Simplifying Li-Ion Battery Charging with an Autonomous Power Manager

Consumer demand for smaller, lower-cost, and easier-to-use battery-powered handheld devices presents a number of challenges to system designers. These challenges are addressed with the PowerPath battery manager family, which features independent, autonomous operation and manages seamless switching between multiple power sources, such as car adapters, FireWire inputs, AC adapters, USB ports, and the battery itself.

Minimizing power consumption, maximizing efficiency, simplifying design, and reducing cost are some of the key challenges facing system designers of today’s battery-powered handheld devices.

Many of today’s portable, battery-powered electronics derive their power from an AC adapter, car adapter, USB port, or Li-Ion/Polymer battery. However, autonomously managing the power path control between these different power sources is technically challenging. Designers have traditionally attempted to accomplish this function in a “discrete” manner, using a large number of MOSFETs, op amps, and other discrete components, but have encountered challenges such as hot-swap and high inrush currents that can cause significant system problems. Until recently, even discrete IC solutions required multiple chips to achieve a practical solution, but integrated power manager ICs can easily address these issues. In addition, the IC's autonomous, stand-alone operation eliminates the need for an external microprocessor to implement charge termination, further simplifying the design.

When using high-voltage (>5.5V) AC adapters such as FireWire, unregulated, and automotive adapters, the voltage difference between the adapter's voltage source and the battery in the handheld device is very large. Therefore, a linear charger may not be able to solve the high power consumption problem, while ICs using a switch-mode topology can improve efficiency and alleviate thermal management issues. It is worth noting that when powered by USB, Li-ion/Polymer batteries, or adapters with inputs below 5.5V, linear chargers/power managers are a more appropriate choice.

PowerPath Control

Devices with PowerPath control can power themselves from USB VBUS or AC adapter power and charge their single-cell Li-ion batteries. To ensure that a fully charged battery is unused when connected to the bus, the IC directly powers the load through the USB bus instead of drawing power from the battery. Once the power source is removed, current flows from the battery to the load through an internal low-loss ideal diode, minimizing voltage drop and power dissipation.

Features of PowerPath control:

1. Draws power from a USB source, AC adapter, or battery;

2. Delivers power to an application circuit connected to the OUT pin and a battery connected to the BAT pin (assuming an external power source is connected instead of a battery);

3. The battery charge current is regulated to ensure that the sum of the charge current and the load current does not exceed the programmed USB input current limit;

4. The AC adapter current can be connected to the output (load side) through an external device (such as a power Schottky diode or FET);

5. A unique feature is that the output powered by the AC adapter can be used to charge the battery while supplying power to the load;

6. The load at the OUT pin has priority over the USB input current.

Ideal Diode

When the output/load current exceeds the input current limit or the input power is removed, a low-loss ideal diode provides power from the battery. Powering the load through the ideal diode (rather than connecting the load directly to the battery) allows a fully charged battery to remain fully charged until the external power source is removed. Once the external power source is removed, the output voltage drops until the ideal diode is forward biased. The forward biased ideal diode will then deliver output power from the battery to the load. The forward voltage drop of an ideal diode is much lower than that of a conventional diode, and its reverse leakage current is also lower. The small forward voltage drop reduces power dissipation and self-heating, thereby extending the life of the battery.

 

The ideal diode has the following characteristics:

1. When the output/load current exceeds the input current limit or when the input power is removed, the ideal diode component will provide power from the battery;

2. Powering the load through the ideal diode (rather than connecting the load directly to the battery) allows a fully charged battery to remain fully charged until the external power is removed, and allows the device to operate normally even with a completely depleted battery;

3. Once the external power is removed, the output voltage drops until the ideal diode is forward biased. The forward-biased ideal diode then provides output power from the battery to the load;

4. If the battery is the only available power source or the load current exceeds the set input current limit, the battery will automatically deliver power to the load through an ideal diode circuit placed between the BAT and OUT pins;

5. The ideal diode (and the recommended capacitor on the OUT pin) enable the IC to handle large transient loads and AC adapter or USB VBUS connect/disconnect modes without using a large body capacitor;

6. The low-loss ideal diode extends the battery run time by reducing the IR drop associated with the power path.

Linear Technology's power manager IC series solves the above design problems. In this field, the two main new products that realize this function are the LTC4085 USB power manager and the LTC408?? power manager for high voltage battery charger.

USB Power Manager - LTC4085

The LTC4085 is a monolithic autonomous power manager, ideal diode controller and independent linear battery charger for portable USB devices. The LTC4085 has PowerPath control function, which can power the system load from USB VBUS or AC adapter power and charge a single lithium-ion/polymer battery. In order to comply with the USB current limit specification, the LTC4085 will automatically reduce the battery charging current when the system load current increases. In order to ensure that a fully charged battery is not used when connected to the bus, the IC delivers power to the load through the USB bus instead of taking power from the battery. Once the power is removed, the current will flow from the battery to the load through a 200mΩ internal low loss ideal diode, thereby minimizing the voltage drop and power consumption. On-board circuitry is provided to drive an optional external GATE PFET connection device to reduce the total impedance of the ideal diode to less than 30mΩ if the application requires it. The

unique feature of the LTC4085 is that it can detect the presence of an AC adapter and use it as a backup power source to charge the battery while supplying power to the system load. The LTC4085 also provides an option to charge the battery at a rated value (up to 1.5A) higher than the USB specification allowable value (100/500mA) when the AC adapter is connected, which can significantly increase the battery charging speed. The total charge time for charge termination is set by an external resistor. When the charge current decreases, the charge timer period will automatically extend to ensure that the battery is always fully charged. Additional features include automatic recharging, NTC thermistor input, automatic switch to battery when the AC adapter input is removed, inrush current limiting, reverse current blocking, undervoltage lockout and thermal regulation. The

LTC4085's float voltage is preset to 4.2V, and the guaranteed accuracy is 0.8% over the temperature range of 0~85℃. The charge current can be easily set with a resistor. For battery preconditioning and temperature-appropriate charging certification, a fully discharged battery will be automatically trickle charged at 10% of the programmed current until the battery voltage exceeds 2.8V. The LTC4085 application circuit is shown in Figure 1.

