Using Linear Technology power manager ICs to simplify Li-ion battery charging - Embedded.com

Using Linear Technology power manager ICs to simplify Li-ion battery charging

Product How-To design article:
Today's high-performance, portable handheld Li-ion/polymer battery-powered products have integrated more features, enabling them to inch closer to being “all-in-one” devices for the tech-hungry consumer.

However, this high level of integration, combined with the desire for portability and flexibility of usage with various power sources, presents a number of challenges for the portable electronic device designer.

These include accurate and efficient battery charging, reduced power dissipation, standalone operation (i.e. no external microprocessor for charge termination), autonomous power management and finally, “instant-on” operation – the ability to power the load even with a dead or missing battery.

Autonomous power manager ICs offer full-featured standalone battery chargers integrated with PowerPath controllers and ideal diode devices that efficiently manage a wide variety of input power sources and reduce power dissipation, all with extremely small form factors.

Design challenges
System designers of today's battery- powered handheld devices have had to contend with the need to minimize power dissipation, maximize efficiency, simplify design and reduce cost.

Portable battery-powered electronics can be powered from a wall adapter, automotive adapter, a USB port or a Li-ion/polymer battery. However, autonomously managing power among these sources presents a technical challenge.

Designers have tried to perform this function discretely by using a bunch of MOSFETs, op amps and other components, but have faced problems with hot-plugging and large inrush currents, which may cause major system problems. More recently, even discrete ICs require several chips to implement a practical solution.

An integrated power-manager IC solves these problems simply and easily. In addition, autonomous standalone operation of the IC eliminates the need for an external microprocessor for charge termination, thereby simplifying the design even further.

With high-voltage sources such as Firewire, unregulated higher-voltage (> 5.5V) wall adapters and automotive adapters, the voltage difference between the adapter's voltage source and the battery in the handheld device is large. Thus, a linear charger may not be able to handle power dissipation.

However, an IC with a switchmode topology can improve efficiency and reduce thermal management issues. Note that a linear charger/power manager is more suitable when powering from the USB, Li-ion/polymer battery or adapters with < 5.5V input.

PowerPath control
A device with PowerPath control provides power to the device itself and charges its single-cell Li-ion battery from the USB Vbus or a wall adapter power supply. To ensure that a fully charged battery remains fresh when the bus is connected, the IC directs power to the load through the USB bus rather than extracting 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. (Figure 1 below).

Figure 1: PowerPath control provides power to the device itself and charges its single-cell Li-ion battery from the USB Vbus or a wall adapter power supply.

PowerPath Control has the following features:

1) It can receive power from a USB source, a wall adapter, or a battery.

2) It can deliver power to an application connected to the out pin and a battery connected to the bat pin (assuming that an external supply other than the battery is present).

3) Battery charge current can be adjusted to ensure that the sum of the charge current and load current does not exceed the programmed USB input current limit.

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

5) It has a unique ability to use the output, which is powered by the wall adapter, as a path to charge the battery while providing power to the load.

6) Load on the out pin gets priority over the USB input current.

Figure 2: The LTC4085 is a monolithic autonomous power manager, ideal diode controller and standalone linear battery charger for portable USB devices.

Ideal diode
A low-loss ideal diode provides power from the battery when output/load current exceeds the input current limit or when input power is removed. Powering the load through the ideal diode instead of connecting the load directly to the battery allows a fully charged battery to remain fully charged until external power is removed.

Once external power is removed, the output drops until the ideal diode is forward-biased. The forward-biased ideal diode will then provide the output power to the load from the battery.

The forward voltage drop of an ideal diode is far less than that of a conventional diode, and the reverse current leakage can also be smaller for the ideal diode. The tiny forward voltage drop reduces power losses and self-heating, resulting in extended battery life. (see Figure 1 ).

Figure 3: Onboard circuitry is provided to drive an optional external PFET to reduce the overall ideal-diode impedance below 30 milli-ohms

Linear Technology's growing family of power-manager ICs solves the design problems outlined above. Two key new products that implement this functionality are the LTC4085 USB power manager with linear battery charger and the LTC4089 power manager with high-efficiency, high-voltage battery charger.

The LTC4085 is a monolithic autonomous power manager, ideal diode controller and standalone linear battery charger for portable USB devices. It features PowerPath control that provides power to the system load and charges a single-cell Li-ion/ polymer battery from the USB Vbus or a wall adapter power supply.

To comply with USB current limit specifications, the LTC4085 automatically reduces battery charge current as the system load current increases.

To ensure that a fully charged battery remains fresh when the bus is connected, the IC directs power to the load through the USB bus rather than extracting power from the battery. Once the power source is removed, current flows from the battery to the load through an internal 200m??low-loss ideal diode, minimizing voltage drop and power dissipation.

Onboard circuitry is provided to drive the optional external gate power field effect transistor (PFET) hookup to reduce the overall ideal diode impedance below 30m??if required by the application.

Figure 4: A device with PowerPath control provides power to the device itself and charges its single-cell Li-ion battery from the USB Vbus or a wall adapter power supply.

USB power managers
The LTC4085 can detect the presence of a wall adapter and use it as an alternate power source to charge the battery while providing power to the system load. It also offers the option of charging the battery at a higher rate (up to 1.5A) than USB specifications allow (100mA/500mA) when the wall adapter is present so that the battery can be charged much faster.

