Power Management 101: Power Peripheral and Battery-Based ICs

May 6, 2009 10:57 AM

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.

Other Power Mangement Articles:

• System Power Supplies
• Power Mosfets
• Converter & Controller ICs
• Application Specific ICs
• Power Management Semiconductors
• Power Peripheral and Battery-Based ICs

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