Buck-Boost Chargers Extend Battery Life

Texas Instruments (TI) has introduced new buck-boost battery charger integrated circuits that integrate proper power management, offering maximum power density and universal and fast charging up to 97% efficiency.

The new BQ25790 and BQ25792 solutions support low quiescent current and offer the flexibility to charge batteries from one to four cells in a series and up to 5A of charging current over the entire input voltage range (3.6V to 24V) for USB Type-C, USB PD and wireless applications. These new solutions will be used in small personal electronic equipment, portable medical devices, and building automation applications.

“USB-Type-C / PD adapters are gaining popularity and are becoming a universal charging standard. The high-power density performance of BQ25790 and BQ25792 can help design engineers leverage increased input power from USB-PD and provide flexibility for different (1-4s) battery configurations,” according to Samuel Wong, product line manager, battery management solutions at Texas Instruments.

Fast charging
Limited battery life compromises the autonomy of smartphone users.  Given their limited size, the newest smartphone batteries are remarkable, but no matter how remarkable they are, their size is still limited. Manufacturers try to make phones increasingly more energy efficient, but those phones keep getting used for increasingly power-hungry applications. The result is that often your battery won’t last until the evening if charged just once.

The fast recharge allows you to quickly restore the battery life of the device using some technical tricks. Battery chargers for fast charging also act on the voltage value so as to significantly increase the power output.

The charging standards are a complicated mix of chemistry and physics, and since each one has its limits, incompatibility itself could be a problem. Smartphone batteries recharge when a current flows through them. Higher current and higher voltages recharge batteries faster, but there’s a limit to what they can take. The charge regulator (IC) protects against dangerous current surges by regulating the overall flow of electricity into and out of the battery.

USB Type-C Power Delivery (USB PD) offers a useful alternative for fast and efficient charging in a wide range of applications. The output voltage range of USB PD is adjustable for different battery-powered devices with different battery configurations to utilize 5 W to 100 W (20 V / 5 A) power spectrum of USB PD.

TI’s buck-boost ICs
Universal charging allows charging from car adapters or USB-PD adapters, bringing a new level of flexibility and convenience to a wide range of devices, especially in the medical field. On-the-go charging (OTG) is supported by bi-directional charging.

To meet consumers’ expectations, designers are looking for solutions that extend battery runtime and utilize maximum battery capacity while efficiently charging and reducing heat dissipation to minimize power loss within the charger IC.

The new TI family of integrated buck-boost charger ICs, including the BQ25790 and BQ25792, uses the USB PD input for greater flexibility when charging 1S – 4S batteries in the 3.6V to 24V input voltage range. The devices use a very low-power-consumption charging IC to extend battery runtime during operation and to save as much battery power as possible when the application is not in use. In addition to the ultra-low power consumption, chargers are equipped with a top-off timer that allows additional charging above a normal charge cycle, allowing the battery a maximum capacity charging.

The BQ25790 and BQ25792 multicell buck-boost chargers with less than 1 µA of quiescent current. Combined with an extremely low battery FET resistor of 8 mΩ, engineers can further maximize battery operating time for applications requiring long periods of operation (Figure 1).

Figure 1: BQ25790 block diagram [Source: TI]

“There are two primary challenges for quiescent current in portable electronic design. The first type of quiescent current is shutdown mode for long shelf life during equipment transportation and storage, said Samuel Wong. He continued, “The BQ25790 and BQ25792 provide a shutdown mode as low as 0.6uA. The shutdown mode not only minimizes IC current consumption, it also cuts off the system completely from the battery to avoid system power consumption. This allows the battery to maintain full capacity for months. The second type of quiescent current is battery mode which is used when a battery is used to power a system. In order to provide a long battery run time, these buck-boost battery chargers are designed to minimize the current consumed by the IC itself.”

The new buck-boost chargers fully integrate the following components: metal oxide field effect semiconductor switching transistors (MOSFETs), a battery FET, current sensing circuits, and a dual input selector switch. The reduction in the number of components is particularly important for applications such as smart speakers, which are smaller in size and have lower prices as market adoption increases.

“There are always trade-offs for component integrations in terms of area, cost and performance. The BQ25790 and BQ25792 integrate four buck boost converter MOSFETs, a battery FET, and two current sense resistors within a <10mm2 (WCSP) or 16mm² (QFN) package, respectively. This dramatically shrinks the solution size by over 50% to pack double the power density in the PCB footprint. With our high efficiency power converter and innovative packaging, the chargers support up-to 155mW/mm². The small integrated solution not only increases power density, it also provides a better user experience with simple design and minimum BOM,” said Samuel Wong.

The buck-boost charger has become increasingly popular in recent years, due to its ability to charge a battery from almost any input source and meet the demands of Type-C and PD USB adapters. A critical advantage of the widespread adoption of USB Type-C is a realistic path to a universal adapter and the corresponding reduction in electronic waste. High power density buck-boost chargers should integrate not only general functional charging blocks but also others such as DC/DC converters to simplify system design. Figure 2 shows a system block diagram for a USB PD charging solution.

Figure 2: Block diagram for a USB PD charging solution [Source: TI’s article]

The DC/DC converter discharges the battery to create a VBUS regulated voltage and power external devices when the adapter is not present. In the absence of the adapter, back-to-back MOSFETs in the discharge power path will switch on, switching the output voltage U3 to VBUS and maintaining the VBUS voltage. This keeps the DC/DC converter always on. As part of the input overvoltage and overcurrent protection circuit, the control logic and a driver circuit for the external back-to-back MOSFETs are also integrated in the charger.

Figure 3: The fully integrated buck-boost charge  [Source: TI’s article]

The fully integrated buck-boost charger shown in Figure 3 can simplify the system-wide design of a USB PD charging solution. As part of the input overvoltage and overcurrent protection circuit, the control logic and driver circuitry for external back-to-back MOSFETs are also integrated into the charger. These features eliminate the unit that supports input power path management and input current detection from the block diagram.

Figure 4: USB Type-C FRS realized by single buck-boost charger [Source: TI’s article]

Figure 5: Buck-boost charger FRS from VBUS sink to VBUS source [Source: TI’s article]

In order to support FRS for the Type-C USB port, this integrated buck-boost charger implements a new backup mode that can monitor the VBUS voltage; the VBUS voltage dropping below the preset threshold indicates the removal of the adapter. Fast Role Swap (FRS) is a great new feature of the USB PD 3.0, where a device that is providing power can quickly change its power role to become an energy consumer in order to maintain a consistent data connection. FRS helps prevent any data loss that can occur when power is unexpectedly removed from a device (Figures 4 and 5).

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