248pW, 0.11mV/°C Glitch-Free ProgrammableVoltage Detector With Multiple Voltage Duplicatorfor Energy Harvesting
Teruki Someya1, Hiroshi Fuketa1, Kenichi Matsunaga2, Hiroki Morimura2, Takayasu Sakurai1, and Makoto Takamiya1
12

University of Tokyo, JapanNTT Device Technology Laboratories, NTT Corporation, Japan

Abstract—A glitch-free programmable voltage detector is
proposed for RF energy harvesting. In energy harvesting
applications, ultra-low power (< 1nW), PVT-variation tolerant,and glitch-free voltage detectors are required to turn on or off theoutput switch which connects an output capacitor of the energy
harvester and the load circuits. The proposed multiple voltage
duplicator (MVD) enables the ultra-low power (248pW at 1V)feature, detection voltage programmability (< 49-mV step), andtemperature-variation tolerant (0.11mV/°C in -20°C to 80°C)
operation in 250-nm CMOS. In the conventional voltage detectors,
when the input voltage is increased from 0V, a glitch of the output
is observed, which mistakenly turns on the output switch andspoils the energy harvesting. To remove the glitch, the glitch-freeprogrammable voltage detector is proposed.

I.

RF energy
harvesting
Chargepump

MCU
RFFig. 1. Block diagram of IoT node with RF energy harvesting.

VIN = VDD

VOUT
VIN1

INTRODUCTION

VREF
Voltagereference

VOUT

VOUT = VIN

Undefined

0VMIN

VDETECT

VIN(a)(b)
Fig. 2. (a) Circuit schematic and (b) input-output characteristic ofcommercially available voltage detector [5-7].

Figs. 2 (a) and (b) show a circuit schematic and an inputoutput characteristic of a commercially available VD [5-7],
respectively. When, the input voltage (VIN) is lower or higher
than VDETECT, VOUT is low or high, respectively. VDETECT isfactory-trimmed by changing the resistive divider. As shown inFig. 2 (b), when VIN is between 0V and VMIN, VOUT is undefineddue to the metastability of the comparator, which is the cause of
the glitch of VOUT. To hide the glitch, the minimum VIN (e.g.
0.8V to 1V) (> VMIN) is specified in the commercially available
VDs [5-7]. The VDs cannot be applied to the energy harvesting
where VIN increases from 0V.
Fig. 3 shows a conventional programmable VD. A resistive
ladder and a selector are added to enable the programmable
VDETECT to compensate for the die-to-die process variations. As
shown in Fig. 3 (b), however, the glitch is observed.

CONVENTIONAL VOLTAGE DETECTORS

Fig. 4 shows a glitch-free VD. By replacing the comparatorin Fig. 2 (a) with two stacked pMOSFETs, the glitch is removed.
In this work, VD [3] with two stacked pMOSFETs with the same
threshold voltage (VTH) is modified to two stacked pMOSFETs
with different VTH to achieve temperature-variation tolerantVDETECT. After the optimization of the transistor sizes, VDETECTof the glitch-free VD is given as follows.

A. Glitch-Free and Programmable Operation
In this section, conventional VDs are reviewed and it isshown that all the conventional VDs do not achieve both theglitch-free and the programmable operation.

