Electronic Design

Single-Cell Li-Ion Systems Spawn Compatible ICs

For many handhelds, lithium-ion (Li-ion) batteries have become the power source of choice. In response, IC manufacturers are developing devices that operate properly using the normal output voltage of these batteries. In other words, the nominal Li-ion output of 4.2 V will drop down as low as about 3 V as it's used. An IC then should be able to work with an input voltage of at least 3 to 5 V so it can accommodate a single Li-ion cell.

National Semiconductor's LM3671 step-down switching-regulator IC for singlecell Li-ion applications can operate with an input voltage range of 2.7 to 5.5 V (see the figure). It can handle up to a 600-mA load over the entire input voltage range. Also, it's available as an adjustable output version as well as in 10 fixed output versions supplying 1.2 to 3.3 V.

Optimized for these applications, it includes automatic intelligent switching between pulse-width modulation (PWM) and pulse-frequency modulation (PFM), reducing current consumption. In the PWM mode, it operates at a fixed 2 MHz (typical). The hysteretic PFM mode extends battery life by reducing the quiescent current to a typical 16 µA during light load and standby operation.

Internal synchronous rectification provides high efficiency during PWM mode operation. In the shutdown mode, the IC turns off and reduces battery drain to 0.01 µA (typical). Housings are SOT23-5 and five-bump micro surface-mount packages in lead-free and leaded versions.

The LM3671 is the first in the switching-regulator IC family with similar and expanded capabilities. The LM3674 is a PWM-only version of the LM3671 that produces lower output ripple. The LM3670 and the LM3673 are lower-current versions. Coming in the first quarter of 2007 is a version with a 3-MHz switching frequency. This allows the use of a 1µH chip inductor, which results in a 20mm2 switching regulator circuit.

The ADP2105, ADP2106, and ADP2107 run from input voltages of 2.7 to 5.5 V, enabling single Li+/Li– polymer cell, multiple alkaline/NiMH (nickel-metal-hydride) cells, PCMCIA, and other standard power sources. The output voltage of the ADP2105, ADP2106, and ADP2107-ADJ is adjustable from 0.8 V to the input voltage, while the ADP2105, ADP2106, and ADP2107-xx are available in preset output voltage options of 3.3,1.8, 1.5, and 1.2 V. Each variation is available in three maximum current levels, 1 A (ADP2105), 1.5 A (ADP2106), and 2 A (ADP2107).

The power switch and synchronous rectifier are integrated for minimal external part count and high efficiency. During logic-controlled shutdown, the input is disconnected from the output, and it draws less than 0.1 µA from the input source. Other key features include undervoltage lockout to prevent deep-battery discharge and programmable soft-start to limit inrush current at startup.

The ADP2105, ADP2106, and ADP2107 are low-quiescent-current, synchronous, step-down dc-to-dc converters in a compact 4- by 4-mm LFCSP/QFN package. At medium-to-high load currents, they use a current-mode, constantfrequency PWM control scheme for excellent stability and transient response. To ensure the longest battery life in portable applications, the ADP2105, ADP2106, and ADP2107 use a PFM control scheme under light load conditions that reduces switching frequency to save power.

Operating from a 2.7- to 5.5-V input, Maxim's MAX8581 and MAX8582 high-frequency step-down converters are optimized to dynamically power the power amplifier (PA) in CDMA handsets. They integrate a high-efficiency PWM step-down converter for mediumand low-power transmission and a 60mΩ (typical) bypass mode to power the PA directly from the battery during high power transmission.

Also, they employ an analog input driven by an external digital-to-analog converter (DAC) to control the output voltage linearly for continuous PA power adjustment. The MAX8581 and MAX8582 use an internal feedback network, and the switching frequency is internally set to 2.5 and 1.5 MHz, respectively.

Fast switching (up to 2.5 MHz) and fast soft-start make it possible to use ceramic 2.2-µF input and output capacitors while maintaining low voltage ripple. The small 1.5- to 3.3-µH inductor size can be optimized for efficiency. These ICs are available in 10-pin, 3- by 3-mm TDFN packages with a 0.8-mm max height.

Texas Instruments' TPS7510x LED drivers feature a 2.7- to 5.5-V input range that suits single-cell Li-ion batteries. The TPS75100, TPS75105, and TPS75103 have nominal default output currents of 10, 5, and 3 mA, respectively. This lowdropout (LDO) matching LED current source is optimized for low-power keypad and navigation pad LED backlighting applications.

The IC provides a constant current to up to four unmatched LEDs organized in two banks of two LEDs, each in a common-cathode topology. Without an external resistor, the current source defaults to factory-programmable, preset current level with µ0.5% accuracy (typical). An optional external resistor can be used to set initial brightness to user-programmable values with higher accuracy.

Brightness can be varied from off to full brightness by inputting a PWM signal on each Enable pin. Each bank has independent enable and brightness control, but current matching is done to all four channels concurrently. TPS7510x is available in a nine-ball, 0.4-mm, ball-pitch wafer chipscale package (WCSP) and a 3- by 3-mm quad flat no-lead (QFN) package.

Charger ICs, Too
Li-ion battery chargers also are available for single-cell systems. Linear Technology's LTC4001 2A Li-ion battery charger IC fits 5-V wall adapters (4.5- to 5.5-V input). It uses a 1.5-MHz synchronous buck converter topology to reduce power dissipation during charging.

Low power dissipation, an internal MOSFET, and a sense resistor allow for a physically small charger that can be embedded in a wide range of handheld applications. The LTC4001 includes complete charge termination circuitry, automatic recharge, and a µ1% 4.2-V float voltage. Input short-circuit protection is included, so it requires no blocking diode.

External components set battery charge current, charge timeout, and endof-charge indication parameters. Other features include shorted cell detection, temperature-qualified charging, and overvoltage protection. The LTC4001 comes in a low-profile (0.75 mm), 16-lead (4 by 4 mm) QFN package.

Battery-Powered Equipment Trends

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