Power Electronics

Power Management 101: Power Peripheral and Battery-Based ICs

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.

Supervisor ICs

Supervisory ICs ensure that the system power supplies operate within specified voltage and time windows. In its most basic form, a supervisory IC compares a power supply voltage with a specific threshold. If the power source reaches that threshold, the supervisory IC generates a pulse that resets the system processor.

Fig. 4-2 shows a simplified diagram of supervisor IC and its associated microprocessor. The voltage monitoring section of the supervisory IC includes a comparator and voltage reference as well as reset generator that can reset the associated microprocessor. Usually, supervisor ICs consist of a family of parts set for different thresholds, such as 1.5 V, 1.8 V, etc. There also supervisor ICs that have adjustable thresholds. This supervisor IC has a watchdog timer that protects against an interruption in software execution. Usually, the watchdog timer is a restartable timer whose output changes state on timeout, resetting the system processor or generating an interrupt.

Many systems require multiple supply voltages that can be monitored with multiple devices, but some of the supervisory ICs can monitor two or more voltages. Typically, the number of threshold voltages required in a system depends on the number of processor and peripheral power supplies.

The reset function of the supervisory IC may provide a power-on-reset (POR) to eliminate problems during power-up or a supply voltage sag. These problems can occur because of a slow-rising supply voltage, a supply voltage that exhibits noise or poor behavior during startup, or recovery from a sag. Typically, the reset circuit's voltage tolerance should not exceed ±2.7% over temperature.

Many supervisory ICs include undervoltage and overvoltage comparators with programmable thresholds. Inputs for these comparators can implement a windowed reset that warns if a particular voltage is either too high or too low.

To ensure the continuity of processor memory contents and other critical functions if a supply voltage is lost, many of the older supervisory circuits are able to switch the memory’s power source to a backup battery.

What are the battery power management ICs?

Fig.4-3 shows a typical battery-based system and the associated ICs. Listed below are details of the ICs employed in such a system.

Battery Charger ICs

Performance and longevity of rechargeable batteries depends on the quality of the charger IC. One type of charger IC (used only for NiCd) applies a fixed charge rate of about 0.1C (one tenth of the rated capacity). A faster charger takes 3 to 6 hours with a charge rate of about 0.3C.

A charger for NiMH batteries could also accommodate NiCds, but not vice versa because a NiCd charger could overcharge a NiMH battery. Lithium-based chargers require tighter charge algorithms and voltages. Avoid a charge rate over 1C for lithium battery packs because high currents can induce lithium plating. With most lithium packs, a charge above 1C is not possible because the protection circuit limits the amount of current the battery can accept.

Multi-Function Battery Power Management ICs

These ICs perform multiple functions in a battery-based system. Among these functions are battery charging, dc-dc conversion, battery protection, battery monitoring, and power source selection.

For example, an IC integrates PWM power control for charging a battery and converting the battery voltage to a regulated output. Also, it can simultaneously charge the battery while powering a system load from an unregulated ac wall adapter. Combining these features into a single IC produces a smaller area and lower cost solution compared to presently available multi-IC solutions. The IC shares the discrete components for both the battery charger and the dc-dc converter, minimizing size and cost relative to dual controller solutions. Both the battery charger and dc-dc converter use a current mode flyback topology for high efficiency and excellent transient response. Optional Burst Mode operation and power-down mode allow power density, efficiency and output ripple to be tailored to the application.

The IC provides a complete Li-Ion battery charger with charge termination timer, preset Li-Ion battery voltages, overvoltage and undervoltage protection, and user-programmable constant-current charging. Automatic battery recharging, shorted-cell detection, and open-drain C/10 and wall plug detect outputs are also provided. User-programming allows NiMH and NiCd battery chemistries to be charged as well.

Battery Monitor ICs

Battery-based systems are sensitive to the amount of usable life left in the battery. This is particularly important for computers where a loss of power could mean a loss of stored data. In addition, most battery-based systems are portable, so their operating environment can vary. That environment can cover a wide range of temperatures, which affect a battery’s efficiency, rate of charge and discharge, and therefore battery life.

One solution to this battery-sensitive situation is to include a means for providing a real time indication of remaining battery life to the system user. Battery monitors are actually data acquisition systems that accumulate data related to battery parameters and then transmit the battery data to a host processor.

Battery monitors are mixed signal ICs that incorporate both analog and digital circuits. These monitors include one or more types of digital memory and special registers to hold battery data. Analog circuits include temperature sensors and amplifiers, as well as some interface circuits.

