Hongjian Tan1, Zhuo Gao1, Guo Li2, Ruiliang Song3, Hao Wei1 and Mingyi Chen1,* (Email: mychen@sjtu.edu.cn)
Department of Micro/Nano Electronics, Shanghai Jiao Tong University, Shanghai, China
2
Chengdu Sino Microelectronics Technology Co., Ltd, Chengdu, China
3
The 54th Research Institute of China Electronics Technology Group Corporation, Shi Jiazhuang, China
2023 21st IEEE Interregional NEWCAS Conference (NEWCAS) | 979-8-3503-0024-6/23/$31.00 ©2023 IEEE | DOI: 10.1109/NEWCAS57931.2023.10198112
1
Abstract—This paper presents an asynchronous singleinductor multi-input multi-output (MISIMO) DC-DC
converter for ambient energy harvesting and management.
The converter enables the simultaneous collection and storage
of three DC ambient energy sources, reducing the number of
off-chip components, and provides three voltage domain
outputs of 1.4 V, 0.8 V, and 0.5 V. The asynchronous control
method triggers the switching by events instead of a fixedfrequency clock, leading to an improved conversion efficiency
especially at light-load conditions. In addition, a hybrid
operation mode based on asynchronous architecture is
proposed, which allows the converter to operate in continuous
conductance mode (CCM) and enables rapid response,
reducing the ripple voltage. The prototype has been
implemented in 180 nm BCD process with a total area of 0.828
mm2. The post-simulation results show that the converter
achieves 1 μW to 40 mW load range and 94.8% peak efficiency
with 30 mV output ripple, demonstrating its potential
capability to be applied to ambient energy harvesting for
Internet-of-things (IoT) devices.
Keywords—ambient energy harvesting, power management,
MISIMO, asynchronous DC-DC converter, Internet-of-Things
(IoT)
I.
INTRODUCTION
With the rapid development of Internet-of-things (IoT)
technology, IoT sensing nodes have been deployed
extensively in many fields, such as consumer electronics [1],
biomedical [2] and smart cities [3-4]. Due to the large
number of sensing nodes and complex deployment
environment, using batteries will bring high maintenance
costs and limited operation time. To extend the operating
time of nodes, the systems-on-chip (SoC) integrated in nodes
are usually designed with ultra-low power technologies.
Energy harvesting technologies are also used to power the
circuits by scavenging ambient sources such as solar energy,
thermoelectric energy [5] and RF energy [6-7]. In addition,
IoT nodes usually have different operation modes, which
possess significant differences in terms of energy
requirements. For example, the node in sleep mode maintains
very low static power for a long time, while in active mode it
requires high instantaneous power [8]. On the other hand, the
ambient energy source has problems such as low output
power and susceptibility to changes in environmental
conditions. In order to minimize the battery volume or
preferably enable an ambient-powered IoT node, the energy
harvesting and management circuit based on the MISIMO
DC-DC converter becomes the best candidate to
simultaneously maximize harvested energy and meet the
different energy demands of the nodes [9].
In state-of-the-art works, a diode-based MISIMO energy
harvesting circuit is presented in [10], where the energy
source with the highest voltage is selected into the circuit by
a diode. A multiplexed selective MISIMO energy harvesting
circuit replaces the diode with a switch and uses a
comparator to select the energy source that exceeds the
voltage threshold [11]. However, neither of them enables the
simultaneous harvesting of multiple energy sources. A
synchronously controlled inductor time division multiplexing
architecture multi-input energy harvesting circuit is proposed
in [12]. Various types of input energy sources are harvested
during different time periods with different switch topologies.
A synchronous MISIMO DC-DC converter applying a dualsource mode is presented in [13], which harvests energy
from two energy sources successively in one operation cycle,
improving energy harvesting efficiency. However, they
suffer from efficiency degradation issues under light-load
conditions because of the additional dynamic power
overhead. A MISIMO energy harvesting circuit presented in
[14] uses a fully asynchronous control method that triggers
the circuit to operate whenever an energy source is available.
