Band-Gap Voltage References in Microelectronic Systems
Voltage references
P. Bruschi – Microelectronic System Design
1
Voltage references are used for:
•
Providing an absolute reference voltage for ADCs and DACs
•
Providing an absolute reference voltage for stimulating sensors or
other external devices that require precise control voltages and/or
currents.
•
Creating constant bias voltages (and currents) when required
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Possible reference voltage sources
P. Bruschi – Microelectronic System Design
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•
Zener Diodes
Problems:
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V
Temperature stability is poor for V
Z
≠ 5-6 V
The reference voltage generated by a Zener diode is noisy
(very wide band noise)
•
Band-gap circuits
In present days, zener diodes are available only in high voltage
processes and are used more for protection than for voltage references
solution
Band-gap voltage reference: principle of operation
P. Bruschi – Microelectronic System Design
3
CTAT
CTAT: Complementary To
Absolute Temperature
PTAT
PTAT: Proportional To Absolute
Temperature
V
BG
We start with a DIODE (BJT) biased with
a current
I
C
V
PTAT
P. Bruschi – Microelectronic System Design
4
Band-gap voltage reference: determination of parameter
b
and estimate of the output voltage
We have to determine
the value of
b
, for which:
Example:
Good news!
This voltage is
compatible with
low-supply
voltage circuits
Band-Gap voltage reference: theory
P. Bruschi – Microelectronic System Design
5
It is not necessary that I
C
is temperature-independent
constant
constant
constant
Gray, Hurst, Lewis, Meyer, "Analysis and design of analog
integrated circuits" 4th edition, 2001 J.Wiley & Sons
V
G0
Band-Gap voltage reference: theory
P. Bruschi – Microelectronic System Design
6
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V
GO
is numerically
equivalent to
E
g0
measured in eV
The derivative of
V
BG
depends
on temperature
Let us calculate the derivative of
V
BG
with
respect to temperature
Band-Gap voltage reference: theory
P. Bruschi – Microelectronic System Design
7
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Typically:
=1
P. Bruschi – Microelectronic System Design
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Band-gap voltage reference: calculation result
T
0
=300 K
T
0
=323 K
1 mV
T
(Kelvin)
V
BG
(V)
-20 °C
+100 °C
A bandgap voltage reference
designed for a higher
T
0
, will also
have a higher output voltage.
Even if the derivative is zero only
at
T
0
,
the total voltage variation is
only a few mV for a wide range
Band-Gap voltage reference: a CMOS compatible Circuit
P. Bruschi – Microelectronic System Design
9
Part 1: PTAT current generator
neglecting the effects
of
V
DS
on
I
D
:
I
1
I
2
I
is proportional to
T
(PTAT) and
independent of
V
dd
.
Common centroid
layout
For n=8
P. Bruschi – Microelectronic System Design
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Band-Gap voltage: a CMOS compatible Circuit
I
I
I
Biased with
I
=1
Deriving a temperature sensor from the Band-Gap circuit
P. Bruschi – Microelectronic System Design
11
Adding this branch, we can obtain
a voltage proportional to the
absolute temperature, which can
be conveniently used to monitor
the chip temperature.
I
I
I
I
P. Bruschi – Microelectronic System Design
12
PTAT current generator: multiple stable states
f
NL
I
2
=I
1
P1 is the correct
operating point. P2 (null
currents) is stable
because the two mirrors
have very small gains
around the origin.
P1
P2
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P
2
Positive feedback
loop
Mention to another very popular bandgap circuit
P. Bruschi – Microelectronic System Design
13
Due to virtual short circuit:
(voltages across
R
1
and
R
2
)
We choose
R
1
=
R
2
I
1
I
2
op-amp
b
The bandgap voltage reference in voltage regulators
P. Bruschi – Microelectronic System Design
14
Voltage references provide stable output voltages unaffected by variations in supply voltage, temperature, and process errors. This summary delves into the principles of band-gap voltage references and their crucial role in creating precise control voltages for ADCs, DACs, and various other applications in microelectronic systems.
