Low power, low noise band-gap circuit using second order...

Details Low power, low noise band-gap circuit using second order... Low power, low noise band-gap circuit using second order...

2002-10-29

2004-04-20

Sherry, Michael (Department: 2838)

Electricity: power supply or regulation systems

Self-regulating

Using a three or more terminal semiconductive device as the...

C323S314000, C327S539000, C327S541000

Reexamination Certificate

active

06724176

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention is generally directed to band-gap reference circuits, and more specifically, to a low power, low noise, fast startup, 1-volt operation band-gap reference circuit using second order curvature correction.
BACKGROUND OF THE INVENTION
Band-gap circuits are well known devices that are used to provide a reference voltage that is relatively constant across a wide temperature range. Exemplary band-gap circuits are disclosed in U.S. Pat. No. 3,887,863 and U.S. Pat. No. 6,278,320. The disclosures of U.S. Pat. Nos. 3,887,863 and 6,278,320 are hereby incorporated by reference into the present disclosure as if fully set forth herein.
The theory of operation of band-gap reference circuits is well known in the art. Two different sized base-emitter diodes are biased with the same current level. Since the diodes are the same size, the diodes operate in different current density. The differences in current density are used to generate a proportional-to-absolute-temperature (PTAT) current. The PTAT current develops a voltage across a resistor, thereby creating a PTAT voltage. The PTAT voltage is proportional to absolute temperature and has a positive temperature coefficient. This voltage is then summed to a base-emitter junction voltage of a diode that has a negative temperature coefficient. The negative temperature coefficient and the positive temperature coefficient cancel each other out, so that the combined voltage across the resistor and the base-emitter junction is constant over temperature.
FIG. 1
illustrates conventional band-gap reference circuit
100
according to an exemplary embodiment of the prior art. Band-gap reference circuit
100
comprises capacitor
195
, current sources
110
and
115
, amplifiers
120
and
125
, N-channel transistors
131
-
133
, resistors
140
and
145
, PNP bipolar junction transistors
151
-
153
, amplifier
160
, P-channel transistor
165
, and resistor
170
. PNP bipolar junction transistors
151
-
153
are connected as diodes and are referred to hereafter as PNP diodes
151
-
153
. According to an exemplary embodiment, PNP diode
151
has an area that is eight times larger than the area of PNP diode
152
(i.e., 8:1 ratio).
Current sources
110
and
115
are current mirrors that generate identical currents I
1
and I
2
, respectively. Amplifier 120 samples the voltage on the drain of N-channel transistor
131
, a high impedance node. Amplifier
125
converts the output of amplifier
120
to a control voltage that is applied to the gates of N-channel transistors
131
-
133
. The control voltage forces transistors
131
and
132
,to deliver equal currents I
1
and I
2
to PNP diodes
151
and
152
, respectively. Capacitor
105
sets the dominant pole of the feedback loop formed by amplifiers
120
and
125
and N-channel transistor
131
.
A temperature independent band-gap reference voltage, V(bg), is established by summing the voltage across a resistor (having a positive temperature coefficient) and the base-emitter voltage, V(be), of a pn junction of a pnp diode having negative temperature coefficient. Typically, the sizes of the pnp diodes are chosen with an 8:1 area ratios (the result of using common centroid matching geometry throughout the industry), as in the case of PNP diodes
151
and
152
, so that the PNP diodes operate at unequal current densities.
Let:
1) PNP diode
151
be denoted as D
1
;
2) PNP diode
152
be denoted as D
2
; and
3) PNP diode
153
be denoted as D
3
.
From
FIG. 1
it can be seen that:
V
(
be
)
D2
=V
(
be
)
D1
+I
1
(
Ri
),  [Eqn. 1]
where Ri is the resistance value of resistor
140
.
The current, i, in a PNP diode is given by the equation:
 
