Manage Batteries Better in Bluetooth Headsets

The Bluetooth wireless standard has experienced a tremendous adoption rate over the last five years. Initial obstacles that hindered first-generation devices have been overcome and new applications have been discovered. While the handset is the largest application for this technology, the headset is the second-largest segment and a fast-growing one. Improvements in industrial design (size and fit), falling prices and new laws requiring hands-free calling indicate a bright future for Bluetooth success in the marketplace.

With this rapid growth have come battery-management challenges for Bluetooth headset designs, with the focus on safety, size and charging time. Although industrial design often takes precedence over design considerations of the battery and battery-charger ICs, new technologies and battery-charging solutions are allowing these two previously contradicting goals to exist together. Modern Bluetooth headsets may now be designed for a small solution size and attractive industrial design, while offering extended talk times and fast and safe battery charging.

To better understand how these solutions are possible, let's take a closer look at typical Bluetooth system implementations. This will be followed by showing how an innovative method for charging Bluetooth headsets via another portable device can be useful.

A Typical Headset

A system diagram of a typical Bluetooth headset is shown in Fig. 1. The Bluetooth system-on-chip (SoC) integrates most of the main functions, such as central processing, the transceiver subsystem and several I/O interfaces. The audio codec and the speaker driver comprise the second-most-important system component. The codec, the speaker driver and the corresponding algorithms determine audio quality and signal integrity. The required memory for the system can be integrated on the SoC or supplied by a dedicated memory chip.

These various blocks in the design usually require two to three different voltage rails for operation: one for powering the digital core, one for powering the I/Os and one for the audio subsystem. These power requirements are addressed by a combination of stepdown regulators and low-dropout linear regulators, all of which are powered directly from the battery. The main areas of focus on the power-system implementation are low noise (because of Bluetooth's RF sensitivity), high efficiency (for offering longer talk time and reducing thermal dissipation) and minimum board space (for accommodating the required industrial design).

Every headset also requires battery-charging circuitry for charging the embedded battery and steering power to the system. Some Bluetooth SoCs incorporate battery-charging functionality. However, external battery-charging solutions are increasingly used to meet new system requirements, like faster charging, reduced thermal dissipation on the SoC and stricter safety features.

Modern headsets use battery packs with Li-polymer cells, which allow more flexible and thinner form factors than Li-ion cells. Using polymer electrolytes, this battery technology allows the use of a thin foil casing, since it does not require external pressure between the electrodes and the separator.

This material advancement has led to products with industrial designs that are both more usable and more attractive for the consumer, because the battery can be manufactured to fit most form factors. Typical battery capacities for mono headsets are 80 mAh to 150 mAh, while stereo headsets use battery packs with capacities up to 500 mAh. Most modern system designs can be charged both via an external wall adapter (a USB or proprietary physical interface) or a USB port on a notebook PC.^[1]

Size and Weight Challenges

Two of the main differentiating features and selling points among Bluetooth headset products are size and weight. How small the product dimensions and how low the weight can be are highly dependent on the battery pack used and its capacity, as well as on the system design. Using battery-charging solutions that are offered in chip-scale packaging (CSP)^[2], and which integrate many of the required system functions such as secondary system and battery protection, allows for minimum board space.

CSP in its simplest form requires three basic steps: the addition of a passivation layer on top of the silicon die, the deposit of an under bump metallization (UBM) stack for forming the UBM pad, and the attachment and reflow of the solder ball. This technology allows the device size to be identical to the die size, which results in a very small chip size compared with DFN or QFN packaging equivalents. In addition to the obvious area savings, CSP is generally electrically superior to wire-bonded packaging, offers an extremely low profile (as low as 0.6 mm) and can have a lower junction-to-ambient resistance, thereby providing better power dissipation.

The benefits of space savings and cost outweigh the challenges associated with layout, given the small pitch (0.4 mm or 0.5 mm) of such solutions. However, most IC manufacturers design the ball array configuration such that inner balls handle digital, low-current signals that require only narrow traces while the power traces are on the outside, which greatly simplifies the layout of such devices. The high and fast-growing adoption rate of this packaging type in high-volume portable applications demonstrates the maturity and manufacturability of this technology (Fig. 2).

System Safety

The inherent sensitivity of Li-based rechargeable batteries has introduced strict safety requirements for new systems. Many new designs incorporate secondary protection features, thereby adding a new layer of protection outside the battery pack. Many of the new safety requirements are proactively addressing extreme consumer and system behavior by taking into account potential failures in the complete system chain: power source, battery pack and device.

