The Secret World of USB Charging

There’s a whole set of USB charging specs you’ve probably never heard of because big-battery smartphones, tablets and 2:1’s demand shorter charge times.

Editor’s note: this particular blog posting is sponsored by Pericom Semiconductor.  

$5 chargers useNow that you can buy $5 USB chargers everywhere (mains- and cigarette lighter-powered), it’s tempting to think of them like LED flashlights: cheap commodity throw-aways. And you would’ve been right…until now.

My recent purchase of an Asus T100 Transformer Windows 8.1/Intel Atom 2:1 tablet hybrid forced me to dig into USB charging (Figure).

My own Asus T100 Transformer Book has a “unique” USB charging profile.  (Courtesy: Asus.)

My own Asus T100 Transformer Book has a “unique” USB charging profile.
(Courtesy: Asus.)

This device is fabulous with its convenient micro USB charging port with OTG support. No bulky wall wart to lug around. But it refuses to charge normally from any charger+cable except for the (too short) one that came with it.

My plethora of USB chargers, adapters, powered hubs and more will only trickle charge the T100 and take tens of hours. And it’s not just the device’s 2.0A current requirement, either. There’s something more going on.

Just Say “Charge it!”

The USB Innovators Forum (USB-IF) has a whole power delivery strategy with goals as shown below. Simply stated, USB is now flexible enough to provide the right amount of power to either end of the USB cable.

The USB Power Delivery goals solidify USB as the charger of choice for digital devices. (Courtesy: www.usb.org )

The USB Power Delivery goals solidify USB as the charger of choice for digital devices. (Courtesy: www.usb.org )

There’s even a USB Battery Charging (UBC) compliance specification called “BC1.2” to make sure devices follow the rules. Some of the new power profiles are shown below:

Table 1: USB Implementers Forum (USB-IF) Battery Charging specifications (from their 1.2 compliance plan document October 2011).

Table 1: USB Implementers Forum (USB-IF) Battery Charging specifications (from their 1.2 compliance plan document October 2011).

The reason for UBC is that newer devices like Apple’s iPad, Samsung’s Galaxy S5 and Galaxy Tab devices–and quite possibly my Asus T100 2:1–consume more current and sometimes have the ability to source power to the host device. UBC flexibly delivers the right amount of power and can avoid charger waste.

Communications protocols between the battery’s MCU and the charger’s MCU know how to properly charge a 3000mAh to 10,000mAh battery. Battery chemistry matters, too. As does watching out for heat and thermal runaway; some USB charger ICs take these factors into account.

Apple, ever the trend-setter (and master of bespoke specifications) created their own proprietary fast charging profiles called Apple 1A, 2A and now 2.4A. The Chinese telecom industry has created their own called YD/T1591-2009. Other suppliers of high-volume devices have or are working on bespoke charging profiles.

Fast, proper rate charging from Apple, Samsung and others is essential as harried consumers increasingly rely on mobile devices more than laptops. Refer to my complaint above RE: my Asus T100.

Who has time to wait overnight?!

USB Devices Available

Pericom Semiconductor, who is sponsoring this particular blog posting, has been an innovator in USB charging devices since 2007. With a growing assurance list of charge-compatible consumer products, the company has a broad portfolio of USB ICs.

Take the automotive-grade PI5USB8000Q, for instance. Designed for the digital car, this fast charger supports all of the USB-IF BC modes per BC1.2, Apple 1A and 2A, and the Chinese telecom standard. The IC powers down when there’s no load to save the car’s own battery, and can automatically detect the communication language to enable the proper charging profile (Figure). Pretty cool, eh?

The USB-IF’s CDP and SDP charging profiles require communication between the USB charger and the downstream port (PD) device being charged. Refer to Table 1 for details. (Courtesy: Pericom Semiconductor.)

The USB-IF’s CDP and SDP charging profiles require communication between the USB charger and the downstream port (PD) device being charged. Refer to Table 1 for details. (Courtesy: Pericom Semiconductor.)

As For My Asus 2:1?

Sadly, I can’t figure out how the T100 “talks” with its charger, or if there’s something special about its micro USB cable. So I’m stuck.

But if you’re designing a USB charger, a USB device, or just powering one, Pericom’s got you covered. That’s a secret to get all charged up about.