It maintains high efficiency over a wide load range. However,
the discontinuous conductance mode (DCM) decreases the
efficiency significantly in the presence of abundant energy.
To address the existing issues, we propose a fully
asynchronous MISIMO DC-DC converter. It has three
benefits as follows: the first is the use of fully asynchronous
control to achieve high efficiency under light-load conditions.
The second is the proposed hybrid operation mode, which
allows both continuous conductance mode (CCM) and DCM
operations. This improves the load range of the converter and
reduces the ripple voltage significantly. The third is that the
hybrid operation mode allows the converter to harvest two
ambient sources in one operation cycle. This improves
energy harvesting efficiency.
The present paper is organized as follows: the
architecture of the circuit and the hybrid operation mode are
described in Section Ⅱ. The implementation and analysis of
each module is presented in Section Ⅲ. Section Ⅳ presents
the post-simulation results of the circuit followed by
conclusions in the Section Ⅴ.
II.
ARCHITECTURE AND OPRATION MODE
A. Overall system architecture
Fig. 1 shows the architecture of the proposed
asynchronous MISIMO DC-DC converter. The power stage
consists of 10 power transistors as input/output switches, to
enable switching of different energy sources and loads. The
asynchronous signal generation module senses source and
load conditions with hysteresis comparators. Meanwhile, it
senses the inductor state using a zero-current detector and
then generates asynchronous status signals. The input status
signals are H1, H2 and H3. The output status signals are L1,
L2 and L3, and the zero-current signal is ZCD. The
asynchronous logic control module receives the status signals
and drives the power stage circuit to switch the operation
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Fig. 1. Architecture of the proposed asynchronous MISIMO DC-DC
converter.
state. A low-voltage self-start circuit initially powers up the
circuit. A low-dropout regulator (LDO) with fast transient
response generates the final output voltage with minimized
ripple voltage.
B. Hybrid operation mode
Conventional MISIMO asynchronous DC-DC converters
work in the Buck-Boost operation mode. If Source 2 reaches
the available state during the operation cycle of Source 1, it
has to wait till the end of this cycle. This eventually causes
the output voltage of Source 2 to deviate from the maximum
power point voltage, resulting in a dropping efficiency. The
same problem exists on the output side, leading to additional
voltage undershoot.
To solve the above issue, a hybrid operation mode is
proposed in this paper. Fig. 2 shows the operation principle.
It has three modes as follows: In mode 1, the energy source
charges the inductor and the inductor current increases. The
inductor supplies energy to the load and the inductor current
decreases in mode 2. The energy source supplies the load via
the inductor in mode 3. The inductor current increases
/decreases when the voltage of the source is higher/lower
than the load. The conventional Buck-Boost mode proceeds
successively to mode 1 and mode 2, while the hybrid
operation mode allows the converter to switch within the
three modes until the inductor current crosses zero. Different
from the conventional Buck-Boost mode, when in mode 2,
once the energy is sufficient, the converter switches
immediately to mode 3 instead of waiting for the end of the
current operation cycle. Thus the efficiency of energy
harvesting increases. The output side adopts a similar
operation mode and thus reduces the output voltage ripple.
The operation flow is illustrated in Fig. 3 and described as
follows:
 When ZCD = 1, if any of the input status signals
changes to 1, the converter switches to mode 1.
 Once this input status signal changes to 0, the cycle
start phase ends. The operation moves to next step.
 The input and output status signals are scanned from
the highest priority to the lowest (H1>H2>H3>HB,
L1>L2>L3). The converter switches operation modes
according to the value of the status signals.
 If any of the status signals changes, the operation
returns to the previous step.
Fig. 2. Illustration of hybrid operation mode and corresponding inductor
current.
START
When the zero current signal is 1, the circuit enters the cycle start
phase once any input status signal becomes 1
MODE1
The input status signal becomes 0 and
the cycle start phase ends
NO
Presence of output
status signal 1
YES
Presence of intput
status signal 1
Presence of output
status signal 1
NO
NO
YES
YES
MODE1
MODE3
ZCD=1
YES
Battery recycles
the energy
MODE2
NO
Any status
signal changes
YES
NO
END
Fig. 3. Flow chart of hybrid operation mode.
