How to achieve fast charging with USB PD in portable devices

Article By : Hank Cao

A reference design comprising charger and controller ICs helps create fast charging solution complying with the USB Type-C and PD...

Today’s consumers want to conveniently and quickly charge their portable devices anywhere, whether in a business center or shopping mall or waiting for a flight or train. However, there are many different types of charging adapters and connectors, which can make determining which charger or chargers to have on hand confusing for the end user. If the consumer has to carry multiple different charging adapters for different portable devices—such as phones, tablets, and laptops—it can also be an overwhelming experience (Figure 1).

Figure 1 A variety of input sources and adapters can overwhelm end users. Source: Monolithic Power Systems

USB Type-C connectors offer numerous benefits compared to the connectors shown above: barrel jack, USB Type-A, and Micro-B. The benefits include a reversible plug, higher power when using USB Type-C power delivery (up to 100 W), dual-role power capabilities (can act as a sink or source), and higher data speeds.

These benefits are rapidly driving consolidation toward using USB Type-C connectors in more consumer devices. It has the added benefit of drastically reducing e-waste by enabling consumers to reuse charging adapters and cables.

Figure 2 USB Type-C connectors may soon become the universally accepted standard. Source: Monolithic Power Systems

When considering portable devices, the higher power capability of the USB Type-C connector is very important because it means the device’s battery can be charged much faster. The previous micro-USB connector used by many portable devices allowed for a maximum power of 7.5 W (5 V with 1.5 A). With USB Type-C, portable devices can charge their batteries at twice this power with up to 15 W (5 V with 3 A).

However, even though a standard USB Type-C power supply can provide up to 15 W, the end user will rarely be able to fully utilize 15 W with a 5V power supply. This is due to two main factors:

  1. Type-C cable can have a maximum 250 mΩ round-trip resistance, and the device’s PCB and connector typically add about 100 mΩ of resistance in series with the cable. This means a 15-W supply providing 3 A at 5 V may only deliver up to 11.85 W (15 W – 3.15 W) of power to the charger due to the IR drop.
  2. Since the fully-charged voltage of a single-cell battery keeps increasing year after year, a 5-V supply creates a “headroom limitation” problem for the charger IC. Consider a device where the battery is in the constant voltage phase. It charges at 4.4 V with a 5-V USB Type-C power supply capable of 15 W, and has the series resistance mentioned in the first example (350 mΩ). In this case, only a 0.6 V drop can exist between the power supply’s output and the battery’s input terminal. This means the maximum available current is about 1.7 A (0.6 V/0.350 Ω), which equates to about 7.5 W of power.

To overcome this limitation, either the total series resistance can be decreased, or the power supply’s output voltage can be increased. Since the series resistance is difficult to reduce, USB power delivery (USB PD) provides a method to increase the power supply’s output voltage to up to 20 V in order to deliver up to 100 W of power.

Higher power with USB PD

Even with the limitations mentioned above, standard 15-W USB Type-C power supplies are sufficient for many portable devices. However, devices such as smartphones, tablets, wireless speakers, power banks, and cameras usually have batteries larger than 3000 mAh, and can therefore benefit from faster charging by using USB PD. To provide the higher power output, a USB PD adapter can increase its output voltage above 5 V in accordance with a command from the portable device. The maximum power a USB PD adapter can deliver is usually between 15 W and 100 W, depending on the voltage and current levels.

Figure 3 shows USB PD source power rules, which are outlined in the USB PD standards to ensure compatibility among different PD adapters (sources) and devices (sinks). The source output voltage must be at a 5-V, 9-V, 15-V, or 20-V discrete level depending on its power rating, while also allowing for a maximum current of 3 A. It is also possible to increase the maximum current level to up to 5 A, but this requires a special, electrically-marked cable to ensure safety.

Figure 3 The source power rules are outlined in the USB PD standard. Source: Monolithic Power Systems

USB Type-C power delivery takes advantage of the Type-C connector’s ability to provide higher power by increasing the output voltage, while still being backwards compatible with 5 V to enable universal charging. For example, if the sink device such as a speaker only requires 18 W of power (9 V at 2 A), then any compliant USB PD adapter rated for at least 27 W is guaranteed to meet this need based on the power rules.

This means end users only need to carry one USB PD power supply for all of their devices, which is more convenient and cost-effective. For manufacturers, this means it’s no longer necessary to bundle a power adapter in the box with their product, which also reduces cost and helps the environment by preventing e-waste.

To achieve USB Type-C PD fast charging, a PD controller IC is needed in both the portable device and power supply. The PD controller communicates with the PD adapter to determine how much power is needed by the portable device. The PD controller then checks the loading statuses and safety fault information in the portable device.

In addition, a PD controller that supports dual-role port (DRP) allows the consumer to charge other devices from their portable devices, such as charging a phone from a laptop. The portable device then functions like a power bank for added convenience.

The charger IC is an important component to implement a successful USB Type-C PD fast charging system. Design engineers looking for fast charging solutions can:

  • Simplify their designs while enabling universal charging—the ability to charge multiple battery-powered devices with different charging configurations and high input voltages.
  • Extend battery runtimes and use the maximum battery capacity to enable the best possible consumer experience.
  • Improve charge efficiency to minimize power loss.
  • Protect the battery, system, and input adapter from failures.

USB PD reference design

A USB PD fast charging reference design includes a high-current switching charger, paired with a USB PD controller to create a turnkey solution (Figure 4).

Figure 4 An illustration of a USB PD fast charging solution comprising charger and controller devices. Source: Monolithic Power Systems

The USB PD controller used in this design is compliant with the latest USB Type-C and PD standards. Controllers such as this that provide full functionality and bill of materials (BOM) integration advantages are ideal for portable devices.

MP2731, an integrated buck charger IC, leverages USB PD input in charging a single-cell battery across an input voltage ranging from 3.7 V to 16 V. With integrated components, this buck charger gives the flexibility to charge compact, size-constrained, battery-powered devices like speakers, cameras, point-of-sale (PoS) systems, and more.

It also provides an I2C interface to program charging configurations flexibly, and monitor the charging status and fault. An ultra-low power consumption charger IC extends the battery’s runtime during operation to conserve as much battery power as possible when the application is not in use. To support the DRP of a Type-C connector, the IC can discharge the battery to build a regulated 5 V up to 3 A at its input. In this mode, the Type-C port is a source port that can power external devices like smartphones.

The charging process and USB On-the-Go (OTG) operation can be monitored by the integrated 8-bit analog-to-digital converter (ADC). Safety features including a fast-charging safety timer, battery temperature monitoring, over-voltage protection (OVP), and over-current protection (OCP) are also critical for battery charging and system operations.

To handle PD fast charging, the charger IC reduces the RDS(ON) of the integrated FETs inside the switching charger to improve charger efficiency at high currents. Figure 5 shows charging efficiency curves for the PD fast charging solution, which has a charging efficiency of up to 91% at a 3-A charging current with 9-V input

Figure 5 A view of the charging efficiency enabled by the charging IC in a USB PD fast charging solution. Source: Monolithic Power Systems

Fast charging capability can support large-capacity batteries, allowing portable devices to support more and more features. However, portable devices typically need to be as small as possible, which drives the need for a fully-integrated switching charger.

— Hank Cao is senior applications engineer at Monolithic Power Systems.

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