Cable voltage drop compensation for backseat USB ports: why does it matter?

Other Parts Discussed in Post: TPS62130

With the increasing amount of electronics in today’s automobile, many new functions are being required for passengers.  While in-seat television screens and side air bags make cars entertaining and safe, sometimes just a simple USB port is needed for charging our numerous mobile devices.  In fact, with typically more than 1 such device per passenger, perhaps multiple USB ports are needed to satisfy everyone in the backseat.  With the Engine Control Unit (ECU) typically located behind the dashboard housing most of the electronics and the passengers in the furthest rear seat, how do we overcome the voltage drop across the lengthy cables in the car and provide a regulated 5V at the USB port?  This blog and its resources sufficiently answer that question, but why do we need a regulated 5V at the USB port?  What does it matter if that voltage contains significant IR drop and is only 4.5V or 4.2V when it gets to the backseat?

There are two main answers to this question: the maximum charge voltage of a single lithium-ion battery and the conservation of power.  All newer portable smart devices, like tablets and smartphones, use lithium-ion batteries.  When a single battery is used, its full charge voltage is usually somewhere between 4.1V and 4.35V, depending on the exact chemistry and charging specifications.  Since most battery chargers in these types of devices are step-down chargers only, a voltage higher than the battery’s full charge voltage is required to power the charger.  In addition, some margin or headroom is also needed.  Even with an input voltage of 4.5V to the charger, there may not be enough headroom to fully charge the battery or the battery may simply take longer to charge due to a longer constant voltage charging period.  This is certainly not desired by the passengers in the backseat.

The second reason is simply the conservation of power.  Most higher power battery chargers found in tablets and smartphones are switching converters instead of linear converters.  The higher efficiencies and less heat created by the switching converter allow faster charging with minimal temperature rise in the portable device.  A switching charger takes the input voltage and current allowed by the source (a USB port, protected by a USB switch, in this case) and converts it to the battery’s voltage and charge current.  For a 100% efficient charger, the product of input voltage and input current must equal the product of battery voltage and charging current.  Therefore, with all other variables being equal, a higher input voltage allows more charging current at the same battery voltage and input current limit setting.  The end result is that the battery charges faster.  This is definitely desirable in today’s portable devices!

Cable voltage drop compensation for a USB port is the principle used in TI Design PMP9756.  Converter TPS62130 is paired with amplifier INA213 (both available in automotive (-Q1) versions) to adjust the output voltage depending on the current delivered.  In this way, a regulated 5V is maintained at the USB port and the battery is charged fully and quickly.  And with the 3A of output current provided by the TPS62130, several USB ports can be powered with just one circuit.

  

What else besides USB ports are you powering that could use this concept?

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