One of the most common failure cases is the use of a faulty, noncompliant or poorly regulated power source (wall adapter, car adapter, etc.). This can lead to overvoltage conditions at the input of the battery-charger IC, and consequently to device failure. A common protection against such failures is the addition of a transient-voltage suppressor, an electrostatic discharge protection diode or another IC that provides such functionality (and protection) between the power source and the system.

Even more significant is the protection against a faulty, stressed or unauthorized battery pack. Each one of these cases can result in a battery overvoltage or overtemperature runaway situation with a high probability of battery explosion. In addition to the battery protection that is located inside the battery pack (for overvoltage, undervoltage and overcurrent protection), it is recommended that additional safety measures be taken. Many systems monitor charging time and battery-pack temperature to ensure stable environmental conditions during the charging process. Furthermore, secondary-battery overvoltage protection for guarding against a protection IC failure is also offered in modern battery-charging solutions.

Some of these protection features, commonly offered by discrete components or additional ICs (see the dotted box in Fig. 3) can be eliminated if integrated in the battery-charging IC. Such integration can result in significant space and cost savings, which also can be seen in Fig. 3.

Battery-Charging Challenges

Battery technology has played a major role in the adoption of handheld equipment in today's society. Nowadays, advancements in the Li-based battery technology allow a 6% capacity increase, on average, per year for a given battery size. This, combined with the fact that newer Blue-tooth SoCs reduce necessary power consumption, allows Bluetooth headsets to use smaller batteries.

Despite this trend, the demand for faster charging is another big differentiating factor for many headsets. Some headset designers are requiring charging rates of 2C or even 3C, a requirement that SoCs with integrated battery chargers cannot accommodate, thereby creating the need for an external battery-charging IC. The C rating for charging is a normalized charging specification based on the fast charge current and battery capacity. Therefore, 2C for a 130-mAh battery translates to a fast charging at 260 mA, and at 3C the fast charge current is 390 mA. These charging rates need to be confirmed with the corresponding battery manufacturer to ensure that they will be safe for the battery and not cause any failures. Fig. 4 demonstrates the charge-time savings by using higher charging rates (i.e., charging currents).

Another consideration for the battery charger is the power dissipation (P_DISS) in the pass element for a linear battery charger such as Summit's SMB139. The linear battery charger regulates the output voltage/battery voltage (V_BATTERY) and battery-charge current (I_{CHARGE CURRENT}) by dissipating the excess power from the input (V_IN) as heat. This describes the power loss of the linear battery charger:

P_DISS = (V_IN - V_BATTERY) I_{CHARGE CURRENT}. (Eq. 1)

Eq. 1 should be used to ensure that the battery-charger IC does not become too hot to cause the battery-charger IC to enter into thermal foldback. This is a condition in which the actual charge current is reduced from the expected value to ensure the IC is not damaged, or that the battery-charger IC does not get hotter than what is desired for comfortable consumer use during charging.

Knowing the package junction-to-ambient resistance (θ_JA), the maximum junction temperature (T_J) and the ambient temperature (T_A), the maximum power dissipation can be calculated as:

Digital control of the battery-charger device (via an I²C or a serial-parallel interface) provides more flexibility in designs that take advantage of higher charge-current rates. The real-time control allows charging to be enabled and well controlled based on certain system and environmental parameters (battery-pack temperature, battery voltage, etc.), making faster battery charging safe and effective.

Portability

While usable battery life for many of the modern electronic equipment has reached acceptable levels, the goal of true portability can only be achieved via innovative system designs that allow a wide range of battery-charging alternatives for the user. An innovative and versatile charging method is enabled by the increasing adoption of the USB On-the-Go standard in new portable consumer devices. While the aim of the On-the-Go standard is to address the need for user friendliness and compatibility by allowing portable devices to be connected to each other without the need for a USB host (most commonly a PC), its power attributes can be used for portable-to-portable device charging.

Such an implementation is shown in Fig. 5. In this case, the cellular phone is the power-providing device and delivers the required 5 V (±5%) and a predetermined current to the connected Bluetooth headset. This power can be used by the headset as the input to its battery charger, allowing its battery to be charged by using power from the cellular phone's battery. Hence, battery charging is not limited by the absence of a USB port (notebook) and/or a wall power source. This implementation addresses a very realistic consumer behavior scenario, given that a high number of cellular phone users also own a Bluetooth headset.