Abstract
A standalone PD controller can help address design challenges like solution size and cost by managing power negotiations without firmware development. This article will briefly show how the assimilation of 5 V, 9 V, 15 V, 20 V, 28 V, 36 V, and 48 V voltage rails provides versatility in power delivery, requiring fewer cables around the house. It then introduces a standalone PD controller that eliminates the need for custom firmware by including port detection and nonvolatile memory.
Introduction
The USB Power Delivery (PD) market continues to grow for portable, battery-operated, electronic devices like cell phones, laptops, wireless speakers, power tools, and much more. USB PD provides a great benefit to consumers because it can provide up to 240 W (in the USB PD Revision 3.1 specification) from the same USB Type-C connector. Figure 1 depicts a cell phone being charged by a USB Type-C connector.
USB PD poses new power requirement challenges because of the variety of voltage and current combinations available—5 V, 9 V, 15 V, 20 V, 28 V, 36 V, 48 V, and 1.5 A, 3 A, 5 A, etc.—to supply the wide range of power the USB PD standard can provide. The power source, such as a wall adapter, and the in-line devices, such as a cell phone, communicate their power capabilities and power needs, respectively, in proper voltage and current levels before the source provides power over the USB cable.
Some solutions require multiple integrated circuits (ICs), including port detectors, microcontrollers, and chargers for power delivery. While these solutions work, they take up space on a board, increase the solution cost, and require custom firmware, which can be time-consuming to create.
A standalone PD controller can help address these challenges by managing the power negotiations without firmware development.
USB-C PD Power Requirements
One of USB PD’s significant benefits is allowing consumers to charge their 2.5 W cell phone and their 25 W cordless power drill using the same cable and power adapter. The days of having drawers filled with different cables or never finding the correct charger will be a thing of the past.
Before we look at USB PD, we must revisit previous USB standards to understand some of the USB PD benefits and challenges. The first USB standards, USB 1.1 and USB 2.0, were for data delivery rather than power delivery. They only allow for a maximum delivery of 5 V and 500 mA across a USB cable.
Over time, consumers began demanding more from USB. They wanted to quickly charge a battery over a USB cable, where a 500 mA maximum current was no longer adequate. The BC1.2 standard answered these consumer demands by allowing the transfer of up to 7.5 W—5 V and 1.5 A—over a USB cable. The BC1.2 standard expands the ability to charge a battery over USB, and each new USB standard after BC1.2 has added to power capacity. Type-C 1.3 extends the power capability to 15 W (max) while USB PD 3.0 upgrades the system wattage to 100 W (max). The most recent specification update, USB PD3.1, extends the power capability even further up to 240 W (max).
BC1.2 and Type-C 1.3 continue to supply the 5 V voltage rail used in all previous versions of the USB standard and have increased the power capabilities to 7.5 W and 15 W, respectively, by increasing the maximum current to 1.5 A and 3 A. USB PD3.0 has also increased both the current and voltage capabilities to reach 100 W (max). It allows two devices to transfer up to 20 V and 5 A across a USB cable. The new PD3.1 specification supports up to 48 V and 5 A.
Figure 2 summarizes the power capabilities, maximum current, and voltage that each USB standard allows.
The voltage rails that a USB PD power source provides are variable. The USB PD 3.1 standard states that not only does a power source have to offer a minimum voltage of 5 V and a maximum voltage of 48 V, but it must also provide a few voltage rails in between.
The USB PD 3.0 standard requires that a power source provides specific voltage rails depending on the power capabilities of the source. Sources that can provide more than 15 W must offer 5 V and 9 V rails. Those that can provide more than 27 W must offer 5 V, 9 V, and 15 V rails. Finally, power sources that can provide more than 45 W must offer 5 V, 9 V, 15 V, and 20 V rails.
The power source also provides different current outputs at each of these voltage rails. A power source with a 5 V rail provides between 500 mA and 3 A at this rail. Those with a 9 V rail transfer currents between 1.67 A and 3 A at 9 V. A power source supplies between 1.8 A and 3 A at the 15 V rail. Finally, the power sources provide between 2.25 A and 5 A at 20 V (Figure 3).
The USB PD 3.1 standard adds three additional voltage rails for power sources. Sources provide fixed voltage rails at 28 V, 36 V, and 48 V to support power levels up to 140 W, 180 W, and 240 W, respectively. A power source must supply up to 5 A for each of these voltage rails.
In addition to the standard voltage and current supplies, the USB PD specification also provides a programmable power supply (PPS) capability. The PPS capability allows in-line devices to request small changes in voltage and current from the power source.
