An On-Chip CMOS Relaxation Oscillator With Voltage Averaging Feedback

1150IEEE JOURNAL OF SOLID-STATE CIRCUITS,VOL.45,NO.6,JUNE 2010

An On-Chip CMOS Relaxation Oscillator With

V oltage Averaging Feedback

Yusuke Tokunaga ,Member,IEEE ,Shiro Sakiyama,Akinori Matsumoto,and Shiro Dosho ,Member,IEEE

Abstract—An on-chip CMOS relaxation oscillator with voltage averaging feedback using a reference proportional to supply voltage is presented.A voltage-averaging feedback (V AF)concept is proposed to overcome conventional relaxation oscillator prob-lems such as sensitivity to comparator delay,aging,and ?icker noise of current sources.A test-chip with typical frequency of 14.0MHz was fabricated in a

0.18m standard CMOS process and measured frequency variations

of 0.16%for supply changes from 1.7to 1.9V

and 0.19%for temperature changes from 40to 125 C.The prototype draws

25A from a 1.8V supply,occupies 0.04mm 2,and achieves

7reduction in accumulated jitter (at 1500th cycle)as compared to a oscillator without V AF.Index Terms—Frequency stability,jitter,MOSFET oscillators,oscillator noise,phase noise,relaxation oscillators.

I.I NTRODUCTION

R

ECENTLY ,on-chip reference oscillators are required for low-cost one-chip applications including biomedical sensors,microcomputers,high-speed interfaces such as DDR I/F and HDMI (for initial negotiation),and SoCs.RC-oscilla-tors,including relaxation oscillators,were developed to realize on-chip oscillators with standard CMOS processes.

Table I shows a comparison of conventional oscillators.Crystal oscillators have the best accuracy of ppm order,oth-erwise RC-oscillators are at least %order more accurate.However,RC-oscillators can allow communication links such as UART if frequency variation is kept

within 1%,which enhances their value as on-chip reference oscillators.Moreover,much faster start-up time is more suitable for intermittent low-power operation than crystal oscillators.

There have been conventional accurate RC-oscillators such as a ring oscillator type with a bandgap reference [2],a relaxation type with BGR [3]and peak holding feedback using hybrid os-cillation with relaxation and ring [1],[4];however,their total frequency variations with voltage and temperature were out

of 1%.

Therefore,to achieve frequency variation

within 1%,we propose a voltage averaging feedback (V AF)concept for accu-rate oscillators with low power and small area.

Manuscript received September 24,2009;revised March 26,2010;accepted April 01,2010.Current version published June 09,2010.This paper was ap-proved by Associate Editor Andreas Kaiser.

Y.Tokunaga,S.Sakiyama,and S.Dosho are with the Strategic Semicon-ductor Development Center,Panasonic Corporation,Moriguchi,Osaka 570-8501,Japan (e-mail:tokunaga.yusuke@https://www.wendangku.net/doc/4c2203431.html,;sakiyama.shiro@https://www.wendangku.net/doc/4c2203431.html,;dosho.shiro@https://www.wendangku.net/doc/4c2203431.html,).

A.Matsumoto is with the Advanced Technology Research Laboratory,Pana-sonic Corporation,Soraku-gun,Kyoto 619-0237,Japan (e-mail:matsumoto.akinori@https://www.wendangku.net/doc/4c2203431.html,).

Digital Object Identi?er 10.1109/JSSC.2010.2048732

TABLE I

C OMPARISON OF O

SCILLATORS

This paper consists of four sections.Section II illustrates a conventional RC-oscillator,the proposed voltage averaging feedback concept and its characteristics of sensitivity and noise reduction.An ef?cient method to compensate the temperature dependency of oscillation is also described.Section III shows performance results of the test chip,and Section IV gives the conclusion.

II.O SCILLATOR A RCHITECTURE

A.Conventional RC-Oscillator

Fig.1illustrates a commonly known conventional relaxation oscillator.Oscillation is performed by iteration of charging and discharging the

capacitor as follows:

1)The

voltage

rises while

charging

by .2)

If ,a comparator sets RS-FF and changes the state of oscillator into discharging phase.

3)

falls while

discharging

by .4)

If

,another comparator resets RS-FF and the state returns to charging phase.The issues of this oscillator are as follows:

i)Variation of comparator’s

delay

results in frequency variation with voltage and temperature.

ii)Aging of current

sources

will degrade the accu-racy of the slope

of

and varies frequency.iii)Flicker noise of current

sources

will degrade low offset-frequency phase noise or accumulated jitter.B.Voltage Averaging Feedback Concept

A simple solution to obtain a stable and accurate oscillation is to

shorten to be neglected,however,this approach needs huge power to comparators so that it does not match with our motivation for low power dissipation.Therefore,we propose a feedback concept called V AF to achieve both accuracy and low power dissipation by maintaining whole oscillation waveform

0018-9200/$26.00?2010IEEE

TOKUNAGA et al.:AN ON-CHIP CMOS RELAXATION OSCILLATOR WITH VOLTAGE A VERAGING FEEDBACK

1151

Fig.1.(a)Conventional RC-oscillator.(b)

Waveform.

