ALLPCB
Overview
More laptops, phones, and PCs are adopting USB-C as a compact, multifunctional connector for bidirectional data transfer and power delivery. With USB-C, consumers can use a single cable for charging, data transfer, and connection to displays or other peripherals. The standard provides sufficient power for a range of portable electronics, from power banks to personal computers and monitors. Designing USB-C charging circuits requires a distinct combination of skills compared with traditional USB variants. This article discusses how buck chargers can simplify USB-C charging system design.
Adoption and capabilities
From wearables to building automation and medical devices, more compact, lithium-ion battery-powered electronics are adopting USB-C. The widespread use of lithium-ion batteries in consumer devices has encouraged USB-C adoption because these devices require power conversion. Devices with at least one USB-C port were expected to grow substantially between 2016 and 2021, with notable adopters including phones, mobile PCs, flash drives, media tablets, and docks.
USB-C is positioned as a universal standard because it replaces a variety of cables and simplifies fast charging, content streaming, and data transfer. The reversible USB-C connector has 24 pins, and USB 3.1 Gen 2 (10 Gbps) is the default protocol for many USB-C implementations. For higher throughput, Thunderbolt 3 over USB-C raises bandwidth to 40 Gbps and can support up to 100 W of power, enabling a single cable to power and transfer large volumes of data to complex devices.
From a design perspective, USB-C enables smaller and thinner devices. However, while the standard simplifies the end user experience, charging circuit design for USB-C introduces new challenges.
What makes USB-C design challenging?
The USB communication protocol is relatively complex, which can make implementation time-consuming. Options to integrate USB communication into a design include a microcontroller with custom USB firmware or a fixed-function communication bridge. Designing to the latest USB-C standard presents unique challenges: signal integrity and speed concerns for embedded designs, bidirectional power flow up to 100 W, and compatibility with many legacy interfaces such as USB 1.1/2.0/3.0, HDMI, Ethernet, DisplayPort, power, and audio.
Many available chargers lack built-in USB-C port control. When a USB-C power source is connected, charging does not start automatically until the port controller has identified the adapter. Therefore, the charger and port controller must communicate. Detecting, reading, and processing the digital signals between the charger and a controller requires host-side software development. Designers must ensure their systems handle a variety of USB-C and legacy USB adapters and correctly manage charger input current limits based on the source capabilities detected by the port controller. Setting the input current limit allows a charger to draw the source's full capability to charge a battery faster, which typically requires software development on the host application processor or microcontroller.
USB-C alone supports up to 5 V at 3 A, sufficient for many applications under 15 W. For a source adapter to present a 5 V Vbus, the USB-C port controller must perform end-to-end detection; otherwise the Vbus is 0 V until negotiation occurs, which differs from traditional USB. USB Power Delivery (USB PD) supports power transfer up to 100 W with multiple voltage levels up to 20 V at 5 A, but this requires more complex USB-C power microcontrollers, and many applications do not need that level of power.
Solution size is another important consideration. Although the USB-C connector is larger than some previous small-form connectors, battery-powered consumer devices are shrinking, and USB-C reduces the number of ports needed on a device. As a result, USB-C charging subsystems increasingly need to meet tight size constraints.
Using integrated USB-C buck chargers to simplify design
For applications under 15 W, Maxim offers a USB-C buck charger IC that integrates a USB-C port controller and charging regulator, reducing the need for a separate port controller IC, minimizing host software development, and lowering BOM cost. The MAX77860 is a USB-C 3 A switching charger that integrates a USB-C port controller and charger IC for 15 W applications. It also includes integrated Configuration Channel (CC) detection, allowing automatic USB port connection detection required for presenting Vbus. The CC pins detect cable connection and orientation and can be used for USB Power Delivery and alternate modes such as DisplayPort and HDMI.
With integrated CC detection, charging can start without host intervention. The device supports 3 A charging current and uses a high switching frequency (2 MHz/4 MHz), enabling a compact 3.9 mm x 4.0 mm package and the use of relatively small inductors and capacitors. Its high-efficiency buck conversion reduces heat generation during charging.
The MAX77860 supports backward compatibility with traditional USB BC1.2 adapters. An integrated six-channel ADC provides accurate voltage and current measurements while freeing host application processor or microcontroller resources. The on-chip port controller provides plug detection, cable orientation detection, power and data role detection, and Vbus source capability discovery. A 5.1 V/1.5 A reverse boost OTG function supports USB On-The-Go mode for accessory devices.
Summary
USB-C connectors are common in consumer electronics because they provide compact, multifunctional cabling for bidirectional power and data transfer. Designers familiar with legacy communication protocols face additional complexity when implementing USB-C, particularly around voltage management, efficiency, and solution size. Buck chargers that integrate USB-C port control and charging functions and that are optimized for these challenges can simplify the design process for a range of portable products.
