Charging lithium-ion batteries requires precise voltage detection

Lithium-ion (Li-Ion) batteries are increasingly popular in portable systems because they add capacity at the same size and weight as older nickel-cadmium and nickel-metal hydride chemistries. For example, a portable computer with a lithium-ion battery may operate longer than a similar computer with a nickel-metal hydride battery. However, designing lithium-ion battery systems requires special attention to the charging circuitry to ensure that the battery is quickly, safely, and fully charged.

The new battery charging IC ADP3810 is designed to control the charging of 1 to 4 cell lithium-ion batteries. Four high-precision fixed final battery voltage options are available (4.2 V, 8.4 V, 12.6 V and 16.8 V); they guarantee a ±1% final battery voltage specification, which is important for Li-ion battery charging. The companion device, the ADP3811, is similar to the ADP3810, but its final battery voltage is user programmable to accommodate other battery types. Both ICs precisely control the charging current to achieve fast charging with currents above 1 amp. Additionally, they feature a precision 2.0V reference and a direct optocoupler drive output for isolation applications.

Li-ion charging: Li-ion batteries typically require a constant current constant voltage (CCCV) type of charging algorithm. In other words, a lithium-ion battery should be charged at a set current level (usually 1 to 1.5 amps) until it reaches its final voltage. At this point, the charger circuit should switch to constant voltage mode and provide the current required to maintain the battery at its final voltage (typically 4.2 V per cell). Therefore, the charger must be able to provide a stable control loop to maintain the current or voltage at a constant value, depending on the state of the battery.

The main challenge in charging lithium-ion batteries is achieving the battery's full capacity without overcharging, which can lead to catastrophic failure. There is little room for error, only ±1%. Overcharging by more than +1% can cause battery failure, but undercharging by more than +1% can result in reduced capacity. For example, undercharging a lithium-ion battery by just 100 mV (-4.2% for a 2.4 V lithium-ion battery) results in approximately 10% capacity loss. Since the margin for error is small, the charging control circuit needs to be processed with high precision. To achieve this accuracy, the controller must have a precision voltage reference, a low-offset high-gain feedback amplifier, and a precisely matched resistor divider. The combined error of all these components must result in a total error of less than ±1%. The ADP3810 combined with these components guarantees an overall accuracy of ±1%, making it an excellent choice for charging lithium-ion batteries.

ADP3810 and ADP3811: Figure 1 shows the functional diagram of the ADP3810/3811 in a simplified CCCV charger circuit. The two "gm" amplifiers (voltage input, current output) are key to the IC's performance. GM1 senses and controls the charging current through the shunt resistor R.CS, and GM2 senses and controls the final battery voltage. Their outputs are connected in an analog "OR" configuration, and both are designed so that their outputs can only pull up the common COMP node. Therefore, either the current amplifier or the voltage amplifier controls the charging loop at any given time. The COMP node consists of a "gm" output stage (GM3) whose output current directly drives the DC-DC converter control input (via an optocoupler in isolated applications).

Figure 1. ADP3810/3811 block diagram in a simplified battery charging circuit

The ADP3810 has built-in precision thin film resistors that accurately divide the battery voltage and compare it to an internal 2.0 V reference. The ADP3811 does not include these resistors, so the designer can program any final cell voltage using external resistors according to the following equation. The buffer amplifier provides a high-impedance input to set the charge current using the VCTRL input, and an undervoltage lockout (UVLO) circuit ensures smooth start-up.

To understand the "OR" configuration, assume a fully discharged battery is inserted into the charger. The battery voltage is well below the final charge voltage, so the VSENSE input of GM2 (connected to the battery) places the positive input of GM2 well below the internal 2.0V reference. In this case, GM2 wants to pull the COMP node low, but it can only pull it high, so it has no effect on the COMP node. Since the battery is dead, the charger begins to increase the charging current and the current loop is controlled. The charging current creates a negative voltage across the 0.25 ohm shunt resistor (RCS). This voltage is sensed by GM1 through the 20 kilohm resistor (R3). At balance, (I'm in charge of R.CS)/R3= -V press/80 com. Therefore, the charging current remains at

If the charge current tends to exceed the programmed level, the input to V.CSGM1 is forced negative, which drives the output of GM1 high. This in turn pulls up the COMP node, increasing the current from the output stage, reducing the drive of the DC/DC converter module (can be implemented using various topologies, such as flyback, buck stage or linear stage), Finally reduce the charging current. This negative feedback completes the charge current control loop.

As the battery approaches its final voltage, the input to the GM2 enters equilibrium. Now GM2 pulls the COMP node high, which increases the output current, causing the charging current to decrease, keeping V sense and V referee equal. The control of the charging circuit has been changed from GM1 to GM2. Since the gain of both amplifiers is very high, the transition region from current to voltage control is very clear, as shown in Figure 2. This data was measured on the 3 V version of the offline charger shown in Figure 10.

Figure 2. Current/voltage conversion of CCCV charger ADP3810

Complete Offline Li-Ion Battery Charger: Figure 3 shows a complete charging system using the ADP3810/3811. This offline charger uses a classic flyback architecture to create a compact, low-cost design. The three main parts of this circuit are the primary side controller, the power FET and flyback transformer, and the secondary side controller. This design uses the ADP3810 connected directly to the battery to charge a 0-cell Li-ion battery to 1.1 V with a programmable charge current from 2.8 to 4 A with an input range of 70 to 220 V ac for general purpose operation. The primary-side pulse width modulator used here is the industry standard 3845, but other PWM components can be used. The actual output specifications of the charger are controlled by ADP3810/3811 to ensure that the final voltage is within ±1%.

Figure 3. Complete offline Li-ion battery charger

The current from the ADP3810/3811 control output drives the photodiode directly connected to the optocoupler, eliminating the need for additional circuitry. Its 4mA output current capability can drive a variety of optocouplers - the MOC8103 is used here. The phototransistor's current flows through the RF, setting the voltage on the COMP pin of the 3845, thus controlling the PWM duty cycle. The controlled switching regulator is designed such that increased LED current from the optocoupler reduces the converter's duty cycle.

While the signal from the ADP3810/3811 controls the average charge current, the primary side should have a cycle-by-cycle limit on the switching current. This current limit must be designed so that the primary power circuit components (FETs and transformers) are not overstressed in the event of a secondary circuit or optocoupler failure or malfunction, or during startup. When the secondary side V CC ADP2/7 rises above 3810.3811 V, it takes over and controls the average current. The primary side current limit is set by a 6.30 ohm current sense resistor connected between power NMOS transistor IRFBC1 and ground.

The ADP3810/3811 is the core of the secondary side and is used to set the overall accuracy of the charger. Rectification only requires one diode (MURD320) and no filter inductor is required. The diode also prevents the battery from back-driving the charger when input power is disconnected. A 1000µF capacitor (CF1) maintains stability without a battery. The RCS senses the average current (see above) and the ADP3810 is connected directly (or the ADP3811 through a voltage divider) to the battery to sense and control its voltage.

With this circuit, a complete offline lithium-ion battery charger is implemented. The flyback topology combines an AC/DC converter with charger circuitry to provide a compact, low-cost design. The accuracy of this system depends on the secondary side controller ADP3810/3811. The device's architecture also works well in other battery charging circuits. For example, a standard DC-DC step-down charger can be easily designed by pairing the ADP3810 and ADP1148. It is also possible to design a simple linear charger using only the ADP3810 and an external pass tube. In all cases, the ADP3810's inherent accuracy controls the charger and guarantees ±1% of the final cell voltage required for Li-ion battery charging.