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8/2/2019 Fundamentals of Voltage References 9302009
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Fundamentals ofvoltage reference
and how to select the right ones
foryour application
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The Need for Voltage Reference
Different Types of Voltage Reference andTheir Characteristics
To Select The Right Voltage Reference
CONTENT
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The Need for Voltage Reference
Every Industry Depends on Standards
All Respected Reference Standards Must RemainStable Over Temperature Extremes
All interconnected systems, includingcontrol systems, are only as good astheir weakest element.
All components (sensors, actuators, communications,computing, interfaces, algorithms, etc) must be
of roughly comparable accuracy and performance.
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Power Supplies AC-DC, DC-DC, Battery, Battery Chargers
Industrial Programmable Logic Control, Motor Control,
Instrumentation, Automatic Test Equipment, HandheldMeasurements
Data Processing Servers, Computers, Peripherals, Handheld
Medical Fluid Testing, X-Ray, Sonogram, AED, Infusion, monitors
Consumer Electronics, Automotive, Entertainment, Audio,Video Servo-Motor Control, LCD/LED Adjustments, VolumeControl
Data loggers/Bar Code Scanners Point of Sale terminals, PuritySafety Temperature monitoring
Communications Base Stations, Networking Equipment, CellPhones, Radios, Fiber Optic Systems, Powerline
DACsADCs
AnalogScaling
CircuitsAnalog
Sensors
Comparator
thresholds
The Need for Voltage Reference
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Different Types of Voltage Reference
A Voltage Reference is a device that generates a
precise voltage output from a widely varyingvoltage input.
Time
Voltage
Time
VoltageVoltage
Reference
Head RoomLow Drop OutVoltage
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VoltageReference
PowerSource Load
Voltage
Reference
PowerSource
1
2
31
2
There are two types: Series and Shunt
3-pin device
2-pin device
Different Types of Voltage Reference
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Shunt Reference
Voltage
Reference
PowerSource
1
2
A Zener Diode
2-pin device
Different Types of Voltage Reference
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Shunt Reference Similar in concept to a zener
diode but with much betterspecifications
Operates in parallel with itsload
Requires an external resistor Regulates by adjusting its
current such that Vsupply drop across External R =Reference Output Voltage
Another way of looking at thisis to keep a constant voltageat OUT, the sum of referencecurrent plus load current mustbe constant.
Different Types of Voltage Reference
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Shunt Reference
Power Regulator 1.5% Accuracy
I
I
Generally RegulatorsHave Voltage TolerancesWider Than 1%
References Are Typically
More Precise
The internal Referencecould be a Zener, Buried
Zener or Bandgap
Different Types of Voltage Reference
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No limit to maximum power supply voltage, Just size external resistoraccordingly. Good choice for high voltage applications.
Lowest operating current, 1uA for MAX6006 series
Same maximum current drawn from the power supply regardless ofload. Current either goes thru the load or the shunt reference. Not agood choice for low power systems if there is a large difference
between typical and maximum load currents.
Because they are simple two terminal devices, shunt regulators can beused in many novel circuit configurations: Negative voltage references,floating voltage references, clipping and limiting circuits.
Shunt Reference Characteristics
Different Types of Voltage Reference
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Shunt Reference Key Parameters:
Minimum Reverse Breakdown Current
Minimum current thru the reference necessary for good regulation
Also known as Minimum Operating Current
Maximum Reverse Breakdown Current
Maximum current the reference can tolerate without damage
Different Types of Voltage Reference
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Shunt Reference Design Equation:
VSUPPLY = Power Supply VoltageVREF = Reference Output VoltageIMO = Reference Min. Operating CurrentILMAX = Maximum Load Current
R1 = External Resistor
R1 = (VSUPPLYmin VREF) / ( IMO + ILMAX)Worst Case Current thru R1 = (VSUPPLYmax VREF) / R1
Worst Case Power in R1 = (VSUPPLYmax VREF)2
/ R1Worst Case Power in Shunt Ref = VREF x (IMO + ((VSUPPLYmax VREF) / R1))
Different Types of Voltage Reference
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Series Reference
Oper a t es in ser iesw i t h t h e lo ad
Regu la t es bya d j u s t i n g i n t e r n a l
r esi st ance such t ha tVsu pp ly in t er n a lv o l t a g e dr o p = Vo u tre fe rence vo l t age
The Internal Reference Usually Is
Bandgap Reference
Different Types of Voltage Reference
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Series Reference
A bandgap reference develops two
voltages: One has a positive temperaturecoefficient (tempco) and one has a
negative tempco. Together, they have azero-tempco sum at the output.
