Power Electronics
LED Drivers Build Better LCD Backlights

LED Drivers Build Better LCD Backlights

Maxim Integrated Products and National Semiconductor have developed LED driver families to address the key drawbacks in conventional white-LED backlighting for small LCD viewscreens. These drivers promises to improve battery life and video quality in converged handsets. The drivers also have their own sets of unique features, allowing the power designer to consider novel handset functions in addition to the required display parameters, such as power consumption or color reproduction, when selecting among the components of the two families.

While each device in these families can drive white LEDs, they are primarily designed to drive discrete red, green and blue (RGB) LEDs. The presence of these three colors and the overlap of their respective spectral peaks combine to generate white light. According to sources at Maxim, this white light is richer than what can be generated by a conventional white LED (in which a phosphor coating is optically pumped by a blue LED). Of course, RGB LEDs also have the capability to produce a wide range of colors by mixing various intensities of each color.

For this reason, handset manufacturers are now starting to use RGB LEDs to improve the color gamut of the display, according to National. To fully exploit this range of colors, both the Maxim and National LED drivers have multiple output channels, each with independent current control through an I2C or SPI bus (or, alternatively, autonomous control).

Maxim’s MAX8647 (Figure 1) has six current-sink outputs that are independently controlled linear regulators. According to Maxim, these regulators incorporate a hybrid linear/PWM control scheme (switching at 1 kHz) for LED currents below 6.4 mA in order to maintain accuracy across the full range of LED drive current (0.1 mA to 24 mA).

The flexibility afforded by having six outputs introduces several interesting possibilities. For example, six LEDs can be used to create a relatively bright backlight. Alternatively, three channels can be used to drive the conventional white backlight (generated from RGB LEDs), while the three remaining channels can be used for cosmetic lighting effects, such as driving an external RGB module. According to Maxim, the latter capability is very popular in Asia.

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The MAX8647 also includes a thermal derating function for LED protection, in which LED drive current is reduced by 2.5%/°C once the ambient temperature exceeds 60°C. This function separate from the LED driver’s overtemperature function, which initiates shutdown at 160°C and releases at 140°C.

The LP5520 (Figure 2) from National is also an RGB backlight LED driver for small-format color LCDs, and operates without the need for optical feedback. Three programmable current sinks with PWM control independently drive each LED chain. The user programmable calibration memory has intensity vs. temperature data for each color. Using temperature data measured from an external temperature sensor placed close to the LEDs, the RGB LED currents are adjusted for perfect white balance independent of the brightness setting or temperature. A second ADC input on the device can be used for ambient light measurement.

This white balance calibration data can be programmed to the memory on the production line of a backlight module. Calibration in a display module can be performed at one temperature, and the device can then produce true white light over wide temperature range. The device can also improve the color gamut from 70% up to 100% the National Television System Committee (NTSC) standard.

While the basic LED current regulation in the two devices is similar, their respective power architectures are somewhat complimentary. The MAX8647 is a flying-capacitor negative charge pump, whereas the LP5520 from National is an inductor-based boost converter (thought the LP5521 uses a positive charge-pump).

Charge-pump architecture allows two ceramic capacitors to replace a much larger boost inductor. In the MAX8647, the inverting charge pump produces a negative voltage that is one-half the magnitude of the positive rail. This leads to a rail-to-rail voltage of 1.5 times that of the input voltage. Each output has an independent LDO that can be independently connected to ground or this negative rail.

The MAX8647 architecture minimizes the in-line resistance (dropout voltage) when connected to ground. According to Maxim, it will operate in this state for 90% of a Li-ion battery capacity. When the positive rail voltage is too low to drive the LED (or the LED forward voltage is too high), the negative rail is engaged on the specific channel to provide sufficient voltage margin. Having such adaptive switching for each of LED independently, the IC therefore provides superior efficiency than conventional charge pumps.

According to Maxim, this part operates with greater efficiency than a magnetic boost converter under certain conditions. This is because the dropout voltage of each linear regulator is less than the dropout sources in a boost converter, such as the resistances in the inductor and the rectifier. Furthermore, when all the linear regulators are tied to ground, the charge pump stops switching, whereas a boost converter generates switching losses even at no load. This results in a quiescent current for the charge pump architecture of 70 μA, compared to current in the milliamp range for a boost architecture, according to Maxim.

However, a boost architecture does have certain advantages, such as the ability to drive very high voltages. National’s LP5520 has a magnetic boost converter that creates up to 20 V for the LED supply voltage from the battery voltage. The output can be set at 1-V steps from 5 V to 20 V. In adaptive mode, the circuit automatically adjusts the output voltage to minimum sufficient level for the lowest power consumption.

Both Maxim and National produce other RGB LED driver products, allowing power designers to select among different control features and power architectures. For example, a sister product to the MAX8647, the MAX8648 negative charge pump, also has adaptive-mode switching to each LED. The MAX8648 groups the control of the LEDs into three zones with serial-pulse dimming.

Complimenting the LP5520, National’s LP5521 is a three-channel LED driver based on a positive (in contrast to Maxim’s negative) high-efficiency charge pump. This enables LED driving over the full Li-ion battery voltage range. The device has a program memory for creating a variety of lighting sequences. When program memory has been loaded, the LP5521 can operate independently of a processor.

Another compliment to the LP5520, National’s LP5522 is a single-wire programmable LED controller that provides constant current flow through a high-side driver. Output current can be set from 1 mA to 20 mA by using an external resistor. If no external resistor is used, output current is set to 5 mA default current. The LP5522 is controlled using only one signal. The signal either directly controls the LED driver, or it launches a previously programmed blinking sequence.

Yet another device from National, the LP55281 is a quad RGB LED driver that can drive 4 RGB LED sets and a single fun light LED. A built-in audio synchronization feature allows user to synchronize the fun light LED to audio inputs. The device uses an inductor-based boost converter to drive the LEDs. It also has an LED test feature, which can be used in production for checking the LED connections.

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