978-1-4673-7472-9/15/$31.00 ©2015 IEEE

VDD1

C1Voltage VOUT
detector

In the applications of Internet of Things (IoT), the energy
harvesting is strongly required to enable energy autonomous IoT
nodes. Fig. 1 shows a block diagram of an IoT node with the RFenergy harvesting [1]. The charge pump converts the input RFsignal (e.g. 920MHz) to the DC voltage (VDD). At the start-up,
the charge pump charges the output capacitor (C1) from 0V.
During the charging, the output switch (M1) [2-3] should beturned off to prevent the load circuits (e.g. MCU and RF circuits)
from discharging C1. M1 is very important in the energy
harvesting, because the charging current (e.g. 1µA [4] and 1nA[1]) of the charge pump is much smaller than the load current(e.g. 1mA). When VDD is increased to the pre-defined voltage(VDETECT) of a voltage detector (VD), VD turns on M1 and theload circuits start the operation. This paper focuses on VD. In
the RF energy harvesting, ultra-low power (< 1nW), PVTvariation tolerant, and glitch-free VDs are required. The supply
current of VD should be less than the charging current of thecharge pump, because VD is directly connected to C1. VDETECTof VD should be constant over PVT variations. When a glitch isobserved in VOUT, M1 is mistakenly turned on, which spoils theenergy harvesting. In this paper, a low power (248pW at 1V),
PVT-variation tolerant, and glitch-free VD is proposed andimplemented in 250-nm CMOS.
II.

OutputSwitch (M1)

VDD

249

Programmable
voltage detector(Fig. 3 (a))

VIN = VDD

VIN

VIN1

SelREF

Glitch-freevoltage detector(Fig. 4 (a))

VOUT

VOUT

VOUT2

VOUT = VIN

IB2
VREF1

No glitch

IBN

0I2(a)

Undefined

0

VMIN

Programmable
VIN
VDETECT

VOUT

M1III. PROPOSED GLITCH-FREE PROGRAMMABLE VOLTAGE
DETECTORVOUT = VIN

A. Glitch-Free and Programmable Operation
Figs. 5 (a) and (b) show a block diagram and an input-output
characteristic of a proposed glitch-free programmable VD,respectively. As shown in Fig. 5 (b), by combining theprogrammable voltage detector with the glitch (Fig. 3) and theglitch-free un-programmable voltage detector (Fig. 4), both theglitch-free and the programmable operation are achieved, when
VMIN(Fig. 3(b)) < VDETECT(Fig. 4(b)) < VDETECT(Fig. 3(b)). Table I
summarizes the four VDs in Figs. 2 – 5 regarding the glitch-freeand the programmable operation. Fig. 6 shows a circuit
schematic of the proposed glitch-free programmable VD. Thevoltage reference is implemented with two stacked pMOSFETs
with different VTH, which is the similar concept to [8] with twostacked nMOSFETs implementation. The VTH difference isapproximately 0.4V. To reduce the step of the programmable
VDETECT, the voltage dividers are added to both VIN and VREF.

High |VTH|
No glitch

VOUT
Low |VTH|
0VIN

VDETECT(a)(b)Fig. 4. (a) Circuit schematic and (b) input-output characteristic of glitch-freevoltage detector.

VDETECT  VTH1 m1
VTH2m2VIN
VDETECTthe same of that of the voltage reference. In reality, however,when the leakage current is comparable to I1, the temperaturedependence of VREF1 is worse than that of the voltage reference.
To improve the temperature dependence, I1 should be increased.
Therefore, the power consumption and the temperaturedependence are the tradeoff. Later, the tradeoff is quantitatively
shown in Fig. 9.

(b)Fig. 3. (a) Circuit schematic and (b) input-output characteristic ofconventional programmable voltage detector.
VIN

Programmable(b)Fig. 5. (a) Block diagram and (b) input-output characteristic of proposed
glitch-free programmable voltage detector.

VOUT = VIN

VOUTM2

VOUT(a)I1 VREF IB1Voltage
reference

VOUT1(1)where VTH1 and VTH2 are VTH of M1 and M2, and m1 and m2 are
the body-effect coefficient of M1 and M2, respectively. Eq. (1)shows that the temperature-dependent term is not included andVDETECT is temperature-variation tolerant. Though VDETECTvaries due to the process variations of VTH1 and VTH2, VDETECT is
not programmable, which is a problem of the glitch-free VD.

