Band-Gap Voltage References and Their Importance
Explore the world of band-gap voltage references, their role in providing stable reference voltages in electronic devices, comparison with Zener diodes, and the theoretical principles behind their operation. Discover how these voltage references play a crucial role in maintaining accurate voltage levels despite varying conditions like temperature and process errors.
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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) Band-gap circuits 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 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 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 - BE = + = 0 BG b BE T = b T dV dT k q - 5 8.56 10 / V K T = @ V (290K) T V b @ 23 BE Example: dV dT @- 2 mV/K BE 0.65 0.025 23 1.225 V 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 I n S kT q S = D B n n V V E kT E q 0 G V V 0 0 g g = = = 0 G SI BT e T = T 4 q k T n T 1.5 constant constant IC= It is not necessary that ICis constant GT 1 B = E GT ( ) ( ) ( ) ln = + G E = ln ln V V T V V BE T GO T V V exp BT GO Gray, Hurst, Lewis, Meyer, "Analysis and design of analog integrated circuits" 4th edition, 2001 J.Wiley & Sons 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 ( ) ( ) ( ) ln = + G E + ln V V V b T BG GO T VGOis numerically equivalent to Eg0 measured in eV E 0 g = V 0 G q 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 1.2 1.2 V E eV V 0 0 g G 1 T dV dT k q ( ) ( ) ( ) ln ( ) = G E + ln BG b T V T The derivative of VBGdepends on temperature dV dT 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 VBGis zero at a given temperature T0 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 = + 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 1 mV T0=300 K P. Bruschi Microelectronic System Design 8
Band-Gap voltage: 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 5 5 R kT R q ( ) n = ln 3 Temp V 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