 When ZCD = 1, it indicates the zero current of the
inductor and ends the current operation cycle.
III.
CIRCUIT IMPLEMENTATION
A. Asynchronous control logic circuits
Asynchronous control logic is implemented to enable the
hybrid operation mode which is triggered by events. Fig. 4
shows the asynchronous control logic circuit. It consists of
input and output side state machines, input and output side
pulse generation modules, a cycle start signal generation
circuit and a dead time control circuit.
When an asynchronous event occurs, H1-H3, L1-L3,
ZCD, the battery supply signal HB and other asynchronous
signals flip from 0 to 1. Then the pulse generation module
generates the corresponding pulses CLKH and CLKL. The
rising edge of the pulse signal triggers the state shift signal in
the finite state machine (FSM). The FSM generates the
switching control signals that drive the power stage. This
changes the operation mode of the DC-DC converter. The
dead-time control circuit inserts a dead time between the
switching signals and prevents simultaneous turn-on of the
power stage. This reduces the switching loss and improves
the efficiency.
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Fig. 4. Architecture of the asynchronous control logic circuit.
Fig. 6. Architecture of the low-power fast-transient-response LDO.
freewheeling cycle when ZCD = 1 and the output side is
disconnected from all loads. EN changes to 0 and the
comparator is turned off. In this manner, the comparator is
only turned on when necessary, so that the average static
power is significantly reduced.

Fig. 5. Schematic of the zero-current detector and the duty-cycled rail-to-rail
comparator.
B. Zero current detector
The zero-current detector detects the zero inductor
current state and generates ZCD signal. This helps to prevent
the energy loss as well as the potential failure state due to the
reverse inductor current. Fig. 5 shows the schematic of the
zero-current detector. It includes a multiplexer circuit, a
duty-cycled rail-to-rail comparator, a signal latching circuit
and an enable circuit.
The positive input of the comparator is connected to the
right side of the inductor, VLN. The multiplexer circuit is
used to select which output side switch is connected to the
negative input of the comparator according to the state of the
output side. The comparator detects the voltage drop across
the selected switch and generates the ZCD signal.
A static comparator is adopted to respond to the zero
inductor current state rapidly. To reduce the static power
consumption, a duty-cycled comparator is used, as shown in
Fig. 5. This comparator has a double folded-cascode
structure with a rail-to-rail input voltage range. It achieves
high gain and guarantees zero current detection accuracy.
Each bias current is connected in series with a CMOS switch
controlled by an enable signal (EN). The logical expression
for the enable signal EN is expressed by (1). Converter enters


C. Fast transient response LDO
To meet the power quality and adjustable voltage
requirements of IoT nodes, a LDO is cascaded after the first
output of the converter. Fig. 6 shows the schematic of the
LDO [15]. It consists of a reference circuit, error amplifier,
slew-rate enhancement (SRE) circuit and output power stage.
A bandgap reference circuit is used to generate the reference
voltage and an adjustable output voltage from 0.8 V to 1.2 V
can be achieved by adjusting the output voltage of the buffer.
The error amplifier consists of two cross-coupled push-pull
common-gate transconductance, resulting in a high slew-rate
and low power consumption. An embedded SRE circuit is
used to further increase the slew-rate which helps LDO adapt
better to the wide load range.
IV.
POST SIMULATION RESULTS
Fig. 7 shows the layout of the proposed MISIMO DC-DC
converter. The prototype has been implemented in 180 nm
BCD process. The overall area of the chip is 0.828 mm2,
with an active area of 0.449 mm2.
Fig. 8 shows the post-simulation transient waveform of
the DC-DC converter at 35 mW source power. The openloop voltages of the three energy sources VOC1, VOC2 and VOC3
are set to 2 V, 1.4 V and 1 V respectively. The simulation
results indicate that the average inductor current reaches
67.62 mA and the converter enters CCM mode under such
load condition. Three output voltages close to the target
value are generated. The output ripple voltage is less than 30
mV, reflecting the excellent ripple control capability of the
hybrid operation mode.
