AN61-1 Application Note 61 August 1994 Practical Circuitry for ...

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Application Note 61in this case 6.5mA. Other currents are obtainable byvarying the 190Ω value. The 1N4002 diode string clampsexcessive voltages when laser conduction first begins,protecting the LT1170. The 10µF capacitor at the V C pinfrequency compensates the loop and the MUR405 maintainsL1’s current flow when the LT1170 V SW pin is notconducting. The circuit will start and run the laser over a9V to 35V input range with an electrical efficiency of about80%.Compact Electroluminescent Panel Power SupplyElectroluminescent (EL) panel LCD backlighting presentsan attractive alternative to fluorescent tube (CCFL) backlightingin some portable systems. EL panels are thin,lightweight, lower power, require no diffuser and work atlower voltage than CCFLs. Unfortunately, most EL DC/ACinverters use a large transformer to generate the 400Hz95V square wave required to drive the panel. Figure 12’scircuit, developed by Steve Pietkiewicz of LTC, eliminatesthe transformer by employing an LT1108 micropowerDC/DC converter IC. The device generates a 95VDC potentialvia L1 and the diode-capacitor doubler network. Thetransistors switch the EL panel between 95V and ground.C1 blocks DC and R1 allows intensity adjustment. The400Hz square wave drive signal can be supplied by themicroprocessor or a simple multivibrator. When comparedto conventional EL panel supplies, this circuit isnoteworthy because it can be built in a square inch with a0.5 inch height restriction. Additionally, all componentsare surface mount types, and the usual large and heavy400Hz transformer is eliminated.0.1µF100V95VV IN 2V TO 12V33pF+I LIM47ΩV INL1100µH+0.1µF100V1MMMBTA42MMBTA42C11µF100VU1LT1108CS8SW1FB2.26MMMBTA92ELPANELGNDSW2R125k30.1k0.47µF200VMMBTA4210k400Hz DRIVESQUARE WAVE= MOTOROLA MURS120T3L1 = COILCRAFT DO3316-104AN61 F12Figure 12. Switch Mode EL Panel Driver Eliminates Large 400Hz TransformerAN61-8
Application Note 613.3V Powered Barometric Pressure SignalConditionerThe move to 3.3V digital supply voltage creates problemsfor analog signal conditioning. In particular, transducerbased circuits often require higher voltage for propertransducer excitation. DC/DC converters in standard configurationscan address this issue but increase powerconsumption. Figure 13’s circuit shows a way to provideproper transducer excitation for a barometric pressuresensor while minimizing power requirements.The 6kΩ transducer T1 requires precisely 1.5mA of excitation,necessitating a relatively high voltage drive. A1senses T1’s current by monitoring the voltage drop acrossthe resistor string in T1’s return path.A1’s output biases the LT1172 switching regulator’s operatingpoint, producing a stepped up DC voltage whichappears as T1’s drive and A2’s supply voltage. T1’s returncurrent out of pin 6 closes a loop back at A1 which is slavedto the 1.2V reference. This arrangement provides therequired high voltage drive (≈10V) while minimizing powerconsumption. This is so because the switching regulatorproduces only enough voltage to satisfy T1’s currentrequirements. Instrumentation amplifier A2 and A3 providegain and LTC1287 A/D converter gives a 12-bit digitaloutput. A2 is bootstrapped off the transducer supply,enabling it to accept T1’s common-mode voltage. Circuitcurrent consumption is about 14mA. If the shutdown pinis driven high the switching regulator turns off, reducingT1PRESSURETRANSDUCER5≈ 10V DURING OPERATIONTO PROCESSOR3.3V10641µF–A2LT1101+A = 10+3.3VA31/2 LT1078–0.05µF10k*1N7525.6VCLK D OUTCS+IN LTC1287 V CC–INGND V REFBRIDGECURRENTTRIM700Ω*50Ω1µF1MCALIB+3.3V22µFMUR1103.3VL1150µH1N4148BRIDGECURRENTMONITOR(0.1500V)100Ω**100k–A11/2 LT1078+1N41482.2µF+V CV INE1LT1172E2V SWGNDFBNC* = 1% FILM RESISTOR** = 0.1% FILM RESISTORL1 = TOKO 262-LYF-0095KT1 = NOVASENSOR (FREMONT, CA)NPH-8-100AHLT10341.2V10k2N3904100kSHUTDOWN3.3VAN61 F13Figure 13. 3.3V Powered, Digital Output, Barometric Pressure Signal ConditionerAN61-9
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