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Extending Battery Life of a White Light LED Driver
by Michael Day, Texas Instruments, Inc.

This article describes how to extend the battery life of a circuit that uses a low power DC/DC boost converter for driving white light LEDs. Power loss in the current sense resistor is discussed and a method for reducing this loss is presented. Design equations are given for calculating the proper resistor values for a white light LED driver that includes provisions for dimming the LED brightness.

A current source, rather than a voltage source, typically drives white light LEDs. Regulation of the LED current is accomplished by monitoring the voltage across a current sense resistor. Any power dissipated by the current sense resistor is lost and reduces battery life. The key to extending battery life is to reduce the losses in the current sense resistor.

Figure 1 shows a TPS61040 low power DC/DC boost converter application that drives four LEDs and includes the capability to dim their light output. The resistor values in Figure 1 were calculated using the equations in the "Analog Dimming with Analog Voltage" section of Texas Instruments literature #SLVU068. For this design, the maximum desired output current is 20mA, and the current may be dimmed from 20mA to 0mA by injecting 0V to 3.3V into the VADJ pin. With these parameters, the full scale voltage across the current sense resistor, R4, with Iout = 20mA is 2V. This results in a power loss of 40mW. The four LEDs in series only require a forward voltage of 12.7V at 20mA, but the circuit is forced to generate 14.7V at 20mA.

Standard TPS61040 White Light LED Driver

The voltage across the current sense resistor R4, along with the injected dimming voltage, must combine to equal the reference voltage in the TPS61040 (1.233V). One way to reduce the full-scale voltage across the current sense resistor is to reduce the reference voltage in the TPS61040. As with most controllers, this is not an option because the reference voltage is internally generated and is not user adjustable. A second way to reduce losses in the circuit is to inject another voltage into the circuit to bias up the FB pin under no load conditions. With an additional resistor, the circuit can be designed such that the full-scale voltage across the current sense resistor is less than 2V for a 20mA output. The bias voltage can be a regulated voltage that is already available in the system. If another voltage is not available, the output voltage of the TPS61040 may be used. However, this increases the quiescent current during shutdown.

Figure 2 shows a TPS61040 application that drives four LEDs and includes the capability to dim their light output. The difference between the two circuits is Vbias and the addition of Rbias. The following equations calculate the correct resistor values for the circuit.

Vcs is the desired maximum voltage across the current sense resistor
Iout_max is the maximum LED current
Rbias is the chosen value for the additional bias resistor
Vadj_min is the minimum voltage used to adjust the LED current
Vadj_max is the maximum voltage used to adjust the LED current
Vfb is the reference voltage of the TPS61040 (1.233V)
Vb is the bias voltage

Improved White Light LED Driver Extends Battery Life

Table 1 provides resistor values for different bias voltages and different current sense voltages. Table 2 shows the operating parameters of the circuit under various conditions. The circuit was operated with Vin=3.6V. The column with Vcs=2V was measured using the circuit in Figure 1. All other data are taken with the circuit in Figure 2. Note that although the efficiency of the circuit is essentially the same under all conditions, the battery current is reduced when the full-scale voltage drop across the current sense resistor is reduced. The battery life is calculated using a 1,000mAmp-hour Li-Ion battery.

Calculated Resistor Values

Measured Data Shows Extended Battery Life

References:

TPS61040 - Low Power DC/DC Boost Converter in SOT-23 Package (SLVS413)
TPS61040EVM-002 White Light LED Bias Supply Evaluation Module (SLVU068)

About the Author:

Michael Day, Power Management Application Supervisor for Portable Power, Texas Instruments. Michael has over 12 years design experience in the field of power conversion. His power design experience ranges from outputs in the milliwatt to megawatt range. Michael currently manages a group that provides power supply solutions targeted at portable power applications. He received his BSEE and MSEE degrees in electrical engineering from the Texas Tech University in Lubbock TX. in 1990 and 1992. He is a member of IEEE. He can be reached at m-day@ti.com.

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