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Presentation Transcript
Voltage references Voltage references are blocks that produce an output voltage that is independent of PVT variations: V: Supply voltage T: Temperature P: Process errors Voltage references are used for: Providing an absolute reference voltage for ADCs and DACs Providing an absolute reference voltage for stimulating sensors or other external devices that require precise control voltages and/or currents. Creating constant bias voltages (and currents) when required P. Bruschi Microelectronic System Design 1
Possible reference voltage sources Zener Diodes Problems: Require additional process steps, but only a small number of components are required for each chip (not convenient) Available voltages are > 3 V Temperature stability is poor for VZ 5-6 V The reference voltage generated by a Zener diode is noisy (very wide band noise) In present days, zener diodes are available only in high voltage processes and are used more for protection than for voltage references Band-gap circuits solution P. Bruschi Microelectronic System Design 2
Band-gap voltage reference: principle of operation We start with a DIODE (BJT) biased with a current IC VBG PTAT VPTAT CTAT CTAT: Complementary To Absolute Temperature = + V V bV BG BE T PTAT: Proportional To Absolute Temperature dV dT @- 2 mV/K ..... - 3 mV/K BE P. Bruschi Microelectronic System Design 3
Band-gap voltage reference: determination of parameter b and estimate of the output voltage dV dT We have to determine the value of b, for which: = + V V bV = 0 BG BG BE T dV dT dV dT dV dT dV dT dV dT Good news! This voltage is compatible with low-supply voltage circuits - BE = + = 0 BG b BE T = b T dV dT k q - 5 8.56 10 / V K T = @ V (290K) T V BE Example: dV dT @- 2 mV/K BE 23 b @ = 1.225 V 0.65 0.025 23 V @ + BG P. Bruschi Microelectronic System Design 4
Band-Gap voltage reference: theory E kT 0 g constant 2 3 in T e I 2 i qA n D = ln V V C = F n D 2 i = E Q n I BE T n I S kT q = D S B n n E kT E V V q V V 0 G 0 0 g g = = 0 G = = 4 SI BT e T T q k T T n 1.5 VG0 constant constant IC= It is not necessary that ICis temperature-independent integrated circuits" 4th edition, 2001 J.Wiley & Sons GT 1 B = E GT ( ) ( ) ( ) ln = + G E = ln ln V V T V V BE T GO T V V exp BT GO GT B Gray, Hurst, Lewis, Meyer, "Analysis and design of analog ( ) T P. Bruschi Microelectronic System Design 5
Band-Gap voltage reference: theory ( ) ( ) ( ) ln = + = + G E ln V V b V V V V T BG BE T BE GO T k T ( ) ( ) ( ) ln ( ) ( ) ( ) ln = + G E + = + G E + ln V b T ln V V V b T GO BG GO T q VGOis numerically equivalent to Eg0 measured in eV E 0 g = V The name "band-gap" of this reference voltage comes from VGO, which is the dominant part Let us calculate the derivative of VBGwith respect to temperature 0 G q 1.2 1.2 V E eV V 0 0 g G 1 dV dT k q kT q T k q ( ) ( ) ( ) ln ( ) = G E + ln BG b T dV dT The derivative of VBGdepends on temperature k q ( ) ( ) ( ) ( ) ln = G E + ln BG b T P. Bruschi Microelectronic System Design 6
Band-Gap voltage reference: theory We impose that the derivative of VBG is zero at a given temperature T0 This is possible, since b is a free parameter that can be chosen to obtain this result. k q ( ) ( ) ( ) ( ) ln G E + = ln 0 b T 0 ( ) ( + ) ( ) ln + = ln( ) G E b T 0 ( ) ( ) ( ) ln = + G E + ln V V V b T BG GO T ( ) ( + ) ( ) ( ln T ) ( ) l n = + V V V T Typically: =1 0 BG GO T kT q 1.24 V ( ) T ( ) = + 0 V V T ( ) = + + 0 0 BG G 1 ln 0 V V V 0 BG G T T 2.5 P. Bruschi Microelectronic System Design 7
Band-gap voltage reference: calculation result VBG(V) = + V V b V BG BE T T ( ) = + + 1 ln 0 V V V T0=323 K 0 BG G T T kT q ( ) T ( ) = + 0 V V 0 0 BG G Even if the derivative is zero only at T0, the total voltage variation is only a few mV for a wide range 1 mV T0=300 K T (Kelvin) A bandgap voltage reference designed for a higher T0, will also have a higher output voltage. -20 C +100 C P. Bruschi Microelectronic System Design 8
Band-Gap voltage reference: a CMOS compatible Circuit Q2Q2 Q2 Q2 Q2 Q2 Q1 Q2 Q2 Part 1: PTAT current generator Common centroid layout For n=8 neglecting the effects of VDSon ID: = = = = 3 1 4 2 M M M M I I I 1 2 I2 I1 = V V 1 2 GS GS ( ) ( ) = = V V V V V V V V 1 1 2 2 H K G GS G GS 2 1 GS GS = V V = = + V V V V R I H K 1 2 1 H BE K BE I I I I ( ) n = R I V V = = ln 1 2 C S V ln V 1 1 2 BE BE T T area area = 1 2 S C 1 2 n 1 R q k T ( ) n = I is proportional to T (PTAT) and independent of Vdd. ln I 1 Substrate PNPs 1 P. Bruschi Microelectronic System Design 9
Band-Gap voltage: a CMOS compatible Circuit 1 R q kT = + V V IR ( ) n = ln I 3 2 BG BE 1 Biased with I =1 ( ) ln n 5 I I R kT R q I = + V V 2 3 BG BE 1 R R ( ) n = + ln V V V 2 3 BG BE T 1 = + V V b V BG BE T P. Bruschi Microelectronic System Design 10
Deriving a temperature sensor from the Band-Gap circuit Adding this branch, we can obtain a voltage proportional to the absolute temperature, which can be conveniently used to monitor the chip temperature. 5 5 I I I I R kT R q ( ) n = = ln 3 Temp V R I 3 1 P. Bruschi Microelectronic System Design 11
PTAT current generator: multiple stable states Positive feedback loop P1 fNL I2 I2=I1 P2 P1 is the correct operating point. P2 (null currents) is stable because the two mirrors have very small gains around the origin. I1 A start-up circuit is necessary to prevent the circuit from being trapped into P2 P. Bruschi Microelectronic System Design 12
Mention to another very popular bandgap circuit Due to virtual short circuit: V V = V = V (voltages across R1and R2) I2 I1 H K 1 2 R R = I I We choose R1=R2 = 1 2 op-amp I I I I = + V V 1 1 C V V I R H K 1 2 2 BE BE T 2 2 C I I I I = = I I ln 1 2 C S I R V V V = 2 S n 2 1 2 T BE BE T 1 2 S C 1 S b ( ) n ( ) n ln R V ln R V = = T = + = + T I I V V I R V R 2 1 1 1 1 1 1 BG BE BE T T P. Bruschi Microelectronic System Design 13
The bandgap voltage reference in voltage regulators P. Bruschi Microelectronic System Design 14
