Use RECOM DC/DC converter as USB charger

The existing USB standard allows the maximum charging current to be 500mA (USB 1.0, 2.0) or 900mA (USB 3.0), and the current limit for fast charging is 1.5A. In order to ensure that the charging device can safely transmit power, a fixed voltage can be applied to the data transmission lines D+ and D- or a resistor can be installed between the two data line connections to inform the portable device that it has been connected to the dedicated charging port (DCP). Unfortunately, different manufacturers have different specifications so there is no universal solution (Figure 1).

The existing USB standard allows the maximum charging current to be 500mA (USB 1.0, 2.0) or 900mA (USB 3.0), and the current limit for fast charging is 1.5A. In order to ensure that the charging device can safely transmit power, a fixed voltage can be applied to the data transmission lines D+ and D- or a resistor can be installed between the two data line connections to inform the portable device that it has been connected to the dedicated charging port (DCP). Unfortunately, different manufacturers have different specifications so there is no universal solution (Figure 1).

Use RECOM DC/DC converter as USB charger Use RECOM DC/DC converter as USB charger Use RECOM DC/DC converter as USB charger
Figure 1: Various DCP resistor configurations of different portable device manufacturers

In order to avoid confusion, the USB3.1 standard has added battery charging specifications, allowing portable devices to communicate with chargers to set variable output voltage, current and power limits to match the charging capabilities of various portable devices and connector models (Type A, B type, micro B type or C type).

Even if the portable device is equipped with a USB-C connector, it can still be charged by the old charger, because the USB standard is backward compatible, but it will take longer to charge compared with the dedicated USB 3.1 charger.

Use RECOM converter as USB charger

A USB connector that shorts the D- and D+ pins together can be regarded as a DCP by many portable devices. In the example below, the cost-effective R-78E step-down converter becomes a low-cost DCP solution:

Use RECOM DC/DC converter as USB charger
Figure 2: Simple DCP solution

The solution shown in Figure 2 can be paired with a 12 or 24 VDC fixed voltage power supply or a 12V lead-acid battery. If you use a 24V lead-acid battery as a power source, it is recommended to use R-78C5.0-1.0 (pin compatible) because the input voltage range will rise to 42 VDC. Battery-powered USB chargers require a series of reverse polarity protection diodes.

Some portable devices will not be able to charge unless the D+ and D- pins are connected to the proper identification voltage and resistance as shown in Figure 1. These devices need to add a charging port controller IC to automatically detect the inserted device and set the correct voltage and current before charging.

In the example below, the TI TSP2514 charging port controller is used in conjunction with the SMD SMD RPMB5.0-2.0 step-down converter to provide up to 1.5A continuous charging current (fast charging) for various portable devices:

Use RECOM DC/DC converter as USB charger
Figure 3: RPMB’s charging port controller IC charges portable devices from different manufacturers

The above example is suitable for 12V or 24V battery power supply voltage, but if it is a higher input voltage range (for example, used with 48V lithium ion battery power supply), a converter that can handle up to 60VDC input voltage is required to cope with high charging Voltage, and RPMH5.0-1.5 is the ideal choice. Connecting a voltage divider resistor between the input terminal and the enable pin can provide undervoltage protection; as shown in the figure below, if the input voltage drops below 20V, the converter will automatically shut down to protect the battery from deep discharge.

Use RECOM DC/DC converter as USB charger
Figure 4: DCP (RPMH) for high input voltage

Some applications require an isolated charging port to avoid a short circuit when the output pin is accidentally connected to the power supply. RS6-4805S is a suitable solution, the small SIP8 package provides up to 1200mA of current. The RS6 series also has a built-in undervoltage lockout function to prevent battery damage caused by deep discharge, 1.6kVDC/1 minute isolation, 100V surge tolerance, and ±1kV transient protection using TVS diodes. When power is needed, the RS12 series provides up to 2.4A through the same SIP8 pin.

Use RECOM DC/DC converter as USB charger
Figure 5: Isolated USB DCP with 100VDC surge tolerance

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