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Backlight technology in tablet PC
Sep 25, 2017

Some tablets are powered by a single lithium-ion battery, others using two lithium-ion batteries. No matter how many batteries are used, all tablet makers want to maximize their battery life. The backlight of the display is one of the most power hungry systems in the tablet computer. In the recently released tablet PC, the number of backlit LED ranges from 20 to 36. This article will guide the reader on how to select the best WLED drive and LED string configuration to meet the tablet application requirements without sacrificing efficiency and battery life.

Tablet PC backlight requirements

Like laptops or netbooks, the tablet backlight drive applications are also based on the DC/DC converter and the ground resistance path for LED. This application generally has the following requirements:

1, RF range, low EMI

2. No visible blinking during brightness height

3. The minimum audible noise is caused by the piezoelectric hum of ceramic output capacitor

4, the display brightness is consistent

5, high light ratio

6, maximum efficiency, maximum battery life

Meet the first requirement, that is, within the RF range, low EMI is relatively easy. Over the past few years has been in power supply designers for the implementation of this goal, they tried many methods, such as: the switching frequency and the harmonic concurrent set in the RF range, the use of shielding inductance, in the right circumstances to design PCB the minimum length but with wider wires, and so on. Some drive IC have integrated the MOSFET gate drive circuit with a hierarchical rise time to reduce noise in the RF range.

The brightness adjustment type greatly affects the latter four requirements. When pulse width modulation (PWM) is used to adjust brightness, the LED current is switched on and off at its maximum current level during the adjustment process to produce an average DC LED current. At this time, as long as the PWM brightness adjustment frequency is much higher than 60 Hz, the backlight flicker is not so obvious. If analog brightness adjustment is used, flicker is not a problem because the LED DC current level drops below its maximum value when the brightness is adjusted.

The third requirement is that the minimum audible noise of the ceramic capacitor is related to the topology of the drive. A simple driver whose current sense resistor serves as the LED current ground path. The converter regulates the voltage of the current detection resistor to control the LED current.

Figure 2 shows the driver for the integrated current sink. The driver samples the voltage of each current sink to ensure that the converter can provide just enough power to maintain the current sink working properly.

Like flashing, there is no problem with analog brightness adjustment because the output capacitor voltage has only a small change to accommodate minor changes in the LED current. However, if the PWM brightness adjustment is used, then the way the driver prevents the discharge of the output capacitor becomes important. The simplest driver also has a resistor between the drive feedback (FB) pin to ground and when the drive's converter is effectively off, the output capacitor begins a sharp discharge when the duty cycle is adjusted at low brightness. Some of the more complex drivers incorporate current wells, which can replace current sense resistors. They simply switch on the absorber and the DC/DC converter for the LED, thereby removing the discharge and recharging paths of the output capacitor.

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