Total charge time for charge termination is programmed by an external capacitor. When charging current is reduced, the charge timer automatically extends to ensure the battery is always fully charged. Additional functions include automatic recharge, NTC thermistor input, automatic switchover to battery when the wall adapter input is removed, inrush current limiting, reverse current blocking, undervoltage lockout and thermal regulation.

The LTC4085's float voltage is preset at 4.2V with guaranteed 0.8 percent accuracy from 0°C to 85°C. Charge current is easily programmed using a resistor. For battery pre-conditioning and qualification, fully discharged cells are automatically trickle-charged at 10 percent of the programmed current until the cell voltage exceeds 2.8V.

Finally, the LTC4085 is housed in a tiny 14-pin 3mm x 4mm DFN package with a 0.75mm profile and is guaranteed for operation from -40°C to 85°C.

There are a number of advantages to offering USB and high-input voltage power and battery-charging capability to handheld devices such as GPS navigators, PDAs, digital cameras, photo viewers, MP3/MP4 players and other multimedia devices.

For instance, USB power does not require a travel adapter on the road. Devices may be powered from a laptop PC or some other device with a USB port for example. High-voltage input sources, such as Firewire, 12-24V wall adapters, or automotive car adapter outputs provide faster charging than USB and allow charging in more locations, such as in the car. This is a key to portability.

The LTC4089 and LTC4089-5 are autonomous power managers, ideal-diode controllers and standalone high-voltage, high-efficiency battery chargers for portable USB devices. For high-efficiency charging, their switching topology accommodates various inputs, including high-voltage power sources up to 36V (40V max.), such as 12V wall adapters, automotive adapters and FireWire ports.

In addition, they accept low-voltage power sources such as 5V adapters and USB. The LTC4089/-5 also features PowerPath control that provides power to the device and charges the device's single-cell Lithium battery from the USB bus or a wall adapter power supply and also allows for instant- on operation even with a depleted or missing battery.

To comply with USB current limit specifications, the LTC4089/-5 automatically reduces battery charge current as the system load current increases. To ensure that a fully charged battery remains topped off when the bus is connected, the IC directs power to the load through the USB bus rather than extracting power from the battery.

Once all power sources are removed, current flows from the battery to the load through an internal 200 milli-ohms low-loss ideal diode, minimizing voltage drop and power dissipation. Onboard circuitry is provided to drive an optional external PFET to reduce the overall ideal-diode impedance below 30 milli-ohms if required by the application, providing even higher efficiency operation.

When the LTC4089/-5's power is supplied from a USB port, the power manager maximizes the power available to the system load to the full USB available power of 2.5W (500mA * 5V), and automatically adjusts the Li-ion/polymer battery charge current with the system load current to maintain the total input current compliance within USB limits.

The LTC4089's switching regulator features Bat-Track adaptive output control, which improves the efficiency of its 1.2A-capable battery charger as the switching regulator's output voltage automatically tracks the battery voltage. The LTC4089-5 provides a fixed 5V output from the high-voltage input to charge single-cell Li-ion/polymer batteries.

The battery charger's float voltage is preset at 4.2V with guaranteed 1 percent accuracy from 0°C to 85°C. Charge current is easily programmed using a single resistor. For battery preconditioning and qualification, fully discharged cells are automatically trickle-charged at 10 percent of the programmed current until the cell voltage exceeds 2.9V.

Total charge time for charge termination is programmed by an external capacitor, and a C/10 charge current detection output is provided. Additional functions include thermal regulation, an NTC thermistor input for temperature- qualified charging, automatic recharging of the battery, reverse current blocking, and under-voltage lockout.

The LTC4089/-5 is housed in a low-profile (0.75mm) tiny 22- pin 6mm x 3mm DFN package, and is guaranteed for operation from -40 degree C to 85 degree C.
Adaptive output control
LTC4089's BAT-Track feature is a form of adaptive output control. It is the integration of a battery charger and switching regulator such that the switching regulator only generates enough voltage to support the battery charger, and no more.

For linear power path products, the difference between the input and battery voltages is lost in the charging process as heat. When a switching regulator is implemented, it is advantageous to drop as much voltage as possible across the switcher, since it can be done with high efficiency (drawing less current from the input than is supplied to the charger).

The BAT-Track feature senses the BAT voltage and adjusts the switching regulator output Vout to be 300mV higher than the battery's voltage Vbat, minimizing power lost to heat. Thus, the battery can be charged adequately to minimize overall power dissipation.

This improves the efficiency of the battery charger. For example, given charge current Ibat = 600mA with Vbat = 3.7V and a charger input voltage of Vin = 5V, then the charger efficiency (by term substitution) is:

100 * Pout / (Pout +Pdis) = 100 * (Vbat * Ibat) / (Vbat * Ibat + Pdis) = 100 * (Vbat* Ibat) / (Vin * Iin) = (3.7V x 600mA) / (5V x 600mA) = 74 percent.

Instead, if the charger input voltage is 300mV higher than Vbat the charger efficiency is:

100 * (Vbat* Ibat) / (Vin * Iin) = (3.7V x 600mA) / ((3.7V + 0.3V) x 600mA) = 92.5 percent.

This efficiency difference will reduce power dissipation significantly. Furthermore, if the battery is excessively discharged and Vbat falls too low, the minimum Vout is 3.6V to ensure the system load gets an adequate supply.

Steve Knoth is Product Marketing Engineer at Linear Technology Corp.

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