526IEEE JOURNAL OF SOLID-STATE CIRCUITS,VOL.37,NO.4,APRIL 2002
A Sub-1-V
15-ppm/
thn thp
00.9
002
.
I n d e x T e r m s —C M O S b a n d g a p v o l t a g e r e f e r e n c e ,l o w v o l t a g e ,t e m p e r a t u r e .
I .I N T R O D U C T I O N L
O W V O L T A G E a n d l o w p o w e r a r e t w o i m p o r t a n t d s i g n c r i t e r i a i n b o t h t h e a n a l o g a n d d i g i t a l s y s t e m .I t i s e x -p e c t e d t h a t t h e w h o l e s y s t e m w i l l b e a b l e t o o p e r e d o w n t o
a s i n g l e 1-V s u p p l y i n t h e n e a r f u t u r e .A v o l t a g e r e f e r e n c e ,a s o n e o f t h e c o r e f u n c t i o n a l
b l o
c k s i n b o t h a n a l o g a n
d d i g i t a l s y s -t
e m s ,s h o u l d b e a b l e t o o p e r a t e
f r o m a s i n
g l e 1-V s u p p l y f o r
b o t h s y s t e m s .I n C M O S t e
c h n o l o g y ,a p a r a s i t i c v e r t i c a l b i p o l a r j u n c t i o n t r a n s i s t o r (B J T )f o r m e
d i n a p -o r n -w
e l l i s c o m m o n l y u s e d t o
i m p l e m e n t a b a n d g a p r e f e r e n c e [1]–[3].T h e m i n i m u m s u p p l y
v o l t a g e n e e d s t o b e g r e a t e r t h a n 1V d u e t o t w o f a c t o r s :1)t h e
r e f e r e n c e v o l t a g e i s a r o u n d 1.25V w h i c h e x c e e d s 1-V s u p p l y
[4],[5]a n d 2)l o w -v o l t a g e d e s i g n o f t h e p r o p o r t i o n a l -t o -a b s o -l u t e -t e m p e r a t u r e (P T A T )c u r r e n t g e n e r a t i o n l o o p i s l i m i t e d b y
t h e c o m m o n -c o l l e c t o r s t r u c t u r e o f t h e p a r a s i t i c v e r t i c a l B J T [2]a n d t h e i n p u t c o m m o n -m o d e v o l t a g e o f t h e a m p l i f i e r [4],[6].T h e f i r s t p r o b l e m c a n b e s o l v e d b y r e s i s t i v e s u b d i v i s i o n m e t h o d s [7],[8]t o s c a l e d o w n t h e 1.25-V r e f e r e n c e v o l t a g e .T h e s e c o n d p r o b l e m c a n b e s o l v e d b y u s i n g B i C M O S p r o c e s s
[6]o r b y u s i n g l o w t h r e s h o l d v o l t a g e d e v i c e s [7],[8].A s s h o w n i n F i g .1(a ),t h e m i n i m u m i n p u t c o m m o n -m o d e v o l t a g e o f a n
a m p l i f i e r w i t h a n n M O S i n p u t s t a g e m u s t
b e l e s s t h a n o n
e
V i s r e q u i r e d (a s s u m i n g V a n
d
V c a n b e e a s i l y f o u n d i n m a n
y t e c h n o l o -g i e s .H o w e v e r ,t h e t e m p e r a t u r e e f f e c t o n t h e b a s e –e m i t t e r v o l t a g e a n d t h r e s h o l d v o l t a g e s h o u l d b e c o n s i d e r e d .T h e t e m p e r a t u r e c o e f f i c i e n t (T C )o f t h e b a s e –e m i t t e r v o l t a g e i s
a p p r o x i m a t e l y 2m V /K ,f o r
M a n u s c r i p t r e c e i v e d A p r i l 13,2001;r e v i s e d N o v e m b e r 16,2001.T h i s w o r k w a s s u p p o r t e d b y t h e R e s e a r c h G r a n t C o u n c i l o f H o n g K o n g S A R ,C h i n a u n d e r P r o j e c t n o .H K U S T 6022/01E .
