PowerManagement ICs
keeping tabs on where power is flowing and load shedding. The de-facto standard calls for at least one percent accuracy in most phases of the measuring
game.Maxim says its newly released Teridian/Maxim 78M6613 is the industry's first system-on-a-chip (SoC) that can do it.
The LTC4227 dual ideal diode with Hot Swap controller fromLinear Technology saves power and PCB space
protect the system from damage caused by removing or inserting one or more data or power cards or boards into a power system's
backplane. They are primarily designed to minimize inrush current. These currents are often significant, owing to the various capacitors on the PCB, many of which need to charge from zero to the power-supply voltage when power is applied.
The hot-swap ICs most often drive a control (external)
MOSFET, which serves as a switch to power the PCB through the backplane voltage. As such, the hot-swap controller's job is to make sure the powerMOSFET's output rises relatively slowly, and there are several ways to do this. The trick is to avoid the condition where the time to power-up is a function of the value of the total equivalent capacitance value on the PCB.
The better chips utilize an external current-sense resistor
(monitoring the current flowing into the cards) to control the MOSFET, as well as thermal circuitry to detect and shut down the controller if theMOSFET gets too hot (as for instance, during a damaging peak pulse if the load is nonlinear or such a load is otherwise encountered during normal operation).
You also need to pay attention to choosing the right current-
sense resistance andMOSFET for a given hot-swap application. In general, choose aMOSFET with the lowest on-resistance (RDSon) consistent with your cost budget and the maximum peak current the circuit can be expected to see under any circumstances. New device arrivals include Linear Technology's LTC4227, a dual ideal-diode and single hot-swap controller chip for inrush current control for two power rails (redundant supply operation). The so-called ideal diodes replace the two Schottky diodes and associated heat sinks usually seen in this application.
AC/DC advances
The biggest news in this area is that SoCs have arrived for energy measuring applications. Ever complex AC/DC power systems increasingly need more compact and sophisticated means for
26 | April 2011
Linear Technology's LTC3880 dual-output high efficiency synchronous step-down DC/DCcontroller delivers a complete digital powermanagement systemwith an I2C- based PMBus interface and onboard EEPROM
The 78M6613, for single-phase AC systems as applied in server supplies in datacom and
building automation, includes a 22-bit A/D converter with independent 32-bit compute engine, 8-bit microprocessor, voltage reference with better than 40 ppm/C accuracy, 32 kb of flash memory, UART, four sensor inputs, and 32 kHz time base. It claims a best-in-class accuracy of 0.5 percent for real-time energy measurements of watt- hours (W-h) over a 2000:1 current range.
Better lighting
While not strictly part of power-management per se, drivers for light emitting diodes (LEDs), largely used as backlights for a liquid crystal display (LCD, which in itself is an energy saving component) are nonetheless part of the largest if not the most popular PMIC category. In this connection, the LED driver is linked with the LCDs and saving power in portables (this is apart from a discussion of pure TV applications).
The Teridian/Maxim78M6613 is a highly integrated, single-phase, fully self-contained AC power- measurement andmonitoring SoC with embedded AC loadmonitoring and control firmware
Drivers for the much higher-current white LEDs (often used in
backlighting arrangements for color LCD systems) are a growing segment of the lighting area. LED applications are also slowly entering the commercial lighting and general illumination market, increasingly called into service as an alternative to traditional incandescent and fluorescent bulb solutions.
One design criterion is keeping the brightness constant in each
LED (driving in series is more than likely to achieve that goal almost automatically, since the current through each device is the same; note the forward voltage drop of each LED can vary substantially).
Ideally, choose a driver chip having boost DC/DC converter
capability to drive white LEDs as well as series-connected LEDs (often the wiring arrangement for portable products).
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