Portable devices such as mobile phones and tablets can achieve faster charging speeds than before. To achieve fast charging times, the voltage on the charging device must be maintained at an appropriate level. If this is not the case, the charger may reduce the charge current to a lower (but still acceptable) level, ultimately extending the overall charging time. A drop in voltage on the charging cable can result in insufficient voltage. Let's take a look at what this would do with a Universal Serial Bus (USB) cable and how to deal with possible problems.
A common USB cable interface contact resistance is approximately 30mΩ. Since there are 4 contacts (two at each end of the cable), this means that the total resistance is 0.12 Ω. Assuming a length of 1 m per power cord and a standard 24 AWG cable, the total cable resistance is 0.166 Ω. The estimated total resistance of the cable and contacts is 0.286 Ω. 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 at 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 a long USB cable with a smaller gauge cable will cause the charge rate to be below the maximum. Some 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 as much as possible (for USB 3.0, the maximum is 5.25V). This solution provides sufficient (though 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 the TPS2511. This device monitors the USB data line and automatically provides the correct electrical signature to the charge-compatible device; it also has a current limit function. Its current sense (/CS) pin pulls low when the output current is half of the maximum current set by the current limit threshold resistor. As shown in Figure 1, connecting this pin to a 5V supply feedback resistor through a resistor (see Figure 33 in the TPS2511 datasheet) will increase the output voltage. This reduces the voltage drop by approximately 50%. See the dual port automotive USB charger reference design for a design example.
Figure 1: Current sensing increases the output voltage at 50% load to account for voltage drops
The block diagram in Figure 2 details a method of linearly increasing the converter output voltage to compensate for cable losses and keeping 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 across the sense resistor and uses this voltage to inject a current into the feedback (FB) pin of the controller. For more information on this method, see "Power Tips: Compensating for Voltage Drops on Cables."
Figure 2: Current Sensing Continuously Adjusts Feedback to Maintain a Stable Voltage on the Load
These techniques are just a few ways to deal with voltage drops and to minimize charging time.
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