Power Management 101: Power Peripheral and Battery-Based ICs

May 6, 2009 10:57 AM

What are the power management power peripheral ICs?

Hot-Swap Controller ICs

Often, equipment users wan to replace a defective board without interfering with system operation. They can do this by removing the existing board and inserting a new board without turning off system power, a process called “hot-swap.” Fig. 4-1 shows a typical hot-swap IC circuit. When inserting a plug-in module or p. c. card into a live chassis slot, the discharged supply bulk capacitance on the board can draw huge transient currents from the system supplies. Therefore, the hot-swap circuit must provide some form of inrush limiting, because these currents can reach peak magnitudes ranging up to several hundred amps, particularly in high-voltage systems. Such large transients can damage connector pins, p. c. board etch, and plug-in and supply components. In addition, current spikes can cause voltage droops on the power distribution bus, causing other boards in the system to reset. Therefore, a hot-swap control IC must provide startup current limiting, undervoltage, overvoltage and current monitoring that prevents power supply failure.

At a hardware level, the hot-swap operation requires a reliable bus isolation method and power management. With today’s power hungry processors, careful power ramp up and ramp down is a must, both to prevent arcing on power pins and to minimize backplane voltage glitches.

Connectors employed in these systems must also allow safe and reliable hot-swap operation. One technique is to use staged pins on the backplane with different lengths. This allows events to occur in a time-sequenced manner as cards are inserted and removed. It enables the power ground and signal pins to be disconnected and then connected in an appropriate sequence that prevents glitches or arcing. After insertion, an enable signal informs the system to power up so that bus-connect and software initialization can begin.

One software sequence of the extraction-insertion process starts with an interrupt signal informing the operating system of the impending event. After the operating system shuts down the board’s functions, it signals the maintenance person or operator via an LED that it is OK to remove the board. After installing a new board, the operating system automatically configures the system software. This signaling method allows the operator to install or remove boards without the extra step of reconfiguring the system at the console.

Load-Share Controller ICs

System integrators can improve system reliability with redundant, paralleled power supplies that share the load. Load-sharing distributes load currents equally among paralleled voltage-stabilized supplies. For the shared supplies to operate efficiently, the power system must ensure that no supply hogs the load current while other supplies are essentially idle. Also, the power system must be able to tolerate the failure of any one supply as long as there is sufficient current capacity from the remaining supplies. This requires the combination of power supplies to behave like one large power supply with equal stress on each of the units.

Individual load-shared supplies require an external controller, otherwise the supply with the highest output voltage will contribute most of the output current. Output impedance of typical power supplies is in the milliohm range so a small difference in output voltages can cause a relatively large difference in output currents. This might cause the supply providing the majority of load current to enter the current-limit mode, increasing its thermal stress, which would decrease system reliability. A load-shared system should have a common, low bandwidth share bus interconnecting all supplies. It should also have good load-sharing transient response and the ability to margin the system output voltage with a single control.

Power Supply Management ICs (Margin, Track, Sequence)

Power supply management ICs perform power management functions for the power supplies employed in electronic systems. These functions can include:

• Power supply sequencing controllers
• Power supply margining controllers
• Power supply tracking controllers

A power supply management IC with power sequencing turns the voltage to each power supply on and off in the proper sequence in a multi-supply systems. The sequencers use (internal or external) MOSFETs to switch power supplies on and off in the appropriate, safe sequence. Power sequencing provides predictable and safe operation for locally-generated power, whereas power-on and power-off sequencing is difficult to control or predict when using externally generated power sources.

One approach for determining the reliability of an electronic system is to test system functionality and performance at the specified upper and/or lower power supply voltage limits of a given design. This allows a system to test the correct operation of electrical components at the upper and/or lower power supply voltage limits specified for a given design. Known as “power supply margining,” it improves the lifetime reliability of an electronic system. This margining function can also be included in power supply management IC.

Some applications require that the voltage difference between two power supplies never exceed a specified voltage. This requirement applies during power-up and power-down as well as during steady-state operation. In other applications it is desirable to have the supplies ramp up and down with fixed voltage offsets between them.  Another possibility is to have the supplies them ramp up and down ratiometrically. A power supply management IC with tracking capability provides this capability.

Tracking employs external resistors to control ramp-up and ramp-down together or with voltage offsets, time delays or differing ramp rates. By introducing currents into the feedback nodes of two independent supplies, the IC causes their outputs to track without inserting any pass element losses. Because the currents are controlled in an open-loop manner, the IC does not affect the transient response or stability of the supplies. Furthermore, it presents a high impedance when power-up is complete, effectively removing it from the dc-dc circuit.

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