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Electronic Design

Simple Charger Targets Lithium-Ion Batteries

Lithium-ion batteries are popular in portable equipment due to their high energy densities. Many handheld products are designed to operate from a single lithium-ion cell with an operating voltage range of 3.0 to 4.2 volts. However, lithium-ion cells require very accurate control of the float voltage to obtain high capacity with long cycle life.

The schematic depicts a simple, inexpensive linear charger that can be used to charge a single lithium-ion cell. The circuit provides constantcurrent/constant-voltage (CC/CV) charging from an inexpensive unregulated 6-V wall adapter. The charger is built around a single LTC1541 IC that contains a voltage reference, op amp, and comparator. The high-accuracy voltage reference (±0.4%) regulates the battery float voltage to ±1.2% as required by most lithiumion battery manufacturers. Even tighter accuracy can be obtained by specifying tighter tolerances for feedback divider resistors R6 and R7.

Transistor Q1 regulates battery charging current. Q1’s base current is controlled by the op-amp output (pin 1) and buffered by transistor Q2 for additional current gain. Diode D1 is needed to prevent reverse current flow when the wall adapter is unplugged or during power-fail conditions. Because this is a linear regulator, the designer must consider power dissipation in Q1.

As shown with a 6-V wall adapter, Q1 dissipates a maximum of about 1 W and can be heatsinked directly to the printed-circuit board. Higher current levels or input voltages will increase dissipation, and additional heatsinking must be provided accordingly.

The battery’s charging current is sensed by R11, and fed to the op amp’s “+” input via R10. IC1’s internal 1.2-V reference voltage is divided to 44 mV by R4 and R2 and connected to the op amp’s “−” input. The op amp compares the current-sense voltage against the 44-mV reference and adjusts Q1’s base drive as needed to regulate the current to 300 mA. The op amp’s ±1.25-mV maximum input offset voltage guarantees accurate charge current regulation with only 44 mV across the sense resistor.

Once the battery charges to 4.2 V, the voltage loop begins to reduce charging current to maintain the desired float voltage. A resistor divider consisting of R6, R7, and R9 generates a feedback voltage to IC1’s comparator (pin 5). Once the voltage at this node reaches 1.2 V, the comparator output goes high, pulling the current-sense signal high via R5. During voltage regulation, the comparator output (pin is a pulsed waveform. However, the low slew rate of the micropower op amp smoothes this signal to a small amplitude triangle wave at pin 1. In fact, the voltage at pin 7 may be monitored by a microprocessor to detect the onset of constant-voltage regulation.

Current-sense resistor R11 is in series with the battery-charging path, and would usually result in voltage regulation beginning at around 4.15 instead of the desired 4.2 V (44 mV dropped across the sense resistor). But, the addition of R3 and R9 compensates for this voltage drop and results in activation of the constant-voltage loop at 4.2 V.

In essence, R3 and R9 create a negative output impedance from the regulator that cancels the 0.15-Ω resistance of R11. By carefully selecting the values of R3, R7, and R9, an even larger negative output resistance can be produced. In addition, it’s possible to compensate for the internal resistance of the battery and its internal protection circuitry. This results in faster recharge times without exceeding the 4.2-V limit within the cell itself.

Because the current-sense loop actually monitors the total current drawn from the wall adapter, the charger will automatically “load share” with the portable equipment. In other words, when the equipment draws no power, all of the 300 mA is available to charge the battery. However, any current drawn by the equipment will simply subtract from the available battery-charging current, keeping the wall adapter load limited to 300 mA.

The charger also may be shutdown by logic control (see the figure). Pulling the shutdown signal high forces the current-feedback signal above the 44-mV threshold, thereby turning off Q1. The charger operates normally when the shutdown pin is in its high-impedance condition—the default state on most microprocessor port pins. R8 may be eliminated if the shutdown feature isn’t needed.

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