“Portable devices such as cell phones and tablets are able to charge faster than ever before. To obtain fast charging times, the voltage on the charging device must be maintained at an appropriate level. If this is not the case, it is possible for the charger to 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 can 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 has a contact resistance of about 30mΩ. Since there are 4 contacts (two on each end of the cable), this represents a total resistance of 0.12Ω. Assuming each power cord is 1m long and a standard 24AWG cable is used, the total cable resistance is 0.166Ω. The expected total cable and contact resistance is 0.286Ω. If the 5V converter is designed to provide a maximum output current of 2.1A, the expected voltage drop across 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 the lower voltage limit, and high current loads create potential problems for obvious reasons. Using a heavier USB cable will help, while an extra-long USB cable, using a smaller wire gauge cable, will keep the charging rate below the maximum. Certain measures must be taken to further increase the charging current.
A common solution is to increase the no-load output set voltage, which is typically 5.0V, to 5.15V to 5.20V (5.25V maximum for USB 3.0) as much as possible. This solution provides sufficient (though still a minimum) headroom at a maximum current of 2.1A. For higher load currents, this approach quickly becomes unsustainable.
Another approach uses a dedicated charge port controller such as the 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 is pulled low when the output current is half the maximum current set by the current-limiting threshold resistor. Connecting this pin through a resistor to the feedback resistor of the 5V supply as shown in Figure 1 (see Figure 33 in the TPS2511 datasheet) will increase the output voltage. This reduces the voltage drop by about 50%. See the Dual Port Automotive USB Charger Reference Design for a design example.
Figure 1: Increased current sensing increases output voltage at 50% load in response to voltage drop
The block diagram in Figure 2 details a method for linearly increasing the converter output voltage to compensate for cable losses 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. See “Power Tips: Compensating for Voltage Drops on Cables” for more details on this method.
Figure 2: Current sensing continuously adjusts the feedback to maintain a stable voltage across the load
These techniques are just a few of the ways to deal with voltage drops and minimize charging times.