Fig. 9 shows the post-simulation results of the transient
response with load change. The harvesting power is fixed.
The load condition switches abruptly from 28.9 μW to 5.6
mW at 1.5 ms and back to the initial condition at 2.5 ms. The
state transfer signal switches more frequently in adapting to
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Peak efficiency = 94.8%
(a)
(b)
Fig. 10. (a) End-to-end efficiency of the DC-DC converter. (b) Simulated
power consumption breakdown under 5 mW load power.
TABLE I.
Technology
Fig. 7. Layout of the asynchronous MISIMO DC-DC converter.
PERFORMANCE COMPARISON TABLE
Liu,
JSSC’18 [13]
Chen,
JSSC’19 [16]
Amin,
JSSC’18 [14]
This Work*
180 nm
180 nm
28 nm
180 nm
Active area
(mm2)
1.8
0.75
0.5
0.449
#No. of input
2+supercap
3+supercap
3+battery
3+battery
#No. of output
2+supercap
1+supercap
3+battery
3+battery
Operation
mode
Synchronous
DISIDO
Buck-Boost,
Dual-source
Synchronous
MISISO
Asynchronous
MISIMO
Buck-Boost
Buck-Boost
Asynchronous
MISIMO
Hybrid
operation
Current mode
DCM
DCM
DCM
DCM, CCM
L/CL (μH/μF)
4.7/1
4.7/10
10/1
100/1
Output
power(μW)
20-4000
2.5-10000
1-60000
1-40000
Peak effciency
84.4%
@Pout=3.8 mW
82%
@Pout=0.12
mW
89%
@Pout=20 mW
94.8%
@Pout=5 mW
Architecture
* Simulation results used in this work
mW, which is dominated by the asynchronous signal
generator.
Fig. 8. Post-layout simulation results under 35 mW-source-power condition.
TABLE I. summarizes the performance in comparison
with the state-of-the-art works. The proposed asynchronous
converter achieves highest peak efficiency and a wide load
range. It is also competitive in terms of the number of
supported sources and loads. The introduction of CCM mode
requires a relatively large 100 μH off-chip inductor.
V.
Fig. 9. Post-layout simulation of the step response of the load switching.
the abrupt change of the load condition, while the output
voltage is maintained. The results demonstrate the high
transient response performance of the asynchronous
architecture.
Fig. 10(a) shows the efficiency curve of the DC-DC
converter. The red/blue curves show the converter efficiency
versus energy harvesting power/load power, respectively.
The converter achieves a peak transfer efficiency of 94.8% at
a load power of 5 mW. The efficiency can be maintained
above 70% in the load range of 10 μW to 40 mW. This
shows that the asynchronous hybrid operation mode
improves the efficiency of the converter over a wide load
range. Fig. 10(b) presents the quiescent power consumption
breakdown of the proposed MISIMO DC-DC converter. The
total power consumption is 274.3 μW under load power of 5
CONCLUSIONS
This paper proposes a MISIMO DC-DC converter for
ambient energy harvesting and management. A fully
asynchronous control method is used and a hybrid operation
mode is proposed. The static power consumption and voltage
ripple amplitude have both been reduced. The converter can
simultaneously harvest three DC ambient energy sources and
supply three output voltages of 1.4 V, 0.8 V and 0.5 V to
meet the demand for different voltage domains. The
maximum power conversion efficiency can reach 94.8% at
5 mW load power. Over 80% conversion efficiency can be
maintained in the load power range of 20 μW to 40 mW,
which has a positive application prospect in IoT self-supply
nodes.
ACKNOWLEDGMENT
This work was supported in part by the National Key
Research and Development Program of China (Grant No.
2019YFB2204500) and in part by National Natural Science
Foundation of China (Grant No. 62174109) and in part by
Shanghai Pujiang Talent (Grant No. 20PJ1408600).
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