16 thoughts on “The Secret World of USB Charging

  1. The USB Power Delivery goals solidify USB as the charger of choice for digital device.But if you’re designing a USB charger, a USB device, or just powering one, Pericom’s got you covered. That’s a secret to get all charged up about.

  2. For more information about battery calibration, refer to the
    Apple Web site. Also, when on battery energy lower the actual screen brightness to boost the quantity of performance you obtain from the laptop computer
    battery. Battery charges that work in the car are also good
    options for anyone taking their laptops on trips.

  3. Pingback: USB hub that can work as a charger

  4. I am getting a ASUS T100 this year, for me to go traveling, So I am looking for a compatible battery pack or something similar, can someone give me some advice of what to get?

  5. Came across your post when searching for discussion on the ASUS T100TA. The organization I work for has over 2000 of these devices. Finding replacement chargers was a pain. We tried about a dozen possible chargers and found just one so far that has worked and charged at the same rate as the one provided by ASUS. It’s a PWR+ brand 2A type charger. It charges at 20% per hour like the original. Here’s a link to it on Amazon:
    http://www.amazon.com/Transformer-T100ta-b1-gr-T100ta-c1-gr-Convertible-Touchscreen/dp/B00GOI1HJA

  6. Chris,

    Having a broken micro USB connector on my ASUS T100 charger, I thought I would cut the wire and solder on a new one.

    So I went to take apart the old micro USB pin and to my surprise, soldered inside the male connector, was a tiny surface mount 20k ohm resistor shorting pins 4 (ID pin) and the ground. I have found no specification for such 20k Ohm on micro USB connectors.

    I’ve concluded this is how the ASUS tablet detects its own charger to draw the full 3 amps of power.

    Hope this helps!
    Alex

    • Hi Alex,

      That’s my discovery too, however I tore the soldered cables from the pinout/plug and now, as I bought the 20kOhm resistor I’m not sure which pins should go where. Maybe you made a photo, or simply know what goes where? Of course, if not, I will do the trial & error part to get this done. My charger is alive, i just need to fix the cable and im good to go :D No need for a new charger.

      Please reply (ventricle.vioner@gmail.com) or here under my comment.

  7. As far as i can tell, the data wires in the asus charger are shorted out, and that tells the unit to pull the full 2 amps. You can confirm it yourself with an ohmmeter.

  8. Great post! I recently got a Transformer Book and was wondering why it is so picky when it comes to charging. The more you know!

  9. I have the same issue with my asus transformer book. Is there any solution??? My cord is starting to wear, and I cannot find one that will charge it quickly. Plus, it really annoys me how the cord is not long enough!! If anyone knows of a solution please help me out :)

  10. So, we have some technical standards presented with no extrapolation as to what practical impact/considerations these represent for the consumer. The transparency of sponsorship is admirable but this does not excuse an article that promises to enlighten but which, in fact, only serves to give a reason to place the vendor’s name and products onto the page. Thus, the power of the web is diluted. Shame.

  11. My T100TAM seems to require the following:
    - D+ and D- lines shorted _and floating_ (i.e. no resistor to gnd or 5V)
    - USB shield sorted to gnd line (pref. at powersupply end so that it may use the shield as extra low-resistance path)
    This gets me at least 1.5A charging from a (non-USB) 5V power supply.

  12. My T100 charges without problems from both my Samsung 2A chargers, if it didn’t refuse to charge from the (also 2A, and also Samsung) car charger – I wouldn’t have come across this article. I don’t know if it charges at the same rate (it’s too slow to care, have to leave it overnight anyway), but it does work with both chargers and any of the 3 cables I have (also from Samsung, came with the phones).

  13. My ASUS T100 charges fully and seemingly as fast when I use my Samsung Galaxy S3 charger plugged into the USB slot on my printer. This is all mysterious to me but as long as it works I’m happy. The Samsung charger plugged to the mains works just as well as the printer shortcut.

  14. Hi guys.
    I have two (well I sold one) of these and I joined this forum just to tell everyone that the ORIGINAL USB cable does NOT charge the device properly.
    As soon as I connected another USB charge cable (an old Nokia cable), with ORIGINAL adapter, it charged properly!
    The orange light comes on with the ORIGINAL cable, but it doesn’t really charge: When turned on, the charge light comes on, but Windows doesn’t recognise it as charging for long. With a “better” cable (and ORIGINAL adapter) Windows shows it charging until charged.
    I hope this helps.

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