Original Title：Use a USB-C Charging Controller to Rapidly Implement Fast Charging Without Firmware

　　The trend toward larger displays, increased performance, and higher data throughput in 5G smartphones drives a need for larger battery capacity with fast charging capability. The challenge for designers is to move beyond conventional charging methods that introduce inefficiencies that can result in overheating at the power levels required to meet increasingly demanding consumer expectations for rapid charging.

　　The introduction of the programmable power supply (PPS) capability in USB Type-C® (USB-C) power delivery (PD) 3.0 helps provide an effective solution, but the required firmware development can still stall product delivery.

　　This article describes the problems associated with fast-charging 5G phones and how USB-C PD 3.0 PPS can help designers efficiently meet requirements for ever-faster charging of larger batteries. It then introduces and shows how developers can use a highly integrated ON Semiconductor USB-C controller that implements USB-C PD 3.0 PPS in a finite state machine (FSM). This eliminates the need for firmware development, thereby accelerating implementation of fast charging for next-generation chargers.

　　More powerful smartphones bring new challenges for fast charging adaptors

　　5G smartphones are expected to account for over 50% of total smartphone shipments by 2023, according to market analysts. In using these phones to take advantage of 5G services, however, users will find that the existing base of phone chargers and charging stations will be a poor match to the fast charging requirements of this new generation of smartphones.

　　As already seen in 5G phones such as the Samsung S20 Ultra 5G, these sophisticated devices offer larger displays, as well as increased processing capability and far greater data throughput than available with earlier generation phones. To match their larger displays and correspondingly higher power consumption, available 5G phones already feature larger batteries. For example, the Samsung S20 Ultra 5G features a 6.9-inch display and incorporates a 5,000 milliamp hour (mAh) battery—25% larger capacity than the previous model.

　　While consumers expect the longer battery life available with larger capacity batteries, they also expect that charge times will become even shorter—rather than 25% longer. For manufacturers looking to meet the growing demand for charging stations in vehicles, homes, and offices, the need to decrease charge time for higher capacity batteries becomes a significant challenge in the face of limitations in the batteries themselves.

　　Manufacturers of lithium-ion (Li-ion) batteries specify strict thresholds for charging current and voltage. A conventional lithium-ion battery rated at 1,000 mAh is typically rated for a 0.7 C charge rate, or 700 mA charging current. Applied to a fully depleted 5,000 mAh battery, a 0.7 C charge rate (or 3,500 mA charging current) would require about 45 minutes just to reach a 50% state of charge.

　　More advanced battery cell technologies can support charge rates greater than 1 C, but both charger and charged device need to accommodate dramatically higher power levels. For example, a 5,000 mAh battery charged at a higher 1.5 C rate would only need about 22 minutes to charge from 0% to 50%, but the 7.5 ampere (A) charging current could stress components and generate excessive thermal load even in highly efficient charging systems. In fact, with the broad acceptance of USB-C as the industry-standard interface for power and other functionality, a compatible charger would be limited in the maximum current it could deliver across a USB-C cable. The maximum current is 5 A for USB-C cables containing an emarker IC that provides cable information to connected devices. (For non-emarker cables, the maximum current is 3 A).

　　Mobile device manufacturers can of course overcome this limitation by inserting a charge pump between the supply input and battery charging circuit. To support a 7.5 A charging system, for example, the travel adaptor could deliver 10 volts at 4 A, relying on a typical divide-by-two charge pump to output 5 volts at about 8 A to the charging circuit. This approach enables a travel adaptor to increase the USB-C voltage (VBUS) while maintaining a USB-C-compatible current level.

　　Increased charging power requires more effective control

　　Support for VBUS levels greater than 5 volts has enabled the use of this high voltage, low current approach. The USB PD 2.0 specification defines a series of fixed power delivery objects (PDOs) that specify combinations of fixed voltage levels (5, 9, 15, and 20 volts) and currents (3 or 5 A).

　　Although USB PD 2.0 fixed PDOs enable higher charging power, setting the charge voltage and current at fixed levels that are too high or too low can result in inefficient charging, unacceptable thermal loads, and stress on components. In practice, charging circuits operate at optimal efficiency when their input voltage (supplied by USB-C VBUS) is slightly above their output voltage (battery voltage). Because battery voltage continually changes during normal operation, however, maintaining this point of optimum charging efficiency is a challenge. As the battery discharges, the difference between battery voltage and USB-C charging voltage (VBUS) will widen, lowering charging efficiency. Conversely, as the battery becomes charged, the charging circuit will need to reduce charging current to protect the battery.

