LED Driver Solutions for Mobile Backlighting

By Gina Roos

Contributed By Hearst Electronic Products


Mobile devices, including smartphones, tablets, and notebook computers, have widely adopted LED backlighting systems thanks to their low-power consumption, energy efficiency, and small and thin packaging. Despite the widespread adoption (or perhaps because of it), LED driver manufacturers continue to enhance their devices to further improve the energy efficiency and battery life of mobile products and to enhance the user experience.

LED driver makers are looking at how users interface with their mobile devices, particularly smartphones and tablets, to develop power products that deliver the best lighting along with higher efficiency. A good example of what they have come up with is the Texas Instruments LM3697, which drives three parallel high-voltage LED strings for display and keypad lighting. The most significant feature of this device is its 11-bit configurable dimming resolution, which significantly improves display lighting for the user.

An older, but still valuable product series is Intersil’s ISL97692/3/4A backlight LED drivers, which improve energy efficiency while increasing battery life of tablets and mobile devices. These drivers feature output current phase shifting, which maximizes efficiency during PWM dimming while eliminating audible noise. As an added benefit, particularly for space-constrained applications, the drivers do not require any input voltage except from the battery, helping to reduce board size and component count.

Let’s now take a closer look at Texas Instruments’ LM3697 11-bit LED driver. This device has several significant features. The first one is the 11-bit exponential dimming. This provides a more pleasing display brightness for the user because it provides 0.3 percent relative step size between brightness codes as the brightness level is ramped from one level to another level (Figure 1). “Because the human eye has a static contrast ratio of about 1 percent, the brightness of a display changing at 0.3 percent gives a very smooth brightness transition no matter how fast or slow the change happens,” says John Woodward, technical product marketing manager at Texas Instruments.

“The reason why the exponential ramp is important is because of the way the human eye senses light,” Woodward says. “A quick linear ramp is not very pleasing to the eye, but if you ramp exponentially and gradually turn it on at the lower levels first and then faster, your eye perceives that in a nice smooth transition. That is really why we ramp exponentially, to give the human eye a nice gradual response. You won’t see any flicker on the screen.”

Image of Texas Instruments LM3697 brightness transition

Figure 1: The Texas Instruments LM3697 features 11 bits of exponential dimming, or 2048 steps, providing a smooth transition from the lowest to the highest brightness level. The chart shows the LED current mapping modes for 11 bits (Courtesy of Texas Instruments).

In addition, the LM3697 has built-in ramp features that make it easy to use the 11-bit exponential dimming. The startup, run-time, and shutdown ramps of the brightness level are all independently programmable with sixteen I²C-compatible programmable levels to ensure the best screen brightness transition when changing from one brightness level to the next, according to Woodward.

Accuracy is another important specification for uniform brightness. “As screen sizes get bigger and higher definition, they need more light, which means more LEDs,” says Woodward. The LM3697, with three separate current sinks (channels) up to 30 mA each, can handle up to twenty-one LEDs and can support smartphones or small tablets up to around seven inches. “When you have so many LEDs in multiple channels the accuracy between the channels need to be as close as possible (matching), so having very good matching between the channels and overall accuracy is important when adding more and more LEDs to deliver uniform brightness across the screen,” says Woodward. One of the key features of the LM3697 is 2.5 percent matching over temperature. TI is targeting tighter specs of ±3% matching over temperature, current, and voltage, which will make next-generation LED drivers even more accurate.

Efficiency is also important. The LM3697 provides backlight dimming for 1-, 2-, or 3-series LED strings with up to 90 percent efficiency. The boost converter with integrated 1 A, 40 V MOSFET automatically adjusts to LED forward voltage to minimize headroom voltage, thus improving LED efficiency.

While dimming and accuracy is important for the visual experience, Woodward points to efficiency as one of the biggest and most important features. A new feature on the LM3697 is the ability to automatically select the switching frequency, which is selectable from 500 kHz to 1 MHz. “The part will automatically adjust the switching frequency based on the brightness current in the LED strings to maximize the efficiency for that operating range. Depending on the use case, you can get about two percent more efficiency out of your backlight, which translates directly into usage time,” he adds.

The LM3697 also features adaptive headroom voltage, which helps increase efficiency. This type of control scales the headroom voltage with the LED current. The lower the headroom voltage the lower the losses, which translates into increased efficiency. The LM3697 is fully configurable via an I²C-compatible interface. The device operates over a 2.7 V to 5.5 V input voltage range and a −40° to 85°C temperature range.

Similarly, the Intersil ISL9769x series of backlight LED drivers offers improved energy efficiency, delivering increased battery life for mobile devices including tablets. These devices can operate from the low input voltage of a single-cell Li-ion battery. The series features ultra-low quiescent current of 0.8 mA with a minimum input voltage of 2.4 V. When operating from a single-cell Li-ion battery, the drivers do not require any input voltage other than the battery. This reduces board size and component count, and make it suitable for use in smartphones, tablets and ultrabook/notebook PCs.

The ISL97692 offers four channels and provides 8-bit PWM dimming with adjustable dimming frequency up to 30 kHz. The ISL97693 supplies six channels with direct PWM dimming control. And the ISL97694A provides six channels and 8-, 10-, or 12-bit PWM dimming with adjustable dimming frequency up to 30 kHz, 7.5 kHz, or 1.875 kHz, respectively, controlled with I²C or PWM input (Figure 2).

Image of Intersil ISL9769x series PWM dimming linearity

Figure 2: The Intersil ISL9769x series offers ultra-low PWM dimming linearity (Courtesy of Intersil).

An optional phase-shifting feature is available for the ISL97692/4A, providing an optimized phase delay between channels. Phase shifting can multiply the effective dimming frequency by the number of channels, allowing above audio-band PWM dimming with 10-bit dimming resolution, according to Intersil.

All three devices use adaptive boost architecture, which keeps the headroom voltage as low as possible to maximize battery life while allowing ultra-low dimming duty cycle as low as 0.005 percent at 100 Hz dimming frequency in Direct PWM mode. The devices also provide ±2.5 percent output current matching, and an adjustable switching frequency from 400 kHz to 1.5 MHz.

In summary, when selecting LED drivers for backlighting mobile devices, designers should look for devices that deliver the best lighting experience for users. This means looking at how the devices approach dimming: does it offer exponential dimming, what is the current matching spec, and does it use adaptive headroom voltage control to lower losses? On top of all these specs, engineers should look at the efficiency of the devices — this is a critical spec in terms of how much power is consumed by the backlighting.

For more information on the devices discussed in this article, use the links provided to access product information pages on the Digi-Key website.

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