i=I
S
(
e
V(be)/V
1
),  [Eqn. 2]
where i is proportional to area. Rearranging terms in Equation 2 gives:
V
(
be
)=
V
T
[ln(
i/I
S
)].  [Eqn. 3]
Substituting V(be) in Equation 3 into Equation 1 gives the expression:
V
(
be
)
D2
−V
(
be
)
D1
=I
1
(
Ri
)=
V
T
[ln(8
i
D1
/i
D1
],  [Eqn. 4]
where i
D1
is the current in D
1
(i.e., PNP diode
151
) and i
D2
is the current in D
2
(i.e., PNP diode
152
). Since i
D1
and i
D2
are equal, Equation 4 reduces to:
I
1
(
Ri
)=
V
T
(ln 8)  [Eqn. 5]
Thus, the current I
1
in PNP diode
151
is:
I
1
=
V
T
(ln 8)/
Ri.
  [Eqn. 6]
It is noted that V
T
, the thermal voltage has a positive temperature coefficient, V
T
=+26 mV, at room temperature. Thus, the current I
1
is proportional to absolute temperature (PTAT).
The current I
1
is mirrored by the current I
3
in N-channel transistor
133
. The current I
3
may be used to establish a band-gap reference voltage, V(bg) for use in biasing, where:
V
(
bg
)=
I
3
(
k*Rr
)+
V
(
be
)
D3
.  [Eqn. 7]
By selecting a suitable multiplier, k, such that dV(bg)/dT=0, V(bg) becomes independent of temperature.
Furthermore, it is possible to generate a reference current, I
4
, that is proportional to V(bg). This is achieved by the feedback loop formed by amplifier
160
, P-channel transistor
165
and resistor
170
, which generate I
4
=V(bg)/Ro, where Ro is the resistance value of resistor
170
.
As
FIG. 1
shows, the band-gap circuit provides a temperature compensated reference voltage output for use by other circuits in a system. A temperature insensitive, high-tolerance band-gap reference circuit is an indispensable building block in modern chip level integrated circuits (ICs). Band-gap reference circuits are used for biasing analog circuits, as a reference level for data converters, to set trip points for comparators and sensors, and the like.
Some applications, such as data converters and low drop-out (LDO) voltage regulators, require low-noise characteristics and a high PSRR (power supply rejection ratio). Prior art devices may employ large value filter capacitor to improve noise and PSRR performance. However, this impacts system cost and board size and, worst of all, slows down turn-on time (i.e., the time it takes for the band-gap reference circuit to stabilize the output voltage after being turned on). For example, many cellular telephones conserve battery power by periodically turning off various circuit-blocks. If the turn-on time is too long, it is not practical to shut off these circuits. This wastes power and impacts system performance. Since band-gap reference circuits are relatively slow to startup, it is necessary that a faster startup technique be incorporated to meet the current needs of cellular telephone and other similar power critical applications.
As mentioned, conventional band-gap reference circuit
100
consumes a relatively large amount of current (>100 microamperes) and is slow to start up (>100 microseconds). Additionally, many modern portable applications, such as cellular telephones and pagers, operate from a +1.2 power supply rail. The V(be) base-emitter voltage drops in band-gap reference circuit
100
leave very little voltage margin with which to operate.
Furthermore, the current (i) in a PNP diode, as defined in Equation 2, exhibits non-linear behavior at high temperature. This is a key element that leads to large variation of band-gap voltage over temperature. Reducing such a variation often requires the introduction of a suitable correction current. Prior art current correction devices require elaborate circuitry and trimming techniques to generate an appropriate non-linear correction current that mitigates the nonlinear behavior of the PNP diode current at high temperature. The result is a flatter band-gap voltage profile over temperature.
Therefore, there is a need in the art for an improved band-gap reference circuit that is capable of operating from a low voltage (e.g., +1.2 volts) power supply rail. More particularly, there is a band-gap reference circuit that uses a simple circuit to generate an appropriate non-linear correction curren

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