple is
11 mV (peak-to-peak) at
mA.
For comparison purpose, a conventional diode-based nonsynchronous buck converter is also implemented and measured.
The simplified schematic of the conventional diode mode buckconverter is shown in Fig. 15. Fig. 16 shows the measured power
efficiency of the diode mode converter and the proposed buckconverter, withof 0.6 V. Efficiency of an ideal LDO is also
shown in Fig. 16. It can be easily calculated as 83% and 58%,

forof 0.5 V and 0.35 V, respectively. The proposed buckconverter has a higher efficiency than an ideal LDO. Comparedwith diode mode converter, the maximum efficiency improvement by the proposed buck converter is 3% and 15%, atof 0.5 V and 0.35 V, respectively. That is because the diodemode converter suffers from diode reverse recovery loss, anddiode conduction loss, especially when
is low, meaning
duty cycle is small and diode conducts more often.
Fig. 17 shows the measured power efficiency of proposedbuck converter with both CCM and DCM operation, and conventional one with only CCM operation.
is set to 0.6 V,
andis set to 0.35 V and 0.5 V. Whenis larger than
2 mA, both proposed and conventional buck converters are in
CCM operation, therefore the same efficiency is achieved. In
contrast, whenis less than 2 mA, the proposed buck converter is in DCM operation, therefore significant efficiency improvement is achieved. The power efficiency atof 100A is improved by 20% and 9%, atof 0.35 V and 0.5
V, respectively. The peak efficiency of 94% is achieved by the
proposed buck converter atof 2 mA, thanks to the proposed low-power CCM/DCM controller and the low-power digital PWM controller.
Table II shows the comparison with the publishedlow-voltage buck converters. Compared with [11], [14]and [15], the proposed buck converter achieves higher peakefficiency and lowerand. While compared with [7],2384IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 49, NO. 11, NOVEMBER 2014

TABLE IICOMPARISON WITH PUBLISHED LOW-VOLTAGE BUCK CONVERTERS

selection. By virtue of the low power controller, the proposedbuck converter achieves a peak efficiency of 94% with an
output current range of 50 A to 10 mA. Thanks to the DCM
operation, the efficiency at an output current of 100 A is
improved by 20% and 9%, with an output voltage of 0.35 V
and 0.5 V, respectively.
REFERENCES
Fig. 16. Measured efficiency of proposed buck converter and diode mode buckV, and (a)V, (b)
V.
converter with

Fig. 17. Measured efficiency of proposed CCM/DCM buck converter and conventional CCM converter.

the proposed buck converter has wider
inductor value.

range and smaller

V. CONCLUSION
In this paper, a low-voltage low-power buck converter is
proposed for wireless sensor network systems. A digital PWM
controller is implemented for voltage regulation. An automatic
CCM/DCM controller is proposed to adaptively select CCM
or DCM operation, therefore improving the power efficiencyof the buck converter. A low-voltage DMBB zero-crossing
detector is proposed to enable DCM operation with both low
supply voltage and normal supply voltage. An ultra-low-power
hysteresis voltage detector is designed for body bias mode

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[13] F. Canales, P. M. Barbosa, and F. C. Lee, “A zero voltage and zerocurrent switching three-level DC/DC converter,” in Proc. IEEE APEC,
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[18] P.-H. Chen, K. Ishida, K. Ikeuchi, X. Zhang, K. Honda, Y. Okuma,
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no. 5, pp. 1252–1260, May 2012.

Xin Zhang (S’06–M’08–SM’13) received the B.S.
degree in electronics engineering from Xi’an Jiaotong University, Xi’an, China, in 2003, and the Ph.D.
degree in microelectronics from Peking University,
Beijing, China, in 2008.
In 2008, he joined the Institute of Industrial Science, University of Tokyo, Tokyo, Japan, as a project
researcher, and was engaged in the research of lowvoltage, low-power power management ICs. In 2012,he was a visiting scholar at the University of California, Berkeley, and then a project research associate
at the Institute of Industrial Science, University of Tokyo. In 2013, he was with
the Institute of Microelectronics (IME), Agency for Science, Technology andResearch (A*STAR), Singapore, as a scientist. Since 2014, he has been a research staff member at IBM T. J. Watson Research Center, Yorktown Heights,
NY, USA, where he works on fully integrated voltage converters for server
CPUs. He has authored or co-authored over 30 technical papers and holds two
Japanese patents. His research interests include power management ICs, wireless power transmission, energy harvesting circuits, and low-voltage low-power
CMOS analog circuits.
Po-Hung Chen (S’10–M’12) received the B.S.
degree in electrical engineering from National
Sun Yat-sen University, Taiwan, in 2005, the M.S.
degrees in electronics engineering from National
Chiao-Tung University, Taiwan, in 2007, and the
Ph.D. degree in electrical engineering from the
University of Tokyo, Japan, in 2012.
In 2012, he joined the Department of Electronic
Engineering at National Chiao Tung University
as an Assistant Professor. His research focuseson power management IC for energy harvesting,fully integrated power management ICs, wireless power transmission, andlow-voltage low-power CMOS analog circuits.

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Yasuyuki Okuma received the B.S. and M.S. degrees in electrical engineering from the Tokyo University of Science, Tokyo, Japan, in 1997 and 1999,
respectively.
In 1999, he joined Central Research Laboratory,
Hitachi, Ltd., Japan, where he was engaged in the
research and development of low power analog
circuit techniques for HDD driver and RF-IC. From2003 through 2006, he was a visiting researcher
at YRP Ubiquitous Networking Laboratory, doing
research in the field of low-power circuits andsystems for ubiquitous computing. From 2009 through 2013, he was a visiting
researcher at the Extremely Low Power LSI Laboratory, Institute of Industrial
Science, University of Tokyo, doing research in the field of power supply
circuits and systems for extremely low power LSI circuits and systems. His
research interests include low-power analog circuits, wireless communication
circuits, and power supply circuits.

