Using Integrated Buck-Boost Chargers for Faster USB Charging & Smaller Universal Charging Solutions

The use of universal battery chargers for lithium-ion (Li-ion) and lithium-polymer (Li-poly) batteries with USB 3.0 power delivery (PD) for on-the-go (OTG) charging is growing in a wide range of applications including drones, smartphones, tablet computers, cordless vacuums, portable medical devices, wireless speakers, and electronic point of sale devices. Across these applications, designers are consistently pushed to reduce charge time and form factor, increase power density, and lower cost.

Buck-boost battery chargers combined with USB PD can enable the development of fast, efficient, universal input charging solutions. But these are not simple devices and designing them to support the USB OTG specification can take a significant amount of time. This adds to cost and can affect design schedules. The design process can be further complicated by the need to adhere to USB fast role swap (FRS) timing and control criteria to ensure that a device that is providing power, can quickly become a power consumer to guarantee an uninterrupted data connection.

For USB PD universal charging applications, designers can address many of these issues by turning to integrated chargers that streamline the design process and support the implementation of full-featured and compact buck-boost charging solutions that deliver high power and fast charging with low parts count and high-power density.

This article briefly considers the need for universal charging based on USB 3.0 and USB Type-C®, and the complexities of implementing buck-boost universal input USB OTG and FRS solutions. It will then review the benefits of using an integrated device, and then introduce an integrated buck-boost charging solution from Texas Instruments with dual input selector and USB PD 3.0 OTG and FRS support. A supporting evaluation module to help designers get started on their next universal input USB PD charger with OTG and FRS capabilities will also be described.

Complexities of universal and OTG charging and FRS

By establishing a standardized connector, USB Type-C has helped enable the development of universal AC power adapters and a reduction in e-waste. But standardized connectors are only one factor. Portable devices have varying numbers of cells in their batteries, and there’s wide variability in adapter power ratings and voltages from 5 to 20 volts. The combination of differing adapter ratings and varying battery voltages means that a USB PD charging solution architecture is complex and challenging (Figure 1).

Figure 1: The internal design of a USB PD charging solution can be complex as it must accommodate widely varying battery cell configurations and adapter voltages. (Image source: Texas Instruments)

First, the USB PD controller (U4) must identify the adapter including; USB battery charging specification revision 1.2 (USB BC1.2), standard downstream port (SDP), charging downstream port (CDP), dedicated charging port (DCP), high voltage dedicated charging port (HVDCP), and even non-standard adapters. Following the communication between the USB PD controller and the adapter, the input power path management and current sensing unit (U1) turns on back-to-back power MOSFETs to connect the input voltage from VBUS to the input of the buck-boost charger (U2). The input power path management unit also senses the input voltage and current through the sensing resistor to support overvoltage and overcurrent protection.

An additional four MOSFETs are in the buck-boost charger unit (U2) to step the input voltage up or down as required by the battery voltage. Another power MOSFET and a current sensing resistor on the output of the buck-boost charger are needed to support the USB PD charger narrow voltage direct current (NVDC) power path management and charging current sensing.

NVDC power path management is a specific control protocol that regulates the system at a voltage slightly higher than the battery voltage, and it does not allow the voltage to drop below the minimum system voltage. The minimum system voltage is the voltage level that allows the system to operate even when the battery is removed or completely discharged. Also, if the system power needs exceed the input adapter rating, a battery supplement mode supports the extra system power requirement and prevents overloading the adapter.

OTG power and FRS

To support OTG power, the DC-DC converter (U3) in Figure 1 is used to discharge the battery to provide a regulated voltage on VBUS to power external devices when the adapter is removed, as required by the USB OTG specification. If FRS is also required, the DC-DC converter must be enabled and held in standby mode continuously, even when there is an adapter connected to the VBUS through the USB Type-C port. If the adapter is disconnected, the back-to-back power MOSFETs connected to the DC-DC converter turn on, and connect the converter’s output to hold up VBUS and enable FRS. A drawback of this approach is that keeping the DC-DC converter in standby increases quiescent current losses for the system.

Integrated 1 to 4 cell buck-boost charger with USB OTG and FRS

As shown, designing a universal USB PD charging solution to support OTG and FRS can be a complex task. For applications that use one to four Li-ion or Li-poly cells, Texas Instruments offers designers the BQ25792RQMR fully-integrated buck-boost charger that supports the full input and output OTG voltage ranges for USB Type-C and USB PD, substantially simplifying the design of a complete USB PD charging solution, including support for FRS (Figure 2). An optional dual input power mux controller can provide support for two different input power sources; a USB Type-C connector at VIN1 and an auxiliary power source at VIN2.

