White light LED backlight dimming solution analysis

W LEDs are popular, and PWM and analog dimming drivers will compete in different fields for different application needs. In view of this, the industry has developed a solution that can switch analog and PWM dimming modes to meet different markets. demand.

White light-emitting diodes (WLEDs) have many advantages over cold-cathode lamps, such as solid-state devices, directional light sources, etc. In addition, WLEDs can operate at lower voltages and dimming over a wider range of brightness. And when it is dimmed, it will show a fairly linear change in brightness.

At present, many electronic devices with built-in displays use WLED type backlights. If you do not really understand the implementation methods and advantages and disadvantages of various methods, it is not easy to find the correct LED dimming method. This article will briefly explain the power supply mode of the LED, and then describe the two LED dimming methods and their advantages and disadvantages. Through this information, you can select the appropriate dimming method and LED driver IC for application.

Configure the voltage regulator converter to achieve WLED power supply

The brightness of the WLED changes linearly with the passing current. To achieve optimum WLED current accuracy and uniform brightness for each serial, the LED driver should adjust the current and voltage through the LED instead of adjusting the current and voltage across the LED. Figure 1 shows how the current-regulated DC-DC converter that resets any adjustable output becomes a stable power supply to drive the series in parallel with the output being greater than the sum of the LED forward voltages and the voltage (VLED) falling. Multiple WLEDs.



Figure 1. Adjustable Output DC-DC Converter Provides Stable Current Through WLED Serial

After adjusting the voltage across the current-sense resistor (RSENSE) (VSENSE) instead of the output voltage (VO), the driver stabilizes the power supply so that the VO can self-adjust as the ΣVLED changes due to voltage and temperature. The voltage drop of WLED ranges from 3 to 4 volts. This drop is directly affected by the LED current and is inversely proportional to temperature. Recent low-power drivers have replaced external sense resistors with one or more current sinks, especially single junction field effect transistors (FETs) (Figure 2).



Figure 2 LED driver with integrated current intrusion

The driver has two functions, not only to adjust the driving voltage of the FET to achieve the appropriate current (relative to the bias current) through the FET, but also to adjust the integrated DC-DC converter (usually a boost converter) The output power is such that the FET has the lowest germanium source voltage required for this current. This type of driver with an integrated boost converter and eight integrated current sinks, one of which is the TPS61195.

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