Design ProcessDesign Process

Analysis & EvaluationAnalysis & EvaluationPart IPart I

Example Design: Solar CandleExample Design: Solar Candlebyby

Prof. BitarProf. Bitar

Analysis & EvaluationAnalysis & EvaluationHomeworks #4 & 5Homeworks #4 & 5

Define ProductInterfaces

Define ProductChanges

HW#4

HW#5

1

Analyze2

Design

3

Build

4

Test

5

Evaluate

Analysis

&

Evaluation

The Customer RequirementsThe Customer RequirementsExplicitExplicit– Cat SafeCat Safe– Look NiceLook Nice– Different ColorsDifferent Colors– AutomaticAutomatic– Six Hour MinimumSix Hour Minimum

ImplicitImplicit– Low CostLow Cost– Reliable / DurableReliable / Durable– Low MaintenanceLow Maintenance

Product RequirementsProduct RequirementsSafety / DurabilitySafety / Durability– Heavy Base (Batteries in Base)Heavy Base (Batteries in Base)– Unbreakable Bulb (LED)Unbreakable Bulb (LED)– Secure to Window Sill, Sash or Window PaneSecure to Window Sill, Sash or Window Pane– No CordsNo Cords– Low Voltage (1.5V or 3V) Low Voltage (1.5V or 3V)

Operation & AestheticsOperation & Aesthetics– Traditional LookTraditional Look– Interchangeable Color LED’sInterchangeable Color LED’s– Flickering OptionFlickering Option

Low Operating CostLow Operating Cost– Long Battery Life (Six Hour Minimum)Long Battery Life (Six Hour Minimum)– Rechargeable Battery Rechargeable Battery – Solar RechargeableSolar Rechargeable– Photo Sensor or TimerPhoto Sensor or Timer

Modified System Block DiagramModified System Block Diagram

Solar CellCharge

ControllerRechargeable

Battery

EfficientDriveCircuit

LED

SwitchingControl

ModeSelection

PhotoSensor

Timer

Technical Research AreasTechnical Research AreasSolar CellsSolar Cells – Availability, cost, size, shape, – Availability, cost, size, shape, output output voltage, output current, silicon types, voltage, output current, silicon types, spectrum spectrum sensitivity.sensitivity.

Charge ControllersCharge Controllers – Voltage Regulation, – Voltage Regulation, Current Current Regulation, etc.Regulation, etc.

Rechargeable BatteriesRechargeable Batteries – NiCad, NiMH, Other?, – NiCad, NiMH, Other?, Overcharging & Under-voltage Concerns, etc.Overcharging & Under-voltage Concerns, etc.

Drive CircuitsDrive Circuits – DC/DC Boost Converters, Charge – DC/DC Boost Converters, Charge Pumps, etc.Pumps, etc.

LED’sLED’s – – Colors, Voltage Levels, Current Colors, Voltage Levels, Current Requirements, Intensities, Illumination Angles, etc.Requirements, Intensities, Illumination Angles, etc.

Photo Sensors Photo Sensors – Types, Interface Requirements– Types, Interface Requirements

What to Consider First?What to Consider First?

Start at the OUTPUT. What does our Start at the OUTPUT. What does our design have to do? Drive an LED.design have to do? Drive an LED.

What current? What Voltage(s)?What current? What Voltage(s)?

Take a look at LED specification sheets…Take a look at LED specification sheets…

Agilent White LED Spec. SheetAgilent White LED Spec. SheetDissectedDissected

LED SpecificationsLED Specifications

Absolute Maximum RatingsAbsolute Maximum Ratings

NOT TO BE EXCEEDED !!NOT TO BE EXCEEDED !!

LED SpecificationsLED Specifications

Electrical CharacteristicsElectrical Characteristics

Normal Operating ConditionsNormal Operating Conditions

LED Specifications – Curves !LED Specifications – Curves !

Relative Intensity vs. WavelengthRelative Intensity vs. Wavelength

LED Specifications – Curves !LED Specifications – Curves !Max. Forward Current vs. TemperatureMax. Forward Current vs. Temperature

Stay well under this curve !!Stay well under this curve !!

LED Specifications – Curves !LED Specifications – Curves !Max. Forward Current vs. VoltageMax. Forward Current vs. Voltage

Check: 20mA @ 3.2V (Typical)Check: 20mA @ 3.2V (Typical)

LED Specifications – Curves !LED Specifications – Curves !

Spatial Radiation PatternSpatial Radiation Pattern

Check Viewing Angle !!Check Viewing Angle !!

Mechanical SpecificationsMechanical Specifications

LED Drive RequirementsLED Drive Requirements“First Stakes in the Ground”“First Stakes in the Ground”

Must supply a constant current of Must supply a constant current of approximately 20mA.approximately 20mA.

