Existing USB standards allow a maximum charging current of 500mA (USB 1.0, 2.0) or 900mA (USB 3.0), with fast charging limited to 1.5A. To ensure safe power transfer from charging devices, a fixed voltage can be applied to the data transfer lines D+ and D-, or a resistor can be installed between these two data line connections to inform the portable device that it is connected to a dedicated charging port (DCP). Unfortunately, different manufacturers have different specifications, so there is no universal solution (Figure 1).
Figure 1: Various DCP resistor configurations from different portable device manufacturers
To avoid confusion, the USB 3.1 standard added specifications for battery charging, allowing portable devices to communicate with chargers to set variable output voltage, current, and power limits to suit the charging capabilities of various portable devices and connector models (Type A, Type B, Micro Type B, or Type C).
Even if the portable device is equipped with a USB-C connector, it can still be charged with an older charger because the USB standard is backward compatible, although it will take longer to charge compared to a dedicated USB 3.1 charger.
Use a RECOM adapter as a USB charger
A USB connector with its D- and D+ pins shorted together can be considered a DCP (Digital Packet Connector) in many portable devices. In the example below, the cost-effective R-78E buck converter serves as a low-cost DCP solution:
Figure 2: Simple DCP solution
The solution shown in Figure 2 can be used with a 12 or 24 VDC fixed voltage power supply or a 12V lead-acid battery. If using a 24V lead-acid battery as the power source, it is recommended to use R-78C5.0-1.0 (pin compatible) because the input voltage range will increase to 42 VDC. Battery-powered USB chargers require a series reverse polarity protection diode.
Some portable devices will fail to charge unless the D+ and D- pins are connected to the appropriate identification voltage and resistor as shown in Figure 1. These devices require an additional charging port controller IC to automatically detect the inserted device and set the correct voltage and current for charging.
In the example below, the TI TSP2514 charging port controller, used with an SMD surface-mount RPMB 5.0-2.0 buck converter, can provide up to 1.5A of continuous charging current (fast charging) for a variety of portable devices:
Figure 3: RPMB's charging port controller IC for charging portable devices from different manufacturers.
The example above applies to 12V or 24V battery supply voltages. However, for higher input voltage ranges (e.g., with a 48V lithium-ion battery supply), a converter capable of handling up to 60VDC input voltage is needed to cope with high charging voltages, and the RPMH5.0-1.5 is an ideal choice. Adding a voltage divider resistor between the input and enable pin provides undervoltage protection; as shown in the diagram, if the input voltage drops below 20V, the converter will automatically shut down, protecting the battery from deep discharge.
Figure 4: DCP (RPMH) for high input voltage
Some applications require isolated charging ports to prevent short circuits when the output pin is accidentally connected to a power supply. The RS6-4805S is a suitable solution, offering up to 1200mA of current in a compact SIP8 package. The RS6 series also features undervoltage lockout to prevent battery damage from deep discharge, 1.6kVDC/1 minute isolation, 100V surge withstand, and ±1kV transient protection using a TVS diode. When power is required, the RS12 series provides up to 2.4A of current through the same SIP8 pin.
Figure 5: Isolated USB DCP with 100VDC surge withstand capability
