Don’t let the USB voltage drop to slow down the charger

Portable devices such as mobile phones and tablets can achieve faster charging speeds than before. To obtain a fast charging time, the voltage on the charging device must be maintained at an appropriate level. If this is not the case, the charger may reduce the charging current to a lower (but still acceptable) level, ultimately extending the overall charging time. A drop in voltage on the charging cable will cause insufficient voltage. Let’s take a look at how this affects Universal Serial Bus (USB) cables and how to deal with possible problems.

Portable devices such as mobile phones and tablets can achieve faster charging speeds than before. To obtain a fast charging time, the voltage on the charging device must be maintained at an appropriate level. If this is not the case, the charger may reduce the charging current to a lower (but still acceptable) level, ultimately extending the overall charging time. A drop in voltage on the charging cable will cause insufficient voltage. Let’s take a look at how this affects Universal Serial Bus (USB) cables and how to deal with possible problems.

A common USB cable interface contact resistance is about 30mΩ. Since there are 4 contacts (two at each end of the cable), this represents a total resistance of 0.12Ω. Assuming that the length of each power cord is 1m and a standard 24AWG cable is used, the total cable resistance is 0.166Ω. The estimated total resistance of cables and contacts is 0.286Ω. If a 5V converter is designed to provide a maximum output current of 2.1A, then the expected voltage drop on the cable will be 0.6V. For a fixed converter voltage of 5.0V, the voltage on the end of the cable will drop to 4.4V. For USB devices, this voltage value is a lower voltage limit, and the cause of potential problems with high current loads is obvious. Using a heavier USB cable will help, while an extra-long USB cable, using a smaller wire gauge cable, will make the charging rate lower than the maximum. Certain measures must be taken to further increase the charging current.

A common solution is to increase the no-load output setting voltage with a typical value of 5.0V to 5.15V to 5.20V as much as possible (for USB 3.0, the maximum value is 5.25V). This solution provides sufficient (although still the minimum) headroom at a maximum current of 2.1A. For higher load currents, this method will soon be difficult to support.

Another method uses a dedicated charging port controller such as TPS2511. This device monitors the USB data line and automatically provides the correct electrical signature to the charging compatible device; it also has a current limit function. Its current sense (/CS) pin pulls down to a low level when the output current is half of the maximum current set by the current-limiting threshold resistor. As shown in Figure 1, connecting this pin to the feedback resistor of the 5V power supply through a resistor (see Figure 33 in the TPS2511 data sheet) will increase the output voltage. This reduces the voltage drop by approximately 50%. For design examples, please refer to the dual-port automotive USB charger reference design.

Don’t let the USB voltage drop to slow down the charger
Figure 1: Increased current sensing increases the output voltage when the load is 50% to cope with the voltage drop

The block diagram in Figure 2 details a method for linearly increasing the converter output voltage to compensate for cable loss and keep the voltage at the end of the cable constant. This solution adds a sense resistor to monitor the output current. A differential amplifier increases the voltage on the sense resistor and uses this voltage to inject a current into the feedback (FB) pin of the controller. For further details on this method, see “Power Tips: Compensating for Voltage Drops on Cables.”

Don’t let the USB voltage drop to slow down the charger
Figure 2: Current sensing continuously adjusts the feedback to maintain a stable voltage on the load

These technologies are just a few ways to deal with voltage drops and shorten the charging time as much as possible.

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