TABLE I SUMMARY OF FOUR VOLTAGE DETECTORS

B. Low Power and Temperature-Variation TolerantOperation
In this section, the power consumption to generate VREF1 andthe temperature dependence of VREF1 in the conventional
programmable VD in Fig. 3 (a) are discussed. The voltage
reference need to drive the leakage current (IB1, IB2, and IBN) ofthe switches in the selector. In an ideal case of the leakage
current = 0 and I1 = I2, the temperature dependence of VREF1 isConventionalCommercial VD(Fig.2)
Programmable VD(Fig.3)Glitch-free VD(Fig.4)
Proposed glitch-freeprogrammable VD (Fig.5)

250

Programmable
VDETECT

Glitch-free
VOUT

No

NoYes

✓No✓✓
Yes ✓

NoYes

Yes

VIN

SelIN

VIN
=VIN1

M1

I1

IA0 V

REF

IA1 V

M2

1M3

Voltagereference

IAN V

N

Voltage divider

VOUTSelREF

VIN

VIN

VOUT1

IC0 VREF’ IB0
IC1 V1’

VOUTB

IB1

VREF1

IC2
VN’
ICN

VIN

IBN

Low VTHVOUT2High VTH

Selector
Glitch-freevoltage detector

Multiple voltage duplicator(MVD)

Programmable voltage detectorFig. 6. Circuit schematic of proposed glitch-free programmable voltage detector.

1VOUT2(Measured)

Voltage [V]0.8
VOUTB(Measured)

0.6
0.4

GlitchVOUT1(Simulated)

0.2
Fig. 7. Die photo and layout of proposed glitch-free programmable voltage
detector in 250-nm CMOS.

0

B. Multiple Voltage Duplicator for Low Power andTemperature-Variation Tolerant Operation
To solve the tradeoff between the power consumption togenerate VREF1 and the temperature dependence of VREF1 in theconventional programmable VD (Fig. 3 (a)), a multiple voltage
duplicator (MVD) is newly proposed and inserted between thevoltage divider and the selector as shown in Fig. 6. MVD is amultiple-input multiple-output voltage buffer. In MVD, M1 isthe current source with the subthreshold leakage current (VGS =0V) of IC0, and the other transistors (M2, M3) are serially stacked.
When the source current (IC0, IC1, IC2, and ICN) is sufficiently
larger than the leakage current (IB0, IB1, and IBN) of the switches,
an interesting characteristic of MVD is that each input voltage
equals to each output voltage (VREF = VREF’, V1 = V1’, and VN =VN’), because IC0 = IC1 = IC2 = ICN and VGS of all transistors inMVD (M1, M2, and M3) are 0V. Thus, in MVD, multiple input
voltages are simultaneously duplicated to multiple output
voltages. The power overhead of MVD is 56pW at 0.8V.
Different from Fig. 3 (a), by inserting MVD between the voltage
divider and the selector, IA0 = IA1 = IAN = 0 and I1 can be greatly
reduced by reducing the transistor size of the voltage reference
and the voltage divider. Thus, the tradeoff between the power

251

00.2

0.4

0.6

VIN [V]

0.8

1

VDETECTFig. 8. Simulated and measured input-output characteristics of proposed
glitch-free programmable voltage detector.

consumption to generate VREF1 and the temperature dependenceof VREF1 in the conventional programmable VD is solved withMVD.
IV.

MEASURED RESULTS

Fig. 7 shows a die photo and a layout of the proposed glitchfree programmable VD fabricated in 250-nm CMOS process.
The core area is 76µm by 120µm. Fig. 8 shows simulated andmeasured input-output characteristics of VOUT1, VOUT2, and
VOUTB of the proposed glitch-free programmable VD in Fig. 6.
The glitch in VOUT1 is successfully removed in VOUTB thanks tothe proposed glitch-free VD. To clarify the tradeoff between thepower consumption to generate VREF1 and the temperaturedependence of VREF1, Fig. 9 shows the power supply current
dependence of the temperature coefficient (TC) of VREF1 withand without MVD. For VREF1 without MVD, the results are
obtained by SPICE simulations of the circuits in Fig. 3 (a), wherethe voltage reference shown in Fig. 6 is used and the transistor

