Cell phones, MP3 players, digital cameras, handheld video games... the list goes on and on. Battery-powered systems are everywhere. One reason for such growth is the availability of batteries and power-management ICs that can support increasingly complex electronic systems. Figure 1 shows a typical power-management subsystem employed in a battery-based system. To be effective, these power-management subsystems must:
To meet these design objectives, the power-management subsystem design begins with the battery, which may be a non-rechargeable primary battery or a rechargeable secondary battery. Primary batteries include alkaline and lithium metal cells. Nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium-ion (Li-ion), and lithium-polymer (Li-pol) are popular rechargeable batteries.
Lithium-ion batteries offer the greatest electrochemical potential and the highest energy density per weight. They're also safe, provided certain precautions are met when charging and discharging. Liion energy density is about twice that of the standard NiCd. Besides high capacity, the load characteristics are reasonably good and behave similarly to the NiCd in terms of discharge characteristics. And, its relatively high cell voltage (2.7 to 4.2 V) enables one-cell battery packs.
Exercise caution when handling and testing Li-ion batteries. Don't short-circuit, overcharge, crush, drop, mutilate, or penetrate them. Don't apply reverse polarity to them, expose them to high temperature, or disassemble them. Finally, always use them with their designated protection circuit.
The Li-pol battery differs from the Liion in its fabrication, ruggedness, safety, and thin-profile geometry. Unlike the Liion, the Li-pol has minimal danger of flammability since it doesn't use a liquid or gelled electrolyte. Also, the Li-pol features simpler packaging and a lower profile than the conventional Li-ion battery.
The ac adapter is a cost-effective power source for charging portable system batteries, since OEMs don't have to design and qualify the supply. Typically, these adapters can power the unit as well as charge the associated battery. The switch-mode adapter provides greater efficiency and smaller size. The linear power-supply adapter is less efficient and larger, but it produces less radiated or conducted EMI. A high-efficiency adapter minimizes heat dissipation, resulting in a smaller and reliable unit.
The PSA-15R 15-W ac adapter from Phihong meets the Energy Star and California Energy Commission (CEC) requirements (Fig. 2). Beginning June 1, all external power-supply products shipping into California must comply with the CEC standard. The CEC also approved new energy-saving standards to slow down the demand for electricity throughout the state. According to the CEC, the energy savings from the new standards over the next 10 years will free the state from having to build three large power plants.
On average, Energy Star-approved models are 35% more efficient than conventional designs. Also, they're often lighter and smaller. The PSA-15R received safety approvals from cUL/UL, TUV, SAA, CE, C-Tick, and CCC (except for 48 V). It provides no-load power consumption of less than 0.5 W, as well as low leakage current with a maximum of 0.25 mA.
Battery chemistries have unique requirements for their charge technique, which is critical for maximizing capacity, cycle life, and safety. Linear topology works well in applications with low-power (e.g., one-or two-cell Li-ion) battery packs charged at less than 1 A. However, switch-mode topology better suits large (e.g., three-or four-series Li-ion or multiple-NiCd/NiMH) battery packs that require charge rates of 1 A and above. Switch-mode topology is more efficient and minimizes heat generation during charging, but it can produce EMI if it isn't packaged properly.
The charge and discharge capacity of a secondary battery is in terms of "C," given as ampere-hours (Ah). The actual battery capacity depends on the C-rate and temperature. Most portable batteries are rated at 1C. A discharge of 1C draws a current equal to the rated capacity. In other words, a battery rated at 1000 mAh provides 1000 mA for one hour if it's discharged at the 1C rate.
Li-ion batteries have a higher voltage per cell, tighter voltage tolerance, and the absence of trickle or float charge when reaching full charge. The charge time for Li-ion batteries charged at a 1C initial current is about three hours. Full charge occurs after reaching the upper voltage threshold, and the current drops and levels off at about 3% of the nominal charge current.