Koichi Ishida (S’00–M’06) received the B.S. degree
in electronics engineering from the University ofElectro-Communications, Tokyo, Japan, in 1998,
and the M.S. and Ph.D. degrees in electronics
engineering from the University of Tokyo, Tokyo,
Japan, in 2002 and 2005, respectively.
He joined Nippon Avionics Co., Ltd. Yokohama,Japan, in 1989, where he developed high-reliability
hybrid microcircuits for aerospace programs. From2005 to 2007, he worked on CMOS analog and RF
circuits at the Tokyo Institute of Technology, Yokohama, Japan. From 2007 to 2012, he worked for on-chip power supply circuits
and organic electronics circuits at the Institute of Industrial Science, Universityof Tokyo, Tokyo, Japan. Since July 2012, he has been with the Chair for Circuit Design and Network Theory, Dresden University of Technology, Dresden,
Germany. His research interests include low-voltage low-power CMOS analog
circuits, on-chip power supply circuits, and large-area flexible electronics.
Dr. Ishida is a member of the IEICE.

Yoshikatsu Ryu graduated from Kobe City College
of Technology, Japan, in 1992.
In 1992, he joined Sharp Corporation, Nara, Japan.
From 1992 to 2001, he was involved in the development of semiconductor processing technology, andfrom 2001 to 2009, he was engaged in circuit designof analog LSIs. He was a visiting researcher at theExtremely Low Power LSI Laboratory, Institute ofIndustrial Science, University of Tokyo, Japan, from2009 to 2012. His current interests are low-voltage
low-power CMOS charge pump circuits.

Kazumori Watanabe graduated from theTomakomai Technical College in 1975.
In 1975, he joined Matsushita Communication
Industrial Co., Ltd. (presently Panasonic Mobile
Communications Co., Ltd.), Yokohama, Japan. He
was engaged in development of a digital system forpagers. From 1997 through 2003, he has managed
development of the technology for cdmaOne cellular
phone. From 2003, he was engaged in system design
development of using high-frequency semiconductor integrated circuit in Panasonic Corporation
Semiconductor Company, Kyoto, Japan. He was a visiting researcher at
the Extremely Low Power LSI Laboratory, Institute of Industrial Science,
University of Tokyo, Japan, from 2009 to 2013.

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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 49, NO. 11, NOVEMBER 2014

Takayasu Sakurai (S’77–M’78–SM’01–F’03)
received the Ph.D. degree in electrical engineeringfrom the University of Tokyo, Tokyo, Japan, in 1981.
In 1981, he joined Toshiba Corporation, where hedesigned CMOS DRAM, SRAM, RISC processors,
DSPs, and SoC Solutions. He has worked extensively
on interconnect delay and capacitance modelingknown as Sakurai model and alpha power-law MOS
model. From 1988 through 1990, he was a visiting
researcher at the University of California, Berkeley,
where he conducted research in the field of VLSICAD. Since 1996, he has been a Professor at the University of Tokyo, working
on low-power high-speed VLSI, memory design, interconnects, ubiquitouselectronics, organic ICs and large-area electronics. He has published more than
600 technical publications, including 100 invited presentations and several
books, and has filed more than 200 patents.
Dr. Sakurai is the executive committee chair for VLSI Symposia and asteering committee chair for the IEEE A-SSCC. He served as a conference
chair for the Symposium on VLSI Circuits and ICICDT, a vice chair for
ASPDAC, a TPC chair for the A-SSCC, and VLSI Symposium, an executive
committee member for ISLPED and a program committee member for ISSCC,
CICC, A-SSCC, DAC, ESSCIRC, ICCAD, ISLPED, and other international
conferences. He is a recipient of 2010 IEEE Donald O. Pederson Awardin Solid-State Circuits, 2009 and 2010 IEEE Paul Rappaport award, 2010
IEICE Electronics Society award, 2009 IEICE achievement award, 2005 IEEEICICDT award, 2004 IEEE Takuo Sugano award and 2005 P&I patent of
the year award and four product awards. He delivered the keynote speech atmore than 50 conferences including ISSCC, ESSCIRC, and ISLPED. He wasan elected AdCom member for the IEEE Solid-State Circuits Society and an
IEEE CAS and SSCS distinguished lecturer. He is an IEICE Fellow and IEEE
Fellow.

Makoto Takamiya (S’98–M’00–SM’14) received
the B.S., M.S., and Ph.D. degrees in electronicengineering from the University of Tokyo, Tokyo,
Japan, in 1995, 1997, and 2000, respectively.
In 2000, he joined NEC Corporation, Japan, where
he was engaged in the circuit design of high speed
digital LSIs. In 2005, he joined the University ofTokyo, Japan, where he is an Associate Professor of
VLSI Design and Education Center. From 2013 to2014, he stayed at University of California, Berkeley,as a visiting scholar. His research interests include
circuit design of low-power RF circuits, ultra-low-voltage logic circuits,
low-voltage power management circuits, and large area and flexible electronics
with organic transistors.
Dr. Takamiya is a member of the technical program committees of theIEEE International Solid-State Circuits Conference and the IEEE Symposium
on VLSI Circuits. He received the 2009 and 2010 IEEE Paul RappaportAwards and the Best Paper Award in the 2013 IEEE Wireless Power Transfer
Conference.