05 voltage & current references
DESCRIPTION
Circuitos Geradores de Referências de Tensão e CorrenteTRANSCRIPT
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
Voltage/Current Reference
Prof. Jader A. De Lima
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
• a voltage reference is an esential part in ADC´s, DAC´s, linear regulators, etc
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
• series reference:
• provides load current through a pass transistor between VIN and VREF• requires less power, as load current is provided as it´s needed
• shunt reference:
• regulates VREF by shunting excess current to ground via a parallel transistor• bias current must be equal or greater than load current
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
• temperature coefficient (TC or tempco)
curvature of the voltage driftFirst-order (linear) TC
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
Voltage Reference
• avalanche is a rather noisy mechanism• Zener reference: best TC @ high voltages e.g. 6.2V→ high supply voltage is required to achieve low TC from Zener reference
TC + 100ppm/oC
Best TC @ high voltagese.g. 6.2V!!
strong TC -0.3%/oC
TC > 0
TC < 0
• simple circuits
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
• the bandgap reference concept:
• output voltage for zero TC is close to bandgap voltage of Si (~1.2V)• VCC must be higher than 1.2V → not suitable for sub-1V applications
CTAT
PTAT
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
• dependence of VBE on temperature (CTAT voltage)
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
( ) 0.002065T 1.265BE1V T = +−no laboratório (Exp 2)
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
• PTAT voltage
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
• Determinar a referência Vout
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
13 2 1 2
2
2 2 12
3 3 2
ln
ln
BE BE t
BE t
IR I V V
I
R R IV V
R R I
φ
φ
= − =
= ∆ =
2 13 2 3
3 2
lnref BE BE t
R IV V V V
R Iφ= + = +
• problem: maintain I1/I2 independent of T
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
Since I1 = I2 (assuming R1 = R2)
• bandgap reference with an opamp
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
Projetar o bandgap abaixo para mínima variação a T = 300oK, admitindo:
R1 = R2IS1= 0.1pAIS2 = 0.4pAI1 = I2 ≈ 1mAβ >> 1Vout ≈ 1.1V∆VBE/∆T = -2mV/oC
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
Projetar o bandgap abaixo para mínima variação a T = 300oK, admitindo:
R1 = R2IS1= 0.1pAIS2 = 0.4pAI1 = I2 = 1mAβ >> 1∆VBE/∆T = -2mV/oC
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
para T = T0
R1 = R2 = 555.2 Ohms
para T0 = 300oK
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
current mirror
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
• Reference Current Generation
supply dependent!
β >> 1
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
Robert J. Widlar
• Widlar current source
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
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EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2014
REFERÊNCIAS:
• Fundamentals of Microelectronics, B. Razavi, John Wiley and Sons, 2006
• Microelectronic Circuits, A. Sedra and K. Smith, Oxford university Press, 5th Edition, 2003