Increasing Li-ion charge current has little effect on shortening the charge time. Although it reaches the voltage peak faster with higher current, the topping charge will take longer. Li-ion batteries can't absorb overcharge, which can cause the cell to overheat. Li-ion constant-current constant-voltage (CCCV) chargers are important to get the maximum energy into the battery without overvoltage.
Linear Technology's LTC4069 is a complete CCCV linear charger for single-cell Li-ion batteries (Fig. 3). Its 2-by 2-mm dual-flat no-lead (DFN) package and low external component count suit it well for portable applications. Also, it's specifically designed to work within USB power specifications. The CHRG pin indicates when charge current drops to 10% of its programmed value (C/10). An internal timer terminates charging according to battery manufacturer specifications. There's a charge current monitor output as well.
No external sense resistor or blocking diode is required due to the internal MOSFET architecture. Thermal feedback regulates charge current to limit the die temperature during high-power operation or high-ambient-temperature conditions.
With the input (ac adapter or USB supply) removed, the LTC4069 automatically enters a low current state, dropping battery drain current to less than 1 mA. With power applied, the LTC4069 can be put into shutdown mode, reducing the supply current to less than 20 mA. And, it includes automatic recharge, trickle charging, softstart, and a negative-temperature-coefficient (NTC) thermistor input used to monitor battery temperature.
Li-ion battery packs require a protection circuit that limits the maximum charge and discharge current and monitors the cell temperature. Ideally, this protection circuit shouldn't consume current when the battery-powered system is off.
Maxim's MAX1666 protects against overvoltage, undervoltage, overcharge current, overdischarge current, and cell mismatch for two-to four-cell Li-ion battery packs (Fig. 4). It does this by checking each cell's voltage in the battery pack and comparing it to a programmable threshold and the other cells in the pack. It's available in four versions—the S version monitors two lithium cells, the A and V versions monitor three cells, and the X version monitors four cells.
GAS-GAUGE AND MONITOR ICs
Portable systems are sensitive to usable battery life. This is particularly important for computers where a loss of power could mean a loss of stored data. Therefore, a real-time indication of remaining battery life is useful. One approach employs a battery monitor that accumulates battery data and transmits it to a host processor. Another approach, the "gas gauge," displays battery life within its associated equipment.
Battery monitors are mixed-signal ICs that integrate digital memory and registers for battery-data storage. Analog circuits include temperature sensors and amplifiers, as well as interface circuits. To measure battery current, monitors usually contain an internal or external current sense resistor. Voltage and current measurements are typically via an on-chip analog-to-digital converter (ADC). Information about overcharge (overvoltage), overdischarge (undervoltage), and excessive charge and discharge currents (overcurrent, short circuit) is particularly important in Li-ion battery systems.
The gas-gauge IC usually is found 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, long storage periods will reduce battery life. Initially, the battery must be fully charged and the counters and registers set to states consistent with a fully charged battery. As discharge occurs, the gas-gauge IC tracks the amount of charge removed from the battery.
Most battery gas gauges compensate for both temperature and charge/discharge rate. Typically, they display the available charge on LEDs. They 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 only is activated at the user's command. All LED segments are lit at full charge. As battery life decreases, the gas-gauge IC extinguishes successive segments on the thermometer display.
The bq2700 and bq27200 (bqJUNIOR) from Texas Instruments are standalone, single-cell Li-ion and Lipol battery-capacity monitoring and reporting ICs for portable applications (Fig. 5). They monitor a voltage drop across a small current-sense resistor connected in series the battery to determine charge discharge activity. Compensations for battery temperature, self-discharge, and discharge rate are applied to the capacity measurements to provide available time-to-empty information across a range of operating conditions.
The ICs automatically recalibrate learn battery capacity in the course discharge cycle from full to empty. Internal registers include current, capacity, time-to-empty, state-of-charge, cell temperature and voltage, and status. The ICs can operate directly from single-cell Li-ion and Li-pol batteries, and they communicate to the system over a harmonic-differential-quadrature (HDQ) one-wire or I2C serial interface.
