Charging smartphone from USB without fear



From time to time many of us are facing the same problem when trying to charge your smartphone or tablet from the USB port - it just not charging or charging slower than it would do from the original charger. Recently I have bought USB car charger to power my Samsung Galaxy Tab Pro 10.1 on the road just to discover it doesn't work. Even though the Galaxy Tab actually sees the charger connected is just marking it as unrecognized power source. The reason is simple - the Galaxy Tab Pro not considers the charging device as "native charger".


What is "native" charger?

The native charger for smartphone or tablet often have a special voltage signature on USB data lines to let the device recognize the charger and figure out the maximum charging current it can consume from the power source. The intent is twofold. First, it is stopping the device from consuming too much current from the charger. Second, it is preventing the charging from unrecognized power sources. The "unrecognized" is the keyword here, as profiting from selling additional device accessories (chargers) is definitely a business strategy.


DCP - Dedicated charging port

USB specification defines a new port type - USB Battery Charging. USB Dedicated Charging Port should have the D+ and D– data lines shorted together with a maximum series impedance of 200Ω. Or just simply shorted. For some mobile devices it is sufficient for start charging.


Samsung devices

Samsung devices require 1.2V voltage on both D+ and D- data lines, see the schematic below. The R1/R2 voltage divider is providing the required voltage on the D+ and D- pins of the USB connector to be recognized as Samsung native charger. That's it. The only issue is that completely discharged Galaxy Tab battery might draw even more than 500 mA from the USB port, when the USB 2.0 specification limiting the maximum current load to 500 mA. USB 3.0 specification makes improvement here and increased the maximum current to 900 mA. The divider resistor values are not critical as far the divider ratio remains the same, i.e. 0.24 or closer.

If you would like to make your own divider you can download the Eagle project files.


Below is Chinese-made USB charging adapter for Samsung devices implementing the schematic above. The label says its TF-USB-P1000 V1.0, but is just a bogus one...


Apple iPhone and iPad devices

Apple's chargers also indicate the maximum charging current by voltages on the D− and D+ lines. The possible configurations are listed in the table.

Configuration # D+ D- Maximum current
1 2V 2V 500mA
2 2V 2.7V 1A
3 2.7V 2V 2.1A
4 2.7V 2.7V 2.4A

The adapter for the configuration #1 will look like this:


USB Charging Port Controller

Texas Instrument's USB Dedicated Charging Port Controller TPS2514 is specifically designed to implement all the charging schemas mentioned above. An auto-detect feature monitors USB data line voltage and automatically provides the correct electrical signatures on D+ and D– data lines. Note that the chip datasheet never mentioned Samsung or Apple in particular apparently due to the copyright issues. Instead, it mentioned 1.2V mode and different Apple charging modes as Divider 1, Divider 2 and Divider 3. There are two modifications of the chip. TPS2514 has Dividers 1 and 2 for configuration #2 and #3 when TPS2514A only Apple Divider 3 for configuration #4. Both chips support DCP and Samsung charging. Note that Divider 1 or Divider 2 is configured by flipping connection to USB data lines. In other words, it is pre-wired and there is no way do it on the fly.



With TPS2514 device implementing the USB charging adapter is simple, see the schematic below. When charging Apple devices the current is limited to 1A as it is implemented Divider 1 charging schema with D+ = 2.0 V and D− = 2.7 V. Also, try to use good quality charging cables. The one particularly bad cable I was using introduced a significant voltage drop forcing my Samsung device switching to the slow charging mode.


Eagle project files are here.

Note: The problem is actually more complex and there is much more beyond providing the right voltage to D+ and D- pins. The most phones/tablets have sophisticated charge controller circuitry and if power voltage goes below 5.25V under the load the charging current is also limited. For instance, my Galaxy Tab Pro tablet changing current is 1.7A with voltage 5.25V using TPS2514 schematic above and connecting to the original Samsung charger rated for 5.3V/2A. When switched to no-brand 5V charger rated to 5V/2.1A the current dropped to 1.37A with charging voltage 5.07V. And finally, when charging from USB 3 desktop port the current is about 0.58A with voltage 4.79V.


QC 2.0/3.0 USB Charging

See QC 2.0/3.0 USB Charger