The PPS capability is most useful to speed up the charging of lithium-ion batteries by optimizing the operating point for the switching charger. During the constant current phase of a charging cycle, the charger provides the battery with a fixed current and the battery’s voltage will slowly increase to the final charge termination voltage. Normally, the input to the charger will be fixed, which creates power losses when the input to the charger is much larger than the battery voltage. The PPS feature adjusts the input voltage of the charger such that it operates near its peak efficiency. With the resulting lower power dissipation, the battery can be charged faster with an increased charging current.
PPS allows for countless voltage and current combinations along a USB cable. Designers who want to use the PPS capability must find a way for a power source and an in-line device to agree on how much power the source should provide.
USB-C PD Design Blocks
It is no small task for charging to begin under a discrete USB PD system. A power source, such as a wall adapter, connects to the in-line device, such as a phone or power drill, through a USB cable. Both devices typically need multiple ICs to implement the back-and-forth communication to get the power source ready to provide the in-line device power (Figure 4).
The CC pin detection IC identifies the cable orientation and source current capability by measuring the voltage of the CC pins. This IC also requests the power source’s voltage and current capabilities and communicates back to the power source when the in-line device selects a voltage and current.
The BC1.2 detection IC supports legacy USB adapters. Although newer devices are more widely adopting USB Type-C, many applications still use older USB specifications. BC1.2-compatible ports have D+/D– pins instead of CC pins to communicate a power source’s power capabilities. The BC1.2 detection IC reads the D+/D– pins to configure charging for applications still using legacy USB standards.
The charger IC safely and effectively charges the battery on the in-line device. The power source will provide a constant voltage to the in-line device, the charger’s input source. The charger will then ensure the battery is charged to the battery’s voltage, current, and temperature specifications.
Finally, the microcontroller unit (MCU) block organizes communication between the other ICs. The MCU communicates with the CC pin detection IC to determine the power capability of the power source. Then, the MCU compares the power source’s capability with the charger and battery’s power needs to determine how much current and voltage the power source should provide. The MCU communicates the final power settings back to the CC pin detection IC to configure the power source correctly. Once the correct current and voltage are confirmed, the MCU will configure and enable the charger.
USB PD requires more elements than legacy USB or standard Type-C designs provide. More ICs lead to higher costs and larger solution sizes. It also requires a complex firmware design to manage the communication between the different elements and meet all the USB PD 3.0 standard requirements. The firmware design alone can create longer development cycles unless a designer has an intimate knowledge of USB specifications.
Standalone PD Controller
Standalone PD controllers can help simplify USB PD designs by having the CC pin detection, BC1.2 detection, and MCU integrated into one IC. The four-IC design now becomes just two, which saves board space and costs.
The most powerful element integrated into the standalone PD controller is the embedded MCU, which integrates all the USB PD 3.0 standard communication protocol and timing requirements. The designer no longer needs to spend development time coming up-to-speed on these specifications.
One example of a standalone PD controller is the MAX77958 (Figure 5). Two unique features of the MAX77958 are the nonvolatile memory and the I2C main port that directly controls a companion charger. Both features help eliminate the need for an external MCU and custom firmware development.
Designers can generate customized scripts for typical applications using a graphical user interface (GUI) and then load them into the nonvolatile memory of the IC. The PD controller automatically executes commands, such as toggling GPIO or sending an I2C command to the charger through the I2C main port.
The customization script is written in the GUI using simple, user-friendly commands. The software translates the customization script to hexadecimal format and writes it to the IC configuration area. Developers can define simple functions and sequences based on the functionality they need for their application.
Figure 6 shows some of the functions a designer can use for programming the customization script. The GUI outputs a binary (bin) and hexadecimal (hex) file based on the customization script. The customization scripts are a unique feature that drastically reduces development time.
Conclusion
The USB PD specification dramatically expands the number of battery-operated devices charged through a USB cable. The specification outlines seven new voltage rail requirements, 5 V, 9 V, 15 V, 20 V, 28 V, 36 V, and 48 V, to help accommodate the wide range of power capabilities. Power sources and in-line devices now need to negotiate a current and voltage level before charging can begin.
Standalone PD controllers integrate most blocks into one IC, which helps simplify the design process. Some even eliminate the need for an external MCU and customized firmware. Standalone PD controllers help accelerate your design development to ensure you are staying ahead of the latest trends in USB PD.
References
“Designing in USB Type-C and Using Power Delivery for Rapid Charging.” Digi-Key, March 2017.
Dunstan, Bob and Richard Petrie. “USB Power Delivery.” USB Implementers Forum, November 2019.
“USB-C Fundamentals.” Analog Devices, Inc., 2020.
“USB Power Delivery.” USB.
“USB Power Delivery: Convenience and Safety.” Renesas.