　　Without the ability to directly reduce the charging levels supplied by the travel adaptor, power dissipation will increase, lowering efficiency and generating heat. As a result, the optimal charging level continually changes, often in incremental amounts, requiring a corresponding incremental level of control on charging voltage and current to achieve maximum efficiency.

　　How USB-C PD 3.0 PPS enhances efficiency

　　Designed to address the growing need for more efficient charging at higher charging power, the USB-C PD 3.0 PPS capability allows the device being charged (sink) to request the charger (source) to increase or decrease the charging voltage and current in mV and mA steps advertised in augmented PDOs. Using this capability, a sink can tune its source voltage and current to optimize charging efficiency.

　　The introduction of PPS dramatically shifts the way the charging process operates. In the past, the source charger both controlled and executed the charging algorithm. With PPS, control of the charging algorithm shifts to the sink, requiring that the source execute the algorithm as directed by the sink.

　　With PPS, a smartphone or other sink communicates with a charging source to optimize power delivery, arriving at a mutually agreeable PD “contract” through a negotiation protocol involving a brief interchange as follows:

　　The source discovers if the connecting cable is 5 A capable

　　The source advertises its source voltage and current capabilities described in as many as seven PDOs

　　The sink requests one of the advertised PDOs

　　The source accepts the requested PDO

　　The source provides power at the agreed-upon voltage and current levels

　　Advanced mobile devices like the Samsung 5G phone mentioned earlier use this capability to provide fast charging using compatible chargers. For manufacturers designing fast charging travel adaptors and building charging stations into other products, implementing this type of charging protocol would typically require development of controller firmware able to execute the protocol and operate associated power devices. For a well-established standard like USB-C PD PPS, however, an FSM solution offers an effective alternative, eliminating the need for firmware development that can delay final product delivery. Using an FSM implementation of USB-C PD 3.0 including PPS, ON Semiconductor’s FUSB3307 adaptive source charging controller speeds development of chargers able to meet the fast charging requirements of next-generation smartphones and other mobile devices with high capacity batteries.

　　Integrated controller for USB-C PD 3.0 compliant fast chargers

　　ON Semiconductor’s FUSB3307 is an integrated power source controller that enables the implementation of USB-C PD 3.0 PPS without the need for an external processor. Along with cable detection, load gate driver, multiple protection features, and constant voltage (CV) and constant current (CC) regulation, the device integrates the complete PD 3.0 Device Policy Manager, Policy Engine, Protocol, and PHY layers in hardware.

　　Designed to support both AC/DC and DC/DC chargers, the FUSB3307 can provide a full set of responses appropriate to a PD power source. As a result, designers can implement a USB-C PD 3.0 compatible supply source with the FUSB3307 and relatively few additional devices and components.

　　When connected to a sink, the FUSB3307 will automatically detect the capabilities of the sink device and connecting cable and will advertise its capabilities in accordance with USB-C specifications. When the sink responds with the selection of a supported PDO, the FUSB3307 will enable VBUS and control power circuits to ensure the requested charging voltage and current levels are delivered to the sink.

　　Because the FUSB3307 integrates a full set of control functionality, the fundamental principles of operation remain conceptually the same for both AC/DC and DC/DC charger design. In response to commands from the sink, the FUSB3307 in the source uses its CATH output pin to drive a feedback control signal to the source's power stage. During charging operations, the FUSB3307 monitors charging voltage using its VFB pin and charging current detected across a sensing resistor using its IS+/IS- pins. These monitored levels in turn feed into internal voltage and current loop error circuits tied to the voltage (VFB) and current (IFB) pins. These signals in turn work to control the CATH pin for CV and CC control. Other pins in the FUSB3307’s 14-pin small outline integrated circuit (SOIC) package support the load gate driver, USB-C connector interface, and protection features.

　　FUSB3307 source controller simplifies charger design

　　Designs for each type of charger will of course use different configurations for the primary CATH output, VFB input, and other pins. In an AC/DC wall charger or AC/DC adaptor, the FUSB3307 would monitor voltage and current on the secondary side and drive control feedback to the primary side (Figure 1).

　　Figure 1: In an AC/DC design for a wall charger or adaptor, the FUSB3307 responds to commands from a sink device for different charging voltages by controlling the PWM controller through an isolating optocoupler. (Image source: ON Semiconductor)

　　In this charge design, the FUSB3307 CATH output pin would typically connect to an optocoupler cathode on the secondary side to deliver a feedback control signal to a primary side pulse width modulation (PWM) controller, such as the ON Semiconductor NCP1568. On the secondary side, the FUSB3307 voltage and current sense inputs would monitor the output from a synchronous rectifier controller, such as the ON Semiconductor NCP4308.

