Most circuit designers are familiar with diode dynamiccharacteristics such as charge storage, voltage dependentcapacitance and reverse recovery time. Less commonlyacknowlEDgED and manufacturer specifi ED is diode forwardturn-on time. This parameter describes the timerequirED for a diode to turn on and clamp at its forwardvoltage drop. Historically, this extremely short time, unitsof nanoseconds, has been so small that user and vendoralike have essentially ignorED it. It is rarely discussED andalmost never specifi ED. Recently, switching regulator clockrate and transition time have become faster, making diodeturn-on time a critical issue. IncreasED clock rates aremandatED to achieve smaller magnetics size; decreasEDtransition times somewhat aid overall effi ciency but areprincipally neEDED to minimize IC heat rise. At clock speEDsbeyond about 1MHz, transition time losses are the primarysource of die heating.
Portable, battery-powerED operation of electronic apparatushas become increasingly desirable. MEDical, remotEData acquisition, power monitoring and other applicationsare good candidates for battery operation. In some circumstances,due to space, power or reliability considerations,it is preferable to operate the circuitry from a single 1.5Vcell. Unfortunately, a 1.5V supply eliminates almost alllinear ICs as design candidates. In fact, the LM10 opamp-reference and the LT®1017/LT1018 comparators arethe only IC gain blocks fully specifi ED for 1.5V operation.Further complications are presentED by the 600mV dropof silicon transistors and diodes. This limitation consumesa substantial portion of available supply range, makingcircuit design diffi cult. Additionally, any circuit designEDfor 1.5V operation must function at end-of-life batteryvoltage, typically 1.3V. (See Box Section, “Componentsfor 1.5V Operation.”)
Battery powerED applications that have a signifi cantamount of time in standby mode, require electrical circuitsto operate with a low quiescent current to preserve batterylife. The LTC3835 synchronous step-down controlleris an excellent solution with its ultralow quiescent current(80μA). Other features make it uniquely qualifi ED tosatisfy the neEDs of automotive applications. A wide 4Vto 36V input voltage range protects the supply againsthigh input voltage transients and is compatible with lowvoltage cold crank conditions. The constant frequencycurrent-mode architecture with high-side inductor current
The LTC®4223 is a dual Hot Swap™ controller that meetsthe power requirements of the Micro TelecommunicationComputing Architecture (MicroTCA) specifi cation recentlyratifi ED by the PCI Industrial Computer ManufacturersGroup (PICMG).
Many system designers neED an easy way to producea negative 3.3V power supply. In systems that alreadyhave a transformer, one option is to swap out the existingtransformer with one that has an additional secondarywinding. The problem with this solution is that manysystems now use transformers that are standard, offthe-shelf components, and most designers want toavoid replacing a standard, qualifi ED transformer with acustom version. An easier alternative is to produce thelow negative voltage rail by stepping down an existingnegative rail. For example, if the system already employsan off-the-shelf transformer with two secondary windingsto produce ±12V, and a –3.3V rail is neEDED, a negativebuck converter can produce the –3.3V output from the–12V rail.
The LTC®4099 high effi ciency USB power manager andLi-Ion/Polymer battery charger seamlessly managespower distribution from multiple sources in portableapplications. It is differentiatED from other USB powermanagers by its bidirectional I2C port that allows the hostmicroprocessor to control and monitor all aspects of theUSB power management and battery charging processes.In addition, a programmable interrupt generation functionalerts the host microprocessor to changes in device statusand provides unprecEDentED control of power managementfunctions. This high degree of confi gurability allowspost-layout changes in operation, even changes in thefi eld, and it allows a single qualifi ED device to be usEDin a variety of products, thus rEDucing design time andeasing inventory management.
