Efficiently dim your LEDs without a sense resistor

When driving a single  LED from a battery  , efficiency is key. Efficiently using battery energy and converting it to light output extends battery life, reduces costs, and reduces hassles for the end user. Efficient switching power conversion starts with efficient switching, and the next and equally important step is dimming. Dimming allows the user to reduce light output when such high brightness lighting is not needed, thereby reducing battery power consumption. Dimming combined with efficient power conversion maximizes battery life in devices such as smartphones and blood glucose meters .

LED current regulation via a) sense resistor and b) MOSFET in the power stage.

A further way to make these devices more efficient at driving LEDs is to improve the efficiency of the switching power stage itself. A significant loss in every LED driver is the loss of the current sensing element (usually the sense resistor). As a resistor, all current passing through it causes losses, draining the battery. In order to tightly control the LED current (which is proportional to the light output), a resistor is often used as the sensing element. The LED current is routed through this resistor and the resulting voltage is measured and regulated by the LED driver. While simple, this is unfortunately also a relatively large loss for a single LED driver.

In most switching power supplies, a MOSFET is provided in the power stage to sense the LED current instead of a resistor. This approach requires more processing of the current signal, but when it is integrated into the LED driver IC, it eliminates the complexity for the user. This approach is simple and can achieve higher efficiency results. In addition, the design does not need to use a larger sense resistor, which may be more expensive! For example, I compared the  input current consumption of the TPS61260 LED driver when using a sense resistor or a MOSFET to sense the LED current. For a full analysis of this design, click here (LEDs Magazine may need to log in to view it), and click here to test it.