　　In a DC/DC charger design used in an automotive application, for example, the FUSB3307 directly controls the DC/DC controller. Here, the FUSB3307 CATH feedback signal is connected to the compensation (COMP) pin of a DC/DC controller such as the ON Semiconductor NCV81599 (Figure 2).

　　Figure 2: In a DC/DC charger design for a car charger, the FUSB3307 directly controls the voltage output of a DC/DC controller, raising or lowering the output as commanded by a sink, such as a 5G phone or other mobile devices. (Image source: ON Semiconductor)

　　ON Semiconductor implements this specific DC/DC charger design in its FUSB3307MX-PPS-GEVB evaluation board for the FUSB3307. Designed to operate from a single DC power supply, the board provides a complete charging source compliant with USB PD 3.0 with PPS, delivering 5 A current (max) at VBUS levels from the standard’s minimum 3.3 volts to its maximum 21 volts.

　　The evaluation board enables developers to explore FUSB3307 interaction with USB PD 3.0 compliant devices, as well as legacy USB PD 2.0 devices. Developers can immediately begin exploring the fast charging process by monitoring VBUS voltage and current delivered by the board to a USB-C PD capable device such as a laptop or smartphone.

　　This approach offers particular insight into the FUSB3307’s ability to interact with an off-the-shelf USB PD 3.0 5G phone, as well as the phone’s use of the USB PD 3.0 PPS protocol to optimize its charging voltage and current. In one demonstration of this capability [1], an off-the-shelf Samsung S20 Ultra 5G is found to issue a series of commands to the FUSB3307MX-PPS-GEVB evaluation board to modify the charging voltage and current in both large and small steps (Figure 3).

　　Figure 3: The ON Semiconductor FUSB3307MX-PPS-GEVB evaluation board demonstrates the ability of the FUSB3307 to respond to an off-the-shelf 5G phone’s commands to fine-tune its charging voltage and current. (Image source: ON Semiconductor)

　　After the board and phone are connected in this demonstration, the 5G phone selects the baseline PDO (5.00 volts and max 5.00 A) as shown in the first 10 seconds of the figure. In this phase, the charging voltage (VBUS) is 5 volts and the 5G phone is sinking about 2 A charging current (IBUS). The 5G phone then requests an augmented PDO that declares the source’s ability to supply 8 volts at 4 A. The FUSB3307 complies with the request and the change is immediate: VBUS jumps to 8 volts as requested and IBUS shows a gradual increase as the 5G phone ramps the increased IBUS current.

　　After this sharp jump in VBUS, the incremental increases in charging power possible with PPS become evident. The 5G phone requests a 40 millivolt (mV) increase in VBUS about every 210 milliseconds (ms), gradually ramping VBUS to even higher levels. When IBUS reaches 4 A (dashed green line in the figure), the FUSB3307 uses the standard PPS protocol to issue an alert message notifying the 5G phone that the requested current limit has been reached. The 5G phone continues to issue requests for further increases in VBUS in 40 mV increments, eventually reaching 9.8 volts. In everyday use, this sort of adaptive source charging capability can achieve maximum charging efficiency required for fast charging without overheating or otherwise compromising the sink device.

　　Using the ON Semiconductor FUSB3307MX-PPS-GEVB evaluation board, developers can immediately explore the use of USB-C PD in existing devices and extend the board’s associated reference design to implement custom fast charging in units compliant with USB PD 3.0. Best of all, implementation requires no firmware development. With the FUSB3307, developers use familiar power supply techniques to build adaptors capable of taking full advantage of the fast charging capabilities of next-generation 5G phones and other compatible devices.

　　Conclusion

　　While 5G phones bring a wealth of new features and capabilities to users, the larger capacity batteries needed to support these devices also challenge designers. In particular, they need to ensure that travel adaptors and charging stations deliver fast charging without overheating the phone.

　　With its fully compliant USB PD 3.0 PPS capabilities—and no need for firmware development—ON Semiconductor’s FUSB3307 adaptive charging controller offers an immediate design solution. By using this controller in combination with familiar power supply devices and components, developers can quickly implement adaptors capable of supporting a rapidly expanding base of USB PD 3.0 capable 5G phones and other mobile devices.

　　Reference

　　Convergence of 5G, Fast Charging and USB-C Programmable Power Supplies