How 5V USB charging works?

Not so long ago, you always had to make sure to charge your devices with the specific chargers or adapters designed for them. Different cables and bulky adapters used to take up a lot of space. Now, 5V USB charging has become very popular to an extent that many battery-operated devices do not send specific adapters with the product.  The only charging instrument needed is a USB cable.

That’s a great news for everyone, but the question is why not all USB charging devices work the same way, or with the same speed? What standards do they follow?  How do smart phones control charging speed?

Similar to many engineering products, USB charging started with a history. Initially, USB port was invented to transfer data from one electronic device to another. The power in this communication was used to power up the USB devices such as Mouse and Keyboard, not to charge any battery. That’s why most older PCs cannot provide enough current to charge your portable device very quickly

As most electrical instruments start to take advantage of this USB communication protocol for charging and providing power, engineers defined and standardize charging methods to the categories below:

• USB 1.0 and USB 2.0:

These standards were defined when USB was not being used to charge batteries. Therefore, current allocation at 5V was only limited to 500mA and 100mA.

• • Low Power:  5V, 100mA
  • High Power: 5V, 500mA
  • Suspended: 5V, 2.5mA

The determination on how much current should be drawn from a compatible USB 1.0 and 2.0 source is based on negotiation and enumeration. Interesting enough, 2.5mA current can still be provided by the source even if the negotiation does not agree on a current and system suspends the current. Once a portable USB device is plugged in to this USB 1.0/2.0. it can draw 100mA while negotiating or numerating for higher current.

• BC 1.1 and further standards

With increase in popularity, USB started a source for battery charging and BC1.1 (and later 2.1,..etc) came into effect. Based on this standard USB sources can supply up to 1.5A current to charge a portable device. This standard categorizes the USB power sources into three different types explained briefly below:

• USB charging Power Sources Based on BC1.1
• SDP (Standard Downstream Port):
  • Same as USB 2.0
  • High Power, Low Power, Suspended
  • Device recognizes the source when D+ and D- are separately connected to Ground through 15K
  • Still needs to enumerate to be USB compliance, but not too strictly
• CDP (Charging Downstream Port):
  • New for Laptops and PCs
  • Charges up to 1.5A
  • Power can be supplied before enumeration
  • Is recognized by handshaking and monitoring D+ and D-
• DCP (Dedicated Charging Port):
  • Wall Warts, Auto adapters
  • No enumeration occurs
  • Charges upto 1.5A
  • Identified by a short between D+ and D-
• Detection methods of Portable Devices

It is extremely important to recognize the USB source type before drawing current from it. Exceeding in drawing more than allowable current could result to permanent damage to the USB device and USB source. Sometime a trip on the fuse would be the result of over current and other times the charger can lock output and prevent charging completely. In order to recognize the USB power sources from each other. each should have certain signatures in their internal electronic circuitry to inform the portable device of the maximum allowable current.

• As it was briefly described above, for SDP, D+ and D- must be grounded through a resistor 14.25K-24.8K to ground. Picture below obtained from BC 1.1 standard shows the diagram for recognizing SDP power sources. In this figure RDP_DWN/RDM_DWN are the pull-down resistors should be selected from 14.25K to 24.8K Ohm.

• DCP is the simplest (almost shorting D+ and D-) USB charger without a need for enumeration. However, if the portable device doesn’t recognize the DCP, it cannot take advantage of its maximum capacity. Picture below shows the diagram when RDC_DAT should be 200 Ohm or less.

• CDP is more complex to detect but more useful since it gives you options to draw simply 500mA or 1.5A.

VDM_SRC is 0.5V to 0.7V and VDAT_REF ranges from 0.25V to 0.4V. As it is shown, more complex internal circuitry inside the power source is required to enable it for enumeration.

It is in advantage of the designer to use certain integrated circuits (ICs) such as USB controller and microprocessor to ease the design for CDP. On the portable side, this is almost critical to use controllers and USB manager ICs to avoid damage and ease enumeration designs.

• Apple and some other chargers

In order to protect its IP, high revenue and reduce competitions, Apple introduced its own detection methods for USB charging. This is why many USB chargers may not be able to charge an Apple product very quickly if they are able to even start to charge at all.

Diagram below shows the detection method used by apple products. In this method Voltage on top of the D+/D- was 2.0, 2.7 or vice versa.

The above figure was obtained from Maxim Integrated datasheet MAX14578E/MAX14578AE.

• Design Tips

It is important to read the BC1.1 or higher standards carefully before initial design of a USB portable device or USB charger. However, reading this article should have given you some enough understanding t get your hands dirty. Specially, if you are planning to design a USB charger, DCP is one of the most straight forward and quick chargers. No extra circuitry is needed for enumeration and it can provide currents upto 1.5A. All you need is to connect D+ and D- on your USB port with resistances less than 200Ohm. The rest is the responsibility of the portable device to recognize the charging capacity.

• Product Suggestions

One great product that we recently found on Amazon was this 6-slot charger that is even compatible with the latest apple products. Personal use of this product revealed great performance.