Circuit design of car USB charger with line compensation function

　　With the popularity of smartphones and tablets and the high power consumption that comes with them, the batteries of such devices can only last for one day of use. More and more occasions and devices are equipped with one or more USB charging ports, and car USB chargers are an important part of them. Due to the large body size and long lines in the car, the voltage of the USB charging port may decrease with the impedance of the line, resulting in insufficient charging current. This paper introduces a design of a car USB charger with line compensation function, which makes the voltage of the USB charging port increase with the increase of current, realizes the constant USB charging voltage, and ensures the charging current of the USB port.

　　DC/DC Buck Converter : The main power stage of the design uses a synchronous Buck converter controlled by TI LM25117-Q1. LM25117-Q1 is an automotive-grade product that provides various protections required by the power circuit, including adjustable input undervoltage, overcurrent, short circuit and overtemperature protection. In addition, LM25117-Q1 can simulate diode control. Through external settings, the synchronous buck converter simulates diode operation when light load is applied. When reverse current is detected in the lower tube, the lower tube is turned off, which improves the light load efficiency of the converter.

　　Charging management circuit: A design scheme for a USB charger with line compensation. The calculation method of line compensation and the design process in mathcad are given. Output charging management TPS2546-Q1 is an automotive-grade product with an integrated USB 2.0 high-speed data line (D+/D-) switch USB charging port controller and power switch. TPS2546-Q1 supports BC1.2 SDP/CDP, DCP mode, and also supports non-BC1.2 standard fast charging mode, such as D+/D- distribution mode 2.0V/2.7V and 2.7V/2.0V, D+/D-1.2V mode. In this design, TPS2546-Q1 is configured in DCP-Auto mode (CTL1=0, CTL2=1, CTL3=1, ILIM_SEL=1), which supports automatic switching between BC1.2 DCP and non-BC1.2 standard fast charging modes, and can support fast charging of mainstream smartphones and tablets in the world. In this mode, when the charging current exceeds the load detection current threshold (ILD typical value 700mA), STATUS is pulled low and LED D3 will light up. When the charging current drops to the typical value of 650mA, D3 will go out after 3 seconds. According to the data sheet, the 22.1kΩ resistor R11 is connected to the ILIM_HI pin, and the current limit value is set to a typical value of 2.275A, a minimum value of 2.12A, and a maximum value of 2.43A to support a maximum charging power of 12W (5V, 2.1A).