Power Tips: how to build a car charger

phone and tablet battery life meme

As my buddy, Brian, mentioned in his blog, these days everyone carries a smart phone or tablet.  They are necessary for business emails, personal calls and keeping up with the world.  There is always a new craze in games, Angry Birds, Candy Crush, Words with Friends, etc.  Our smart phones predict the weather and give us directions.  All of these features and functions make our lives easier and more productive. 

If you’re like me, you charge your device overnight and start off the day with a full battery.  However, because of power draining elements like screen size and display brightness, todays batteries rarely last as long as I need or desire.

A convenient way to combat this problem is to charge the phone during commutes.  DC/DC chargers in cars are becoming not only popular, but necessary.

DC/DC chargers come in a few different varieties.  Many cars have built in USB ports that can be used to charge these devices.  Aftermarket and stock radios also provide this convenience while allowing you to listen to music stored on your device.  Finally, if none of those options are available, you can find after-market chargers that fit in an accessory port or cigarette lighter.

Typically, these designs use the car battery as input; this means that the input voltage range can be 6V to 14V under normal operating conditions and transients can go up to 80V or higher.  The output is usually 5V at currents up to 2.1A per port.  There are three main components to this type of design:

  1. Input condition and filtering. The input filtering and conditioning circuits typically used to reduce EMI and protect the downstream DC/DC converter.  Some popular choices for the converter are the TPS54240 and TPS40170.  These two ICs offer a wide input range and variety of features that make the design process easy for the engineer. 
  2. DC/DC conversion.
  3. USB charging and protection. Protection being especially important. Who wants to have their hundreds of dollars device damaged by a faulty charger?  The actual battery and charger for the smart phone or tablet are inside the device.  In order for the charge to be able to provide maximum charging current, the charger needs to be able to identify itself.  The TPS2511 and TPS254x devices help with this identification as well as provide the necessary USB protection.

With the above in mind, I created two USB DC/DC charger designs you can use for free: PMP7388 and PMP7390

For a single port charger in a form factor that fits flush with the surface of a cigarette lighter plug, use the PMP7388.

single port charger in a form factor that fits flush with the surface of a cigarette lighter plug

Uses the TPS54240 and TPS2511 for a non-synchronous buck converter with integrated high side FET.  The TPS54240 can handle input voltages up to 40V, to protect against high surges on the input a conditioning circuit used to clamp the input voltage to 39V.  The output is capable of providing 2.1A and offers “universal charging” via the TPS2511

For a dual port charger, use the PMP7390.

The PMP7390 dual port charger using the TPS40170 synchronous buck converter designed with external FETs to give high efficiency and low thermal dissipation.

Uses the TPS40170 synchronous buck converter designed with external FETs to give high efficiency and low thermal dissipation.  The output is capable of providing 5V at 5A and offers “universal charging” via the TPS2511 for each port.

Please comment below with any questions/suggestions on implementing or tweaking this design to meet your deepest charging desires

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