Must supply a minimum of 3.2V for Must supply a minimum of 3.2V for white LED’s.white LED’s.

Should work for other color LED’s, Should work for other color LED’s, since they have lower voltage levels.since they have lower voltage levels.

Customer Requirement CheckCustomer Requirement Check

NOTE: Each decision MUST map to a NOTE: Each decision MUST map to a customer requirement !!customer requirement !!

Question: Which customer requirements Question: Which customer requirements are met by designing for an LED current of are met by designing for an LED current of 20mA at a minimum of 3.2V? 20mA at a minimum of 3.2V?

Answer: Multi-Color LEDS, Reliable, Answer: Multi-Color LEDS, Reliable, Durable, Low MaintenanceDurable, Low Maintenance

Update System Block DiagramUpdate System Block Diagram

Solar CellCharge

ControllerRechargeable

Battery

EfficientDriveCircuit

LED20mA

3.2V(min)

SwitchingControl

ModeSelection

PhotoSensor

Timer

What to Consider Next?What to Consider Next?

The Battery (or Batteries)The Battery (or Batteries)– Battery Capacity: How long does the candle need to Battery Capacity: How long does the candle need to

operate? (Six Hours Minimum)operate? (Six Hours Minimum)– Battery Voltage: What is the minimum battery Battery Voltage: What is the minimum battery

voltage? ( 3.2V )voltage? ( 3.2V )– Battery Availability: What rechargeable technologies Battery Availability: What rechargeable technologies

will work in this application? (NiMH, NiCd, Lithium Ion, will work in this application? (NiMH, NiCd, Lithium Ion, Sealed Lead Acid)Sealed Lead Acid)

– Battery Shape: Needs to fit in candle stem or base.Battery Shape: Needs to fit in candle stem or base.– Battery Cost: I don’t want to spend a lot on batteries!Battery Cost: I don’t want to spend a lot on batteries!

Battery Selection - Two Viable OptionsBattery Selection - Two Viable OptionsMajor Factor? Price!Major Factor? Price!

  Voltage/Cell Capacity Size Price (1) Price (1k)

NiCd 1.2V 700mAhr AA 1.78 0.852

NiMH 1.2V 1600mAhr AA 3.48 1.775

Problem: Need at least 3 batteries in series Problem: Need at least 3 batteries in series to achieve 3.2V minimum. to achieve 3.2V minimum.

Energy Considerations…Energy Considerations…Need: 20mA x 3.2V x 6hrs = 384 mW hrs Need: 20mA x 3.2V x 6hrs = 384 mW hrs (ie: 1382.4 Joules)(ie: 1382.4 Joules)

One NiCd: 700mAhrs x 1.2V = 840 mW hrs One NiCd: 700mAhrs x 1.2V = 840 mW hrs (ie: 3024 Joules)(ie: 3024 Joules)

Conclusion: A single NiCd has more than Conclusion: A single NiCd has more than enough energy to do the job ! (But the enough energy to do the job ! (But the voltage will have to be “boosted”).voltage will have to be “boosted”).

Battery SpecificationsBattery Specifications

Battery SpecificationBattery SpecificationDischarge CurveDischarge Curve

Battery SpecificationBattery SpecificationCharge CurveCharge Curve

Update System Block DiagramUpdate System Block Diagram

Solar CellCharge

Controller

RechargeableBattery

1.2V NiCd

EfficientDriveCircuit

LED20mA

3.2V(min)

SwitchingControl

ModeSelection

PhotoSensor

Timer

What Next? LED Drive CircuitWhat Next? LED Drive Circuit

Solar CellCharge

Controller

RechargeableBattery

1.2V NiCd

EfficientDriveCircuit

LED20mA

3.2V(min)

SwitchingControl

ModeSelection

PhotoSensor

Timer

Searching LED Driver IC’s…

www.digikey.com– LED Drivers (1,319 IN STOCK!)

Narrowing Search (down to 47)– Step-Up (Boost)– Vmin ≤ 0.8V (why?)– Internal Switch– Number of Outputs (1)– Small Qty. Packaging (cut tape / tube)

Case…avoid DFN, QFN, LLP, BGA

Possible LED Drivers

Manufacturer Part Number Unit

Linear Technology LTC3490ES8#PBF $4.20

LT1932ES6#TRMPBF $4.17

Diodes/Zetex ZXLD383ET5CT-ND $1.56

Diodes/Zetex - ZXLD383ET5CTDiodes/Zetex - ZXLD383ET5CT

Typical Application CircuitTypical Application Circuit

Update System Block DiagramUpdate System Block Diagram

Solar CellCharge

Controller

RechargeableBattery

1.2V NiCd700mAHr

ZetexLED

Driver85% Eff.

LED20mA

3.2V(min)

SwitchingControl

ModeSelection

PhotoSensor

Timer