Outline DCDC conversion and voltage regulation Buck Regulating wallwartbattery output to desired voltage Boost Size constrained systems Run off single AAA for example Sources Pluggedin ID: 1031060
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1. Power supplyDesign considerations
2. OutlineDC-DC conversion and voltage regulationBuck: Regulating wall-wart/battery output to desired voltageBoost: Size constrained systems (Run off single AAA, for example)SourcesPlugged-inAC-DC conversion Short answer: use third party suppliesDC-DC conversionBattery-basedHigh current draw vs. Low current drawRechargeable vs. non-rechargeable
3. Voltage regulation – Why?Variable voltage drop based on current drawInternal resistance, wire resistanceSpurious resets on current spikesSignals are typically in supply rangeRail-to-rail Ground to VCCNarrower range is commonSupply noise Noisy signals
4. Imaginary perfect regulatorEfficiency = 100% Input power = output powerVin * Iin = Vout * IloadVout is constant; no rippleWhat do we have to give up in practice? Efficiency, High quality (low ripple), low cost/footprintPick any two.EfficiencyLow ripple/quality of regulationCost/footprint
5. Linear regulators (including LDOs)Two major categoriesLinear regulators (including LDOs)Switching regulators (aka switchers)Linear regulatorsWasteful (thermal waste of energy)Power dissipation = Vin * Iload Useful work = Vout * IloadSmall voltage buck and/or low load current5V 3.3V, EfficiencyLow ripple/quality of regulationCost/footprintLinear regulators
6. Switching regulatorsHigh efficiency85%-96%NoisyAverage voltage is well-regulatedA/C noise componentEfficiencyLow ripple/quality of regulationCost/footprintSwitching regulator ($)Switching regulator ($$$)
7. TipsFor fixed voltage with very low dropout, look for custom regulatorE.g. MCP 1700 with < 180mV dropout, up to 250mACan get 3.3V regulation on LiPo batteries (3.7V)Good old 78xx (fixed and variable regulators in family) 2V dropoutSwitching regulatorsPre-packaged drop-in regulator (e.g., OKR-T/3-W12-C)7805 drop-in replacement (e.g., Murata 78xxSR)Using discrete regulator ICsDelicate designs. Use passives and layout recommended in datasheet
8. Heat SinkPay attention to heat thermal issuesMore important in Linear regulatorsFollow datasheet recommendationsWill need understanding of current demands of your project
9. Boost DC/DC conversionCommon case: Buck (step-down) regulationExtreme low power:Operate on 1 AA battery or button cellBoost to more reasonable supply.Typically low-current draw Maybe a sensor or low-duty cycle applicationLots of boost converter ICs Attiny43U (Microcontroller with built-in boost converter)Operates down to 0.7V
10. Battery power: Terminology“Cell” single chamber of electrochemical reactionBattery: array of cells Array size possibly 1Primary: irreversible chemical reactionChemical energy electrical energyNon rechargeableSecondary: chemical energy electrical energy chemical energyRechargeable
11. Characteristics of interestForm factor : AA, AAA, 18500, 2032 etcVoltage Current drawCapacityNot simple; depends on current drawLeakage/Self-dischargeEnergy loss on the shelfEnergy density, power density
12. ChemistryPrimaryAlkalineCell voltage 1.5VButton cell1.35-1.55V (Silver/Zinc/Mercury)Lithium – Family of chemistries1.5-3.7VSecondaryLithum-ion (totally different from Lithium) 3.6-3.7VNiCad 1.2VNiMH 1.2VLithium-ion polymer (LiPo) 4.2-2.7V (Nom: 3.7V)LiFePO4 3.2-3.3 V
13. Capacity500 mAH @ 3.7V50 mA for 10 hours25 mA for 20 hours500 mA for 1 hour5A for 6 minutesNo!Depends on discharge profile
14. Non-rechargeableAlkaline : Good old stuff AA, AAA, C, D : 1.5VLithium : Typically sold as ultra long-lifeDo not ignoreUseful when:Relatively long useful lifeRelatively low current draw OR Relatively rare unplugged operation
15. Other advanced issues: Wireless charging: Inductive couplingQi – broadly used in mobile industrySolar chargingIn-circuit rechargingCareful - charging profilesSerious safety issue (High energy density in personal devices)Microcontroller-driven power managementBattery monitoring issuesThermoelectric
16. Backup batteryVery common use-caseNormal use and battery recharge when plugged inBattery operation when not plugged inSeamless transitionTwo assumptionsRechargeable battery Safe chargingWhat if non-rechargeable? (E.g., smoke alarms)What if trickle charging is inadequate?
17. Simple backup battery
18. Power Management ICsE.g. TI BQ24072 (For Li+)Charging + Dynamic power-pathfor battery backupOther similar ICs for other ChemistryNote: Thermistor input
19. Inductive couplingAir core couplingHigh lossesFairly widespread standard : QiNot available in small quantitiesSome hobby parts availableMagnetic material core Equivalent to transformer, but with separable coilsSecondary coil in toothbrush; Primary coil and core in charging base
20. Solar chargingAlways use to drive battery chargerUse battery to provide stable power supplySize capacity to ensure statistical guarantees of availabilityAssume panel sized to fully-charge battery in 4 hoursProbability of 4 hours of direct sunlight each day = 0.7 (say)Probability of 4 hours of direct sunlight in two days = 0.91 = 1 - (1-0.7) (1-0.7)Probability ...... 3 days > 0.97If battery sized for three days of operation, 97% probability of never running out.Similar process for more sophisticated weather/climate models
21. Power management Sleep/low-power statesImportant for battery-powered systemsSelective Some peripherals/sub-blocks in low-power states when not usedWhole chipQuestions to ask? Programmed wakeup? Via interrupts? Need physical wakeup?Is duty cycle low enough?
22. Voltage Regulator (Selected data) LM 117Adjustable voltage regulatorVout to Adj = 1.25V (Invariant)Negligible current through AdjEverything else followsReason about voltage Vout
23. Other Linear regulators7805 : Very similarSimpler for fixed outputVout-Adj voltage = 5V Very similar reasoningCan be used to design adjustable regulatorsCan be used as current regulatorsConstant current source (LED drivers)
24. Drop-in Replacement Switching regulator (DC-DC)Pin compatibleExamplesReCom R-78Cxx-1.0Equivalent parts from Murata
25. Step-Down Switching Converter: LM 2675 (fixed) 3.3, 5, 12 VDC and (adjustable) 1.21 – 37 V versions Up to 1 amp Up to 96% efficient Five external components
26. Battery monitoringSimple voltage-based approachPlateaus in discharge curvePossibly depends on chemistryFuel gauge ICsSource: Energizer.com
27. Heat sinkIdea of THERMAL RESISTANCE Measured in ºC/W (temperature rise per watt dissipated) Lower thermal resistance is betterThermal resistance is ~ inversely proportional to price1W dissipationDesign goal: heat sink/junction temperature not to exceed 10º C above ambient temperature Need a thermal resistance of approx 10º C/ 1 W 10Airflow reduces thermal resistance