How to extend a power supply for droop compensation

February 21, 2014 // By Carsten Thiele
How to extend a power supply for droop compensation
Carsten Thiele, Analog Senior Field Application Engineer at Texas Instruments explains how to extend a power supply for droop compensation.

Wouldn't it be nice if the existing power design could easily be extended with the capability to compensate for the voltage drop across known cables and switches on load condition? This article describes the solution applicable for nearly all power designs where the feedback divider is accessible. For help with design, calculations are developed and given in a cookbook manner. The description is based on the practical example of a car's centre console USB charging port, which is powered from the electronics located somewhere in the dashboard. To charge mobile digital devices, USB current capability has to be 2A or more. However, the stringent USB port supply voltage limits is often directly contradicted by the use of cheap thin cables, which experience a huge voltage drop.

Voltage drop on power lines and connectors

Figure 1 Real Equivalent Circuit Diagrams

According to the real equivalent circuit diagram Figure 1 the voltage drop in a system is illustrated. The voltage V load is dependent on the current I load , the wire resistance R wire and the connector resistance R con. Basically, everything that is in series with the power line – for example additional switches - needs to be taken into account and V load will drop accordingly. Figure 2 illustrates this characteristic.

 Figure 2 Voltage Drop

If R drop is known and fixed in the system, with the following approach the power supply could be modified to compensate the voltage drop to keep V load constant.

Car Centre Console USB Port Charger Example
Using the practical example of a car centre console USB-Port (Figure 3) the need for compensation is demonstrated. The Infotainment Head-Unit contains the electronics and is located in the dashboard. The USB-Port is a passive implementation and located in the centre console connected via a 3m cable. To keep cost and weight down the wire diameter or cross-sectional area of the cable needs to be minimized.

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