The Circuit Designer's Companion - diagramas.diagram...

Power supplies 249turn-onovershootturn-offunregulated voltageVoltagerailnoise oroscillationslow rampCondition/undervoltagePower failFigure 7.20 Power rail supervision waveformsprotection, can be gathered up into a single power supply supervisory circuit, andseveral ICs are on the market for this purpose. Examples are the MC3423, ICL7665 and7673, TL7705 and MAX690 series. These chips are basically a collection ofcomparators and delay circuits, integrated into one package for ease of use.Unfortunately, there are a multiplicity of types and few second sources, and the partscost may be greater than you would suffer when using standard comparators such as theLM339. In many cases you may still prefer to design your own supervisory circuit fromsuch standard components.A typical application will require the supervisory circuit to have inputs from the dcoutput rail for overvoltage protection, the reservoir capacitor for undervoltage warning,and the low-voltage ac input for power fail detection. Outputs will go to the crowbardevice and the load circuit. Bear in mind that the supervisor needs to operate reliablydown to very low supply voltages.RegPower failUndervoltageOvervoltageCrowbarPower switchSupervisorResetPower failFigure 7.21 Configuration of power supply supervisorPower control7.4 Mechanical requirements7.4.1 Case size and constructionIf you are designing a supply as an integral part of the rest of the equipment thengenerally you won’t need to consider its mechanical characteristics separately from theequipment design. If you are buying in a standard unit, or designing yourself a modularunit which will be used for different products, then case construction becomesimportant. Standard products tend to fall into one of four categories:• open frame, chassis mounting