The positive tempco is usually derivedfrom the difference of two VBE's runningat different current levels.
The negative tempco uses the naturally negative tempco of the VBE voltage (seeFigure). In practice, the tempco sum is not exactly zero.
Depending on design details like the IC circuit design, packaging, and manufacturing
test capabilities, these devices can usually achieve a Vout tempcos between 1 and100ppm per degree C.
Bandgap Basics
Different Types of Voltage Reference
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Series Reference
BGVREFV
R
RV )1(
4
3+=
TBEBGVM
R
RVV )ln(
2
1
2
2 +=
Bandgap Basics (continuous)
CurvatureCorrection
(Removes 3rd Order Effects)
PROM
DACTemp.Sensor
+P
VREF Out
Each Step Is More Precise, We
Stop When It Is Just Good EnoughTo Meet Requirements
Laser Trim
(Removes 1st Order Effects)
Different Types of Voltage Reference
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Series Reference Characteristics
Supply voltage must be low enough not todamage IC but high enough to allow regulation
IC must be sized to handle the power dissipation
in its series pass element. With no load current, the only power dissipationis due to the quiescent current of the reference. Good choice when there is a large variation
between maximum and typical currents
Different Types of Voltage Reference
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Series Reference Key Parameters:
Maximum Supply Voltage Maximum input voltage the reference can tolerate without
damage
Minimum Supply Voltage Minimum supply voltage necessary for good voltage
regulation Quiescent Supply Current
Current used by the reference with no load connected
Maximum Power Dissipation (Not in Parametric SearchEngine) Maximum Power Dissipation the reference can tolerate without
damage
Different Types of Voltage Reference
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Series Reference Equations:
Worst Case Power Dissipation = ((VSUPPLYmax VREF) x ILOADmax) +
(VSUPPLYmax x IQ)
IQ = Quiescent CurrentVSUPPLY = Supply VoltageILOAD = Load CurrentVREF = VOUT
Different Types of Voltage Reference
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Series or Shunt?
The Parametric Table On www.maxim-ic.com Helps
Select The Right Reference
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Series or Shunt?All VoltageReferences
Just Shunt
Just Series
HighLightedProductAreas
All VoltageReferences
Just Shunt
Just Series
HighLightedProductAreas
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HiddenColumns
CompareCheck /
Uncheck
Choose to Narrow
Selected Parts
Download as an ActiveExcel File for Usewithout Internet Access
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Design Example - To Decide Series or Shunt
We want the lowest power solution Vsupply_max = 5.5V
Vsupply_min = 3.0V
Vref = 2.5V
ILmax= 1uA
Series Reference Iquiescent = 5uA
Shunt Reference IMO
= 1uA
Select The Right Reference
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Design Example - To Decide Series or Shunt
Worst Case Series Ref Power Dissipation = ((Vsupply_max Vref)x ILOAD_max) + (Vsupply_max x IQ)
WC_P_Series = ((5.5V 2.5V) x 1uA) + (5.5V x 5uA) = 30.5uW
Select The Right Reference
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Design Example - To Decide Series or Shunt
R1 = (Vsupply_min Vref) / ( Imo + ILmax) R1 = (3.0V 2.5V) / (1uA + 1uA) = 250K
Worst Case Current thru R1 = (Vsupply_max Vref) / R1 WC_I_R1 = (5.5V 2.5V) / 250K = 12uA Worst Case Power in R1 = (Vsupply_max Vref)^2 / R1 WC_P_R1 = (5.5V 2.5V)^2 / 250K = 36uW Worst Case Power in Shunt Ref = Vref x (Imo + ((Vsupply_max
Vref) / R1)) WC_P_SHUNT = 2.5 X (1uA + (5.5V 2.5V)/250K) = 32.5uW Worst Case Total Power = 36uW + 32.5uW = 68.5uW
Worst Case Power Dissipation Shunt
Select The Right Reference
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Conclusion
Better than 0.1% initial accuracy and a 25ppm TempCo,probably select a series reference.
Want the lowest operating current, consider a shunt reference.
Be careful when combining a shunt reference with a widelyvarying power supply or load.
For power-supply voltages higher than 40V, a shunt reference
may be your only choice.
Consider shunt references when building a negativereference, floating reference, clipping circuit, or limiting circuit.
Select The Right Reference
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Thank You and
Good Luck!