15
VIN = 0.8V

0.8

Change of VDETECT [mV]

Temperature coefficientof VREF1 [mV/ºC]

1-20ºC to 80ºC

0.6
w/o MVD (Fig. 3 (a))(Simulated)with MVD (Fig. 6)(Measured)

0.4
0.2

5 dies10Normalized at 25ºC

50-511mV/100ºC=0.11mV/ºC-10-150-20

0200
400
600
800
1000
Supply current to generate VREF1 [pA]

20406080

Temperature [ºC]
Fig. 11. Measured temperature dependence of VDETECT among 5 dies ofproposed glitch-free programmable voltage detector.

Fig. 9. Power supply current dependence of temperature coefficient of
VREF1 with and without proposed multiple voltage duplicator (MVD).

1TABLE II COMPARISON WITH PREVIOUS VOLTAGE DETECTORS

25ºC
Varied VIN1 by SelIN

VDETECT [V]0

CMOS process
Max
Operatingsupply voltage Min
MaxDetection
Min
voltage(VDETECT)
Step

0.80.6

Power (25ºC)
Temperature
coefficient of
VDETECT

0.4
12 V
3
VariedREF1 by SelREF

4Temperature range

Fig. 10. Measured programmable VDETECT in proposed glitch-freeprogrammable voltage detector.

AP4400A
[7]
N/A
5.5V
0.8V
4.2V
2.0V

JSSC'12
[3]
65nm
N/A
0V
0.46V
(Fixed)< 130mV
100mV(Trimming) (Trimming)
180nW
@1.2V

68nW
@3.4V

0.055mV/ºC 0.75mV/ºC-40ºC to85ºC

1.6nW@0.46V
2.0mV/ºC(Calculated)-15ºC to85ºC

N/A

VLSI'12This Work
[9]
180nm
250nm
1.0V
N/A
0V
0.85V
3.58V
(Fixed)0.52V< 49mV(Programmable)
286pW
248pW
@3.6V
@1.0V
1.5mV/ºC 0.11mV/ºC
0ºC to80ºC-20ºC to80ºC

operation, which is essential for the energy harvesting. This
work demonstrates the programmable VD with less than 49-mV
step to compensate for the die-to-die variations for the first time.
The proposed MVD enables the programmable VD with thelowest power of 248pW and the competitive TC of 0.11 mV/°C.

gate width of the voltage reference is varied to change I1 in thesimulation. As the power supply current is reduced, TC
increases, which is the tradeoff between the power consumption
and TC. For VREF1 with MVD, a measured result is shown. By
introducing MVD, the tradeoff is solved, and both the low
supply current (92pA) and low TC (0.10mV/°C) are
simultaneously achieved. Fig. 10 shows a measured
programmable VDETECT in the proposed glitch-freeprogrammable VD. The range of VDETECT is from 0.52V to0.85V and the step of the programmable VDETECT is less than
49mV. Fig. 11 shows a measured temperature dependence ofVDETECT among 5 dies. The results are normalized at 25°C. The
measured TC of VDETECT is 0.11mV/°C in - 20°C to 80°C.

REFERENCES
[1]
[2]

[3]

In Table II, this work is compared with the previously
published VDs. This work achieves the minimum operating
voltage of 0V thanks to the glitch-free operation, which isessential for the energy harvesting. This work demonstrates theprogrammable VD with less than 49-mV step to compensate forthe die-to-die variations for the first time. The proposed MVD
enables the programmable VD with the lowest power of 248pW
and the competitive TC of 0.11mV/°C.
V.

TPS3839
[5]
N/A
6.5V
0.9V
4.38V
0.9V

[4]
[5]
[6]
[7]
[8]

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[9]

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