Charging your 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. Recently I have bought USB car charger to power my Samsung Galaxy Tab 10.1 on the road just to discover it doesn’t work. Even though the Galaxy actually sees the charger connected is just marking it as unrecognized power source. I was trying to power up my Motorola Droid 3 phone – it won’t work either. The reason is simple – the Galaxy Tab doesn’t recognize 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 pins 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.

Charging your Smartphone from USB without fear

Samsung devices

I have figured the simple solution that will make the Samsung Galaxy pad device charge from USB port, see the schematic below. The R1/R2 voltage divider is providing the 1.2V 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 then 500 mA from the USB port, when the USB 2.0 specification limiting the maximum current load to 500 mA. Just be aware.

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.

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