BATTERY-BASED POWER-SUPPLY ICs
Power supplies for battery-based systems must minimize overall pc-board space. Power-supply ICs may use external or on-chip power MOSFET switches. On-chip devices minimize external components, but they can increase the junction temperature and degrade thermal performance.
It's important to minimize the supply's power dissipation, which increases battery run time. In this case, a shutdown pin can help by disabling the power supply, cutting battery drain. When the IC comes out of the shutdown mode, it must do so without generating a transient that upsets the system.
Also available in most battery-based supply ICs, undervoltage lockout (UVLO) disables the power supply if the battery output voltage drops too low. Most of these supply ICs additionally guard against overcurrent, protecting both the IC and system components. This involves a current sensor that monitors load current and cuts power for an overload.
For all switching power supplies, layout is an important design consideration, especially at high peak currents and high switching frequencies. Without a carefully completed layout, the supply IC could become unstable or produce EMI. This requires wide and short traces for the main current path and for the power ground tracks.
The input capacitor, output capacitor, and inductor should reside as close as possible to the IC. Also, the feedback divider should be as close as possible to the control ground pin of the IC. In laying out the control ground, use short traces separated from the power ground traces.
On Semiconductor's NCP1422, a high-frequency step-up switching converter IC, targets battery-operated products requiring up to 800 mA (Fig. 6). It integrates a synchronous rectifier that provides better efficiency than an external Schottky diode. A 1.2-MHz switching frequency permits the use of a low-profile, small inductor and output capacitor.
When the IC is disabled, the internal conduction path from LX or BAT to OUT is fully blocked and the OUT pin is isolated from the battery. This true-cutoff function reduces the shutdown current to typically only 50 nA. A "Ring-Killer" eliminates high-frequency ringing in the discontinuous conduction mode.
In addition, the NCP1422 features a low-battery detector and open-drain low-battery detector output, logic-controlled shutdown, cycle-by-cycle current limit, and thermal shutdown. With all of these functions on, the quiescent supply current is typically 8.5 mA. This IC comes in the compact and low-profile DFN-10 package. With a 2.5-V input, efficiency is 94% for 3.3-V output at 200 mA and 88% for 3.3-V output at 500 mA.
Recent multifunction battery power-management ICs perform battery charging, dc-dc conversion, battery protection, battery monitoring, and power-source selection. For example, TI's TPS65800 provides flexible charge and system power-path management for a USB-port and ac-adapter supply (Fig. 7). It also has multiple power-supply outputs and several circuit options for applications powered by one Li-ion or Li-pol cell (see the table).
The TPS65800 features two efficient step-down converters that can provide the core voltage and peripheral I/O rails in a processor-based system. For maximum efficiency, these converters enter a low-power mode at light loads.
The TPS65800 also powers the system while independently charging the battery, reducing the battery's charge and discharge cycles. This allows proper charge termination and system operation with an absent or defective battery pack. The system can instantaneously turn on an external power source for a deeply discharged battery pack.
The TPS65800 automatically selects the USB port or the ac adapter as the system's power source. In the USB configuration, the host can select from the two preset charge rates of 100 and 500 mA. The IC dynamically adjusts the USB charge rate based on system load to stay within the 100- or 500-mA charge rates.
In the ac adapter configuration, an external resistor sets the magnitude of the charge current. It charges the battery in three phases: conditioning, constant current, and constant voltage. Charge is terminated based on minimum current. An internal charge timer provides a backup safety for charge termination. The TPS65800 automatically restarts the charge if the battery voltage falls below an internal threshold.
An eight-channel, 10-bit, successive-approximation ADC with external trigger capabilities performs single, multiple, and continuous readings, returning maximum, minimum, or continuous average values. It also has three programmable general-purpose input-output (GPIO) ports.
GPIO3 is programmed by default as a trigger for the ADC. Two general-purpose, pulse-width-modulation (PWM) drivers and an RGB driver with programmable current provide a highly integrated solution that's suitable for handheld and other portable applications.
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