T h e a u t h o r s a r e w i t h t h e D e p a r t m e n t o f E l e c t r i c a l a n d E l e c t r o n i c E n g i n e e r i n g ,T h e H o n g K o n g U n i v e r s i t y o f S c i e n c e a n d T e c h n o l o g y ,C l e a r W a t e r B a y ,H o n g K o n g S A R (e -m a i l :e e m o k @e e .u s t .h k ).
P u b l i s h e r I t e m I d e n t i f i e r S 0018-9200(02)02563-5.
e x a m p l e ,m n -w e l l C M O S t e c h -n o l o g y [10].A t h i g h t e m p e r a t u r e s
,
,a n d t h e r e f e r e n c e c i r c u i t w i l l n o
p r o p e r l y .T h u s ,e i t h e r n a t i v e n M O S t r a n s i s t r a n s i s t o r s w i t
h
l e s s t h a n 0.2V
i s r e q u i r e d t o i m p l e m e n t a 1-V r e f e r e n c e .
T o a d d r e s s t h e a b o v e -m e n t i o n e d d e s i g n p r o
b a n d g a p r e f e r e n
c e c i r c u i t i n a s t a n
d a r d C M O s
e n t e d i n t h i s p a p e r .T h e k e y
f e a t u r e o f t h e p c i r c u i t i s t h a t n o l o w t h r e s h o l d v o l t a
g e d e v d e s i g n t e c
h n
i q u e s f o r a c h i e v i n g a g o o d p e r p r e s e n t e d i n e t a i l .I I .P
R O P O S E D S U B -1-V B A N D G A P V OLTAGE R EFERENCE IN CMOS T ECHNOLOGY The structure and the complete schematic of the proposed
sub-1-V bandgap voltage reference are shown in Figs.2and 3,
respectively.The reference core circuitry is modified from the
one proposed by Banba et al.[7].The main differences are that
an amplifier with a pMOS input stage is used and the inputs
of the amplifier are connected to
nodes
and
given
by
is the emitter area ratio,
is injected
to
reference groupIEEE JOURNAL OF SOLID-STA TE CIRCUITS,VOL.37,NO.4,APRIL 2002
527
Fig.1.Bandgap voltage references in CMOS technology using an amplifier with (a)nMOS input stage,and (b)pMOS input
stage.
Fig.2.Proposed sub-1-V bandgap voltage reference.
on the resistor ratio (ratio
of
.There-fore,the minimum supply
voltage
is set to a small value.This
structure is suitable for any CMOS technology to implement low-voltage bandgap reference.Moreover,there is no increase on the total resistance compared to the one proposed by Banba et al.
A.Operation in High-Gain Region by Forward Biasing the Source–Bulk Junctions of pMOS Transistors
A high-gain amplifier with ultralow offset voltage is very im-portant in the proposed bandgap reference to ensure that the
nodes
(4)
where
is the body bias coefficient,
and
(region B in Fig.4),and this allows the amplifiers to operate in the high-gain region.
In [12],the forward-biased junction is defined by the voltage drop across a Schottky diode.In order to eliminate the use of the Schottky diode and allow the reference circuit to be compat-ible to any CMOS process,a temperature-independent voltage
across
is used to forward bias the source–bulk junc-tions of all pMOS transistors.The temperature-dependent bulk
528IEEE JOURNAL OF SOLID-STATE CIRCUITS,VOL.37,NO.4,APRIL
2002
Fig.3.Complete schematic of the proposed sub-1-V bandgap voltage reference (the source–bulk junctions of all pMOS transistors are forward biased by
V
V B.Low-Voltage Amplifier With DC Level-Shifting Current Mirror
A dc level-shifting current mirror [5]must be used in order to make the low-voltage amplifier function properly,especially
for some technologies
with
.The circuit shown in Fig.5(a)is part of the amplifier in Fig.3without dc level-shifting current mirror.The drain–source voltage of MA08is given
by
.
