1 Introduction

LEDs are becoming the mainstream device for backlighting. There are multiple LEDs in a typical backlight design, so consider the consistency of the brightness of each LED. In practical applications, the LED device can be "differentially screened" according to the voltage (Vf). The Vf value of the LED is an important variable in the backlight design. When multiple LEDs are connected together, the currents of the individual tubes must be matched for the entire component to produce consistent brightness.

The brightness of the LED is determined by the current flowing through it. The easiest way to achieve a constant current is to connect multiple LEDs in series. This eliminates the need for differential screening, and the series LEDs ensure that the current flowing through each LED is the same. However, the overall forward voltage in series is the sum of Vf of a plurality of LEDs, and an LED driving circuit with a boost function with an inductance is required, which increases the cost. Moreover, failure of one LED can cause the entire backlight to fail. In addition, the series drive method has strong electromagnetic interference, which is especially disadvantageous for communication products.

The design method of connecting multiple LEDs in parallel does not require a booster circuit with inductance, and the electromagnetic interference is small and the fault tolerance is strong. However, in order to ensure the uniformity of brightness, the LEDs must be differentially screened. As the number of LEDs used increases, so does the cost due to differential screening. Even after differential screening, there is still a range of variation in the Vf value of the LED, which affects the uniformity of brightness.

This article focuses on how to improve the uniformity of brightness in LED parallel design.

2, LED backlight circuit model

The three LEDs are connected in parallel to form a backlight, and the total current is controlled by a triode circuit, as shown in Figure 1. NLED with brightness uniformity is defined as:

Where Imax and Imin are the maximum and minimum values ​​of the individual LED currents;

Figure 1 LED backlight drive circuit

In the figure, D1, D2, and D3 are light-emitting diodes, and T1 is an NPN-type transistor. The resistance values ​​of R1, R2, and R3 can be adjusted according to the design requirements of the total current I, and the I/O or DAC is the control port.

The forward voltage of an ideal LED is:

In fact, the parasitic resistance has a great influence on Vf:

Where: I is the forward current; n is the ideal factor; k is the Boltzmann constant (1.38x10-23 J /K); T is the temperature (298K or 25°C); q is the electronic quantity ( 1.602x10-19) C); Is is the saturation current; R is the parasitic resistance; this article takes Xidiancheng's LED as an example, and its characteristic parameters are shown in Table 1.

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