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:
Minimize battery size and weight while maximizing available run time
Provide the appropriate regulated output voltage over the specified input voltage range and load current
Minimize overall space and weight for associated components
Minimize heat dissipation to eliminate the need for sophisticated thermal management that adds size, weight, and cost
Allow a circuit board layout that minimizes electromagnetic interference (EMI)
Maximize system reliability
BATTERY SELECTION 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.
AC ADAPTERS 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-CHARGER ICs 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.
BATTERY-PROTECTOR ICs 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.
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