Led-driver Considerations

 Many of today’s portable electronics require backlight
LED-driver solutions with the following features: direct
control of current, high efficiency, PWM dimming, over-
voltage protection, load disconnect, small size, and ease of
use. This article discusses each of these features and how
they are achieved, and concludes with a typical circuit
that implements each of these features.
Direct control of current
LEDs are current-driven devices whose brightness is pro-
portional to their forward current. Forward current CAN be
controlled in two ways. The first method is to use the LED
V-I curve to determine what voltage needs to be applied to
the LED to generate the desired forward current. This is
typically accomplished by applying a voltage source and
using a ballast resistor as shown in Figure 1. However, this
method has several drawbacks. Any change in LED forward
voltage creates a change in LED current. With a nominal
forward voltage of 3.6 V, the LED in Figure 1 has 20 mA of
current. If this voltage changes to 4.0 V, which is within
the specified voltage tolerance due to temperature or manu-
facturing changes, the forward current drops to 14 mA.
This 11% change in forward voltage causes a much larger
30% change in forward current. Also, depending upon the
available input voltage, the voltage drop and power dissi-
pation across the ballast resistor waste power and reduce
battery life.  
 Practical solution
The TPS61042 is an excellent example of a modern
LED-driver control IC. Figure 4 is a block diagram of the
TPS61042 with a highly integrated control IC. Q1 is a low-
resistance, integrated power FET The low resistance of
this component contributes to an extremely high efficiency.
The 0.25-V reference voltage reduces losses in the current-
sense resistor. PWM dimming is easily implemented with
this IC by applying a PWM signal to the CTRL pin at frequen-
cies as high as 50 kHz. Q2 implements the integrated load-
disconnect circuitry. Since it is integrated, this circuitry is
perfectly synchronized to the PWM dimming frequency.
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Typical TPS61042 LED-driver solution
Typical TPS61042 LED-driver solution

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