If ,the drain–source
voltage of MA08is less
than
.The drain–source voltage of MA08may be less than the saturation voltage
if
is small.
Since
in the proposed sub-1-V bandgap reference,MA08may operate in triode region and this reduce the gain of the amplifier.
A dc level-shifting current mirror using the parasitic vertical BJTs,shown in Fig.5(b),can solve this problem.The drain–source voltage of MA08is now given
by
.This ensures that MA08
will operate in the saturation region even
when
V ,providing
that
is not greater
than by more than 0.6V .
C.Startup Circuitry
The startup circuit of the proposed reference circuit is formed by MS1–MS4.MS1and MS2form a function of inverter.
The
ratio of MS2is chosen to be much less than one,and
the
.
The drain voltages of MS1and MS2are pulled low,and this turns on MS3and MS4to inject current to the bandgap core circuitry (by MS3)and to the amplifier (by MS4).The drain voltage of M2increases,and the amplifier also starts to operate.Then,the amplifier forces the drain voltage of M1to increase by pulling down the gate voltages of M1–M3to inject current.Once the gate voltage of MS1decreases,the drain voltages of MS1and MS2pull high and cut off MS3and MS4.
The
ratio of MS2is critical since the loop of the refer-ence core is destroyed if MS3and MS4cannot be completely cut off after startup.To ensure a complete cutoff of MS3and
MS4,
the
should be chosen at maximum supply voltage and operating temperature.Since accuracy is not important,it is suggested to choose 1.2times the simulated channel length to avoid the effect of process variations.D.Effect of Offset Voltage and Noise
Bandgap voltage reference in MOS technology suffers from the effect of MOS transistor offset due to
the mismatches of tran-sistor size and threshold voltage.This leads to drifts of the ab-solute value of the reference voltage and also its temperature
IEEE JOURNAL OF SOLID-STA TE CIRCUITS,VOL.37,NO.4,APRIL2002
529
Fig.5.Current mirror in the amplifier.(a)Without dc level shifting(b)With dc level shifting. dependence even after trimming.In the proposed bandgap ref-
erence,the effect of the offset
voltage
is amplified by the resistor
ratio.
However,this can be reduced by increasing the emitter area ratio
(is used in this design),and thus the required resistor
ratio
of
[13],[14].Moreover,the systematic offset can be minimized by
transistor size and bias current in ratio,while the random offset
can be reduced by symmetrical and compact layout[3].
The noise of the proposed reference,which is similar to the
offset voltage,is increased by the resistor
ratio.How-
ever,most of the noise is wide-band thermal noise and can be
reduced by an RC low-pass filter.
The degradation of the noise performance and the effect of
offset voltage is a tradeoff that enables the reference circuit to
operate at a lower supply voltage.
III.E XPERIMENTAL R ESULTS
The proposed sub-1-V bandgap voltage reference shown
in Fig.3was successfully fabricated in AMS
0.6-
/sq.)is used to reduce
the chip area,and the circuit occupies0.24mm
2.2mV at room temperature when the supply voltage changes
from0.98to1.5V.The measured TCs from0C
at different supply voltages
(
C and increases slightly to25
ppm/
A is drawn
when
C.
At
C.This is due to larger threshold voltages at lower
temperatures,thereby forcing the amplifier to operate out of the
high-gain region.The reduction of the gain of amplifier causes
530IEEE JOURNAL OF SOLID-STATE CIRCUITS,VOL.37,NO.4,APRIL2002
TABLE I
C OMPARISON OF L OW-V OLTAGE B ANDGAP R EFERENCES
the rapid drop of the reference voltage at low temperatures.
Therefore,the measured minimum supply voltage is0.98V.
At the lowest supply ,
44dB and that at
10MHz is
C CMOS bandgap voltage reference,
which consumes a maximum of18
C,has been presented.The main features are that no
low threshold voltage device is needed and thus the circuit
is reproducible in any CMOS technology.If low threshold
voltage pMOS transistors(

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