fundamentals of voltage references 9302009

8/2/2019 Fundamentals of Voltage References 9302009

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11Maxim Confidential

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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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



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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



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Shunt Reference

Voltage

Reference

PowerSource

1

2

A Zener Diode

2-pin device



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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.



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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



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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



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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



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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))



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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



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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



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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)



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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



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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



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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



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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

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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



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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



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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



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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.



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Thank You and

Good Luck!

fundamentals of voltage references 9302009

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