Figure 1: LTC4085 USB power manager application circuit

 

For handheld devices such as GPS navigation devices, PDAs, digital cameras, digital photo readers, MP3/MP4 players, etc., providing USB and high input voltage sources and battery charging capabilities has many benefits. For example, USB power allows you to have the convenience of not having to carry a travel charger on the road, and you can power your device from a laptop PC or some other device with a USB port. High voltage input power sources such as FireWire, 12~24V AC adapter or car adapter output can provide higher charging speeds than USB and allow charging in more places (such as cars), which is one of the keys to improving device portability.

Convenience and high power features

LTC408? and LTC408?-5 are autonomous power managers, ideal diode controllers and independent high voltage, high efficiency battery chargers for portable USB devices. To achieve efficient charging, their switching topology can adapt to a variety of inputs, including high voltage power sources up to 36V (maximum 40V) (such as 12V AC adapters, car adapters and FireWire ports). In addition, they also accept low voltage power sources such as 5V adapters and USB. The LTC408-5 has PowerPath control function, which can power the device from the USB bus or AC adapter power supply and charge the single lithium-ion battery of the device, and can also achieve "instant on" operation when using a battery that is exhausted or seriously insufficient. The LTC408-5 typical application circuit is shown in Figure 2.

Figure 2: LTC408 typical application circuit

To comply with USB current limit specifications, the LTC408?-5 automatically reduces the battery charge current as the system load current increases. To ensure that a fully charged battery is terminated at full charge when connected to the bus, the IC delivers power to the load through the USB bus without drawing power from the battery. Once all power is removed, current flows from the battery to the load through a 200mΩ internal low-loss ideal diode, minimizing voltage drop and power dissipation. On-board circuitry is provided to drive an optional external PFET to reduce the total impedance of the ideal diode to less than 30mΩ if the application requires it, further improving operating efficiency.

When the LTC408?-5's power is provided from a USB port, the power manager maximizes the power available to the system load, increasing to 2.5W (500mA×5V) of the full USB available power. Furthermore, it automatically adjusts the charge current of the Li-Ion/Polymer battery based on the system load current to keep the total input current consistent with the USB limit.

The LTC408?? switching regulator features a BAT-Track adaptive output control feature that greatly improves the efficiency of its battery charger, which can provide 1.2A charging current, because the output voltage of the switching regulator automatically tracks the battery voltage. The LTC408?-5 provides a fixed 5V output from a high voltage input for charging a single-cell lithium-ion/polymer battery. The floating voltage of this battery charger is preset to 4.2V and is guaranteed to be 1.0% accurate over the temperature range of 0~85℃. The charge current can be easily set by a resistor. The total charge time for charge termination is set by an external capacitor, and a C/10 charge current detection output is provided. Additional features include automatic thermal regulation, NTC thermistor input for temperature-appropriate charging, automatic recharging of the battery, reverse current isolation and undervoltage lockout. The LTC408?-5 uses a flat (only 0.75mm in height) tiny 22-pin 6mm×3mm DFN package and is guaranteed to operate normally over the temperature range of -40~85℃.

BAT-Track Adaptive Output Control

LTC408???AT-Track function is a form of adaptive output control. It is the integration of battery charger and switching regulator, so that the switching regulator will only generate enough voltage to support the battery charger, and no excess voltage. For linear power path products, the difference between the input voltage and the battery voltage is lost as heat during the charging process.

When implementing a switching regulator, it is beneficial to produce as large a voltage drop as possible across the conversion switch because it can be done efficiently (the current absorbed from the input is less than the current delivered to the charger). The BAT-Track function is responsible for detecting the BAT voltage and regulating the switching regulator output VOUT to 300mV higher than the battery voltage VBAT, thereby minimizing the heat caused by power loss, properly charging the battery and minimizing the total power consumption. This greatly improves the efficiency of the battery charger. For example, when the charging current IBAT = 600mA, VBAT = 3.7V and the charger input voltage VIN = 5V, the efficiency of the charger (using the term substitution method) is: 100×POUT/(POUT＋PDIS)=100×(VBAT×IBAT)/(VBAT×IBAT＋PDIS)=100×(VBAT×

IBAT)/(VIN×IIN)=(3.7V×600mA)/(5V×600mA)=74%. On the contrary, if the charger input voltage is 300mV higher than VBAT, the efficiency of the charger is: 100×(VBAT×IBAT)/(VIN×IIN)=(3.7V×600mA)/((3.7V＋0.3V)×600mA)=92.5%.

This efficiency difference will significantly reduce power consumption. Furthermore, if the battery is over-discharged and VBAT drops to a too low voltage value, the minimum VOUT should be 3.6V to ensure that the system load can obtain a sufficient supply voltage.