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I know that by standard no device should ever try to draw more than 500mA from USB.

But a lot of phones and tablets can somehow identify their purpose made chargers and draw far more than 500mA from it.

How is this done?

I see two solutions, one digital and one analog.

Digital solution is to build some logic into the charger and use data lines to handshake.

Analog solution is to use a 5.2V or higher charging voltage so the logic is as simple as "if input voltage is greater than 5.0V, then it's our VERY powerful charger, forget about 500mA limit".

I'm using a LTC3556 power management chip. The input current limit can either programmed via I2C or by adjusting a resistor. http://www.linear.com/product/LTC3556

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  • \$\begingroup\$ You should read Wikipedia: en.wikipedia.org/wiki/USB Especially the bits about the different port types and current ratings. 500mA is a lie. \$\endgroup\$
    – Majenko
    May 8, 2015 at 14:12
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    \$\begingroup\$ It's not a lie, just a very ignored standard. That said, the new standard is the usb dedicated charging port, or the apple standard \$\endgroup\$
    – Passerby
    May 8, 2015 at 14:17
  • \$\begingroup\$ Ok, not a "lie", but only "true" in certain circumstances and has largely been superseded. Like, you can't normally draw 500mA form a bus powered hub, and 500mA is the limit in a now obsolete standard. \$\endgroup\$
    – Majenko
    May 8, 2015 at 14:28
  • \$\begingroup\$ The majority of the USB host ports available to me are all USB 2.0. They cannot supply more than 500 mA. I agree that saying it is a "lie" is inaccurate enough to be misleading. There are a lot of 500 mA ports out there in the field which cannot reliably supply more than 500 mA. \$\endgroup\$
    – user57037
    May 9, 2015 at 0:45

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Dedicated chargers short the two data lines together. The phone (or whatever) can detect this and knows it can draw more current.

Source: http://www.maximintegrated.com/en/app-notes/index.mvp/id/4803

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    \$\begingroup\$ Yeah. There is a USB battery charging specification covering this. A lot of chargers will also back off if the VBUS voltage starts to sag. In other words, if the charger recognizes that VBUS input Voltage has drooped to 4.4V, it will reduce the charge current as much as necessary to prevent it drooping further. This feature is not so important when connected to a dedicated charger, but it could be helpful to avoid overloading a USB host device. \$\endgroup\$
    – user57037
    May 9, 2015 at 0:49

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