Figure 2: The BQ25792 fully integrated buck-boost charger simplifies the design of a complete USB PD charging solution. (Image source: Texas Instruments)

The BQ25792 supports a wide range of inputs, including:

• A 3.6 to 24-volt input voltage range.
• Detects USB BC1.2, SDP, CDP, DCP, HVDCP, and non-standard adapters.
• Detection of the maximum power point of unknown input sources.

The BQ25792 includes integrated input current sensing that enables the charger to regulate the input current and provide input overcurrent protection to prevent overloading the adapter. In addition, the control and driver circuitry for the external back-to-back power MOSFETs is integrated as part of the input overvoltage and overcurrent protection circuit, replacing the functions of the input power path management and current sensing unit (U1) in Figure 1.

Integration of the four MOSFETs that are in the buck-boost charger unit (U2) in Figure 1 enables the BQ25792 charger to support OTG charging. The charger operates in standard charging mode when the adapter is present. If the adapter is disconnected, the power flow is reversed, going from the battery to VBUS. The BQ25792 is compatible with the full USB PD 3.0 voltage range specification, from 2.8 to 22 volts, programmable in 10 millivolt (mV) steps.

Novel method to support FRS

Support for FRS on the USB Type-C port is implemented through a function called backup mode that eliminates the need for the DC-DC converter (U3) in Figure 1. The BQ25792 supports an ultra-fast switch over for the buck-boost charger section from forward charging mode to reverse OTG mode without the bus voltage dropping out of specification.

Under normal operation, the adapter connects to the BQ25792 through the VIN1 port, charging the battery while providing power to the system and any powered accessories through the PMID output. If the adapter becomes disconnected, the battery can still provide power to the system, but the accessories connected to the PMID pin could lose power.

Using backup mode, the charger constantly monitors the VBUS voltage. When VBUS drops below a threshold level (indicating the loss of the input from the adapter), the charger quickly changes from charging mode to OTG mode, discharging the battery, regulating the VBUS voltage, and implementing FRS, without the need for an additional DC-DC converter. Implementing FRS with the backup power mode in the BQ25792 ensures that any accessories connected to the PMID pin do not lose power when the VBUS voltage drops (Figure 3).

Figure 3: Implementing FRS using the backup power mode in the BQ25792 ensures that any accessories connected to the PMID pin do not lose power when the VBUS voltage drops. (Image source: Texas Instruments)

The BQ25792 offers a choice of a 1.5 megahertz (MHz) or 750 kilohertz (kHz) switching frequency to enable designers to tradeoff solution size and efficiency depending on the needs of the application. Using a 1.5 MHz switching frequency allows the use of small inductor (1 micro Henry (µH)) and capacitor values. Using a 750 kHz switching frequency achieves higher efficiency but results in a larger solution size due to the larger inductor (2.2 µH) and capacitors.

To extend battery life and minimize power loss, the system is powered off during idle, shipping, or storage. In “shipping mode” the I2C is still enabled, but the charger system clock slows down to minimize the device quiescent current. Under normal operation, for battery-only operation, the quiescent current is 21 microamperes (µA). In “shipping mode” the quiescent current drops to 600 nanoamperes (nA).

To help designers get started using the BQ25792, Texas Instruments offers the BQ25792EVM evaluation board (EVB) to implement a synchronous buck-boost charger, delivering up to 5 A of charging current with 10 mA of resolution for 1 to 4 cells (Figure 4). This EVB includes an interface to switch modes between charge and USB OTG. In addition, users can monitor charger status, voltages and currents, and any faults with an integrated analog-to-digital converter (ADC).

Figure 4: The BQ25792EVM evaluation board can be used to implement a synchronous buck-boost charger delivering up to 5 A of charging current. (Image source: Texas Instruments)

Additional features of this EVB include:

• USB auto-detect, USB PD and wireless input, support for inputs from 3.6 to 24 volts
• Dual input source selector to drive bidirectional blocking NFETs
• USB OTG powering with 2.8 to 22-volt output with 10 mV resolution
• Under 1 µA quiescent current in shutdown/shipping mode
• Numerous test points, jumpers, and sense resistors to support voltage and current measurements

Conclusion

As shown, designing a universal USB PD charging solution can be a complex task, and designing chargers to support the USB OTG specification can take a significant amount of time. The design process can be further complicated by the need to adhere to USB fast role swap (FRS) timing and control criteria. The result can be additional cost and compromised design schedules.

To avoid this, integrated buck-boost chargers are available that support the needs portable device designers have for USB PD, OTG charging, adherence to USB FRS timing and control criteria; and reduced charge times for lithium batteries, smaller form factors, increased power densities, lower costs, and faster time to market.

Recommended reading

1. USB Type-C™, USB PD, and USB 3.1 Gen 2: New Standards in Speed and Power