To measure battery current, the monitors usually include either an internal or external current sense resistor. Voltage and current measurements are usually via an on-chip A/D converter.

Among the monitored battery parameters are overcharge (overvoltage), overdischarge (undervoltage) and excessive charge and discharge currents (overcurrent, short circuit), information of particular importance in li-ion battery systems. In some ways a battery monitor assumes some of the functions of a protection circuit by protecting the battery from harmful overcharging and overcurrent conditions.

Battery “Gas Gauge” ICs

The “gas gauge” IC calculates the available charge of the battery while compensating for battery temperature because the actual available charge is reduced at lower temperatures. For example, if the gas gauge IC indicates that the battery is 60% full at 25°C, then the IC indicates 40% full when cooled to 0°C, which is the predicted available charge at that temperature. When the temperature returns to 25°C, the displayed capacity returns to 60%. This ensures that the indicated capacity is always conservatively representative of the charge available for use under the given conditions.

Depending on the battery type, the gas gauge IC also adjusts the available charge for the approximate internal self-discharge of the battery. It adjusts self-discharge based on the selected rate, elapsed time, battery charge level, and temperature. This adjustment provides a conservative estimate of self-discharge that occurs naturally and that is a significant source of discharge in systems that are not charged often or are stored at elevated temperatures.

The gas gauge IC is usually packaged within the battery pack. Because specific inputs on the gas gauge IC connect directly to the battery, those inputs must consume very little power. Otherwise, battery life will be reduced during long storage periods.

The battery gas gauge continuously compensates for both temperature and charge/discharge rate. Typically, it displays the available charge on LEDs and also can send the charge data to an external processor via an I/O port. The LED presentation usually consists of five or six segments of a “thermometer” display. To conserve battery power, the display is only activated at the user’s discretion.

Battery gas gauge ICs employ mixed signal, analog and digital circuits. One technique is to use analog circuits to monitor battery current by measuring the voltage drop across a low-value resistor (typically 20mW to 100mW) in series with the battery. This provides the charge input to the battery and the charge subsequently removed from the battery. Integrated over time, the scaled voltage drives internal digital counters and registers. The counters and registers track the amount of charge available from the battery, the amount of charge removed from the battery since it was last full, and the most recent count value representing “battery full.”

Battery Protector ICs

An added requirement for Li-ion battery packs is a protection circuit that limits each cell’s peak voltage during charge and prevents the voltage from dropping too low on discharge. The protection circuit limits the maximum charge and discharge current and monitors the cell temperature. This protects against overvoltage, undervoltage, overcharge current, and overdischarge current in battery packs

Ideally, the protection circuit should consume no current when the battery-powered system is turned off. However, the protector always consumes some small current. A single-cell rechargeable Li+ protection IC provides electronic safety functions required for rechargeable Li+ applications including protecting the battery during charge, protection of the circuit from damage during periods of excess current flow and maximization of battery life by limiting the level of cell depletion. Protection is facilitated by electronically disconnecting the charge and discharge conduction path with switching devices such as low-cost N-channel power MOSFETs

Battery Power Supply ICs

Virtually all battery-based systems are intended for portable operation. As such, their power supplies have requirements that dictate the associated power supply controller IC configurations. This also means that the controller ICs should require very few external components and any that are used should be low-cost types. Also, to minimize size and weight, the IC should be packaged in some form of small outline package. In addition, the application will determine whether the controller should provide step-up, step-down or some other topology.

One tradeoff in selecting a controller IC is whether it employs external or on-chip power MOSFET switches. On-chip devices minimize external components, but have the potential for increasing the junction temperature and degrading thermal performance. Depending on the package employed, this could also reduce the current carrying capacity of the IC. Some controller ICs described below have on-chip power MOSFETs, others require external MOSETs.

One design consideration is reducing power dissipated by the power supply, which in turn increase battery run time. All controller ICs described below have a shutdown pin that disables the power supply, cutting battery drain. This can be done in many systems that have a normal “sleep” mode. When the IC comes out of the shutdown mode, it has to do so without upsetting the system.

Also available in most battery-based controller ICs is undervoltage lockout (UVLO) that shuts down the power supply if the input voltage drops below a specific threshold. Therefore, if the battery output voltage drops too far, the power supply will shut down. Another characteristic of these controller ICs is protection against overcurrent, which protects both the controller IC and the system components. This is accomplished by sensing current to the load and cutting power for an overload condition.

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