Fig.2.Proposed V AF concept.

including .Applying V AF to a oscillation with ?rst order lag is our solution for conventional issues;for i)V AF achieves os-cillation independent of comparator’s

delay ,for ii)and iii)oscillation without current source means aging free and signi?-cant reduction of ?icker noise.

1)Oscillation Mechanism:Fig.2shows our concept which comprises a complementary relaxation oscillator and an active ?lter for V AF,where the same complementary design is

chosen

Fig.3.Circuit schematics of (a)feedback ampli?er and (b)

comparator.

Fig.4.(a)Waveform and (b)modeling of oscillation.

for comparators and a feedback ampli?er for rail-to-rail input and easiness to obtain enough phase margin (see Fig.3).

The relaxation oscillator part is considered as a voltage-con-trolled oscillator with a control

signal .Oscillation

waveforms

and are summed up

to and transferred to the active ?lter part.At this condition,the active ?lter maintains

DC voltage of whole oscillation at the reference

voltage

as shown in Fig.

4(a).

and are virtually shorted in a low-frequency domain

de?ned by a time constant

of

.For simplicity,let us assume the feedback ampli?er of the active ?lter as ideal;the half circuit of the relaxation oscillator part can be simpli?ed as Fig.4(b).

The oscillation waveform

under

is (1)

1152IEEE JOURNAL OF SOLID-STATE CIRCUITS,VOL.45,NO.6,JUNE

2010

Fig.5.Normalized frequency characteristic.

As mentioned above,the V AF loop equalizes DC of the os-cillation waveform

with

as (2)

In Fig.

2,

is generated by a voltage divider of rail-to-rail voltages.Therefore,we obtain the following simpli?ed

equation:

(3)(4)

Equation (3)means that the oscillation frequency is just de-?ned by a time

constant

and .Thus,the normalized fre-quency (whose basis

is

)

with is plotted as shown in Fig.5,which is general for any design using V AF.The vari-ation

of has little effect

on because V AF automatically ad-

justs

to keep the equilibrium condition of (2),which results in a stable desired frequency.

If is

shorter,

will become higher to keep the same frequency,

while is

longer,will become lower in a similar fashion [see Fig.6(a)and (b)].This fact enables us to choose low-power design for comparators and results in low power and accurate oscillation.

Equation (3)also means that oscillation frequency does not

have any sensitivity

to

because is stif?y ?xed by the re-sistive divider.A numerical analysis was performed

where was set to be 25ns.As shown in Fig.7,we con?rmed that the cross point

of and averaged integration

of

is always at 25ns with

different .

The oscillation frequency is tunable by applying a digital trimming con?guration to the resistive divider,which com-pensates absolute variations

of

and .With temperature independent capacitance (e.g.,metal-oxide-metal capacitance in standard CMOS processes),(3)implies that the sensitivity to temperature is just dominated

by so that it is important to choose a resistive device with low sensitivity.

Peak holding feedback [1]is another solution for oscillation independent of comparator’s delay.However,its discrete sam-pling behavior will cause aliasing issues,therefore,we chose the continuous feedback of V

AF.

Fig.6.Waveforms in equilibrium situation where (a)comparator’s delay is short and (b)comparator’s delay is

long.

Fig.7.Numerical analysis of oscillation @ =0:5.

2)Aging Free Oscillation:Aging degradation of MOS tran-sistors is inevitable and a non-negligible issue for reference os-cillators,therefore,oscillators using current source for oscilla-tion have concern in principle about their accuracy at the end of life.However,the proposed design uses passive devices (R and C)for oscillation so that its accuracy lasts to the end of life.Of course,the feedback ampli?er suffers from aging degradation.However,because the same aging effect will occur to transistors of the differential pair,the random offset will not change from the time of production to the end of life.The systematic offset may be degraded;however,high enough gain of the feedback ampli?er minimizes this issue.

3)Noise Consideration:For conventional oscillators,the main noise sources are current sources and comparators.Al-though there are oscillators with ?rst order lag oscillation,comparators still remain as a signi?cant noise source.The only way to reduce their noise is to apply huge power (for thermal noise)and large size (for ?icker noise)to comparators;however,huge power does not match with low-power applications and large size of gates increases the ratio of gate parasitic capacitors

including

which is sensitive to voltage and temperature.Of course,large parasitic capacitors of comparators result in more power dissipation of oscillation.

However,the proposed V AF concept releases these issues from comparator design.Thanks to its negative feedback effect,the phase noise spectrum of the oscillator in the low-offset fre-quency domain is suppressed by the high-pass ?lter-like transfer function of V AF.Therefore,we can keep the comparators small and low power.The low-offset characteristic of phase noise is