Dynamic Backlighting Boosts Contrast Ratio, Trims Power For Small LCDs

Fig 1. The additional resistor and low-cost FET in the lower right portion of this circuit modulate feedback resistance using an external PWM control signal to the MAX6948 WLED driver. A microcontroller supplies a 5-kHz PWM signal that provides duty cycles from 0 to 100%.

Fig 2. The MAXQ2000 evaluation kit (right) generates the PWM control signal. The MAX6948 evaluation kit (left) adds an extra FET to dynamically control the lighting. A close-up view of the modified portion of the board shows the additional FET mounted on a small board. Below it are the LEDs (small yellow rectangular blocks).

Fig 3. When the system receives an external PWM signal with a 15% duty cycle, it adds the signal to the 50% duty-cycle drive signal generated by the driver chip (a). When the PWM signal changes to an 85% duty cycle, the LEDs brighten due to the wider pulses added to the 50% duty-cycle pulses internally generated by the driver (b). A magnified view of the 85% duty-cycle PWM pulses riding on top of the internally generated 50% duty-cycle PWM signal shows how the external PWM affects the current going through the LED string.

Fig 4. Modulating the resistance of R_B between the values of 3.3 and 30 O generates a forward current of between 30 and 3.3 mA. That puts the operation squarely in the linear range of the LED’s current-to-luminescence relationship (solid portion of the curve).

Backlit liquid-crystal displays (LCDs), found in products ranging from tiny mobile phones to large televisions, often fall victim to complaints about the contrast ratio. This is particularly the case with the black level (the absence of light), which LCDs need as a backlight for illumination. It simply doesn’t look natural.

With dynamic backlighting, though, the backlight on LCD screens can be customized to increase the contrast ratio by varying the backlight intensity. With more handheld devices capable of playing video files, an improved viewing experience becomes a key selling point. 

Most of today’s LCD TVs and handheld devices employ screen-edge backlights implemented with static cold-cathode fluorescent lamps or LEDs. However, future large-area high-end LED backlit LCD TVs will divide the screen into many rows and columns of cells, with each cell composed of groups of RGB LED clusters. By independently controlling the light output of each cell (based on the image content), dynamic backlighting improves the image contrast ratio.

As backlight and driver costs trickle down, next-generation high-end TVs could apply the dynamic control to each RGB LED cluster within a cell, creating even finer-grain control of the backlighting. All the system needs to do is monitor the video content and feed a control signal back to the backlight controller to dynamically adjust the LED brightness in each cell.

For a cell-phone LCD, the small screen needn’t be divided into rows and columns of cells. However, backlighting can also leverage the “snooping” of the video content to dynamically adjust the brightness according to the average intensity of the video screen at that particular moment, enhancing the viewing experience. This technique could also extend cell-phone battery use time, since the backlight may not have to operate at full brightness while a video is playing.

LED backlighting drivers (such as the MAX6948 and many others) have an internal pulse-width-modulation (PWM) block. The block controls the intensities of attached LEDs according to commands from a cell phone’s baseband controller via a serial interface like I²C.

Subsequently, the baseband controller can use the internal PWM block of the driver to dynamically adjust the backlighting in response to a user’s key press or to ambient light changes. However, the limited communication speed and the delay caused by the serial interface’s protocol overhead would certainly eliminate the controller’s use of that internal PWM block for dynamic backlighting.

Varying the backlight intensity corresponding to video content requires the PWM signal’s duty cycle to change dynamically with video content. This signal can be generated according to the average intensity level of each frame using existing video-signal-processing circuits inside a cell phone’s baseband controller. The PWM signal can be sent to a backlight LED driver via the baseband controller’s general-purpose I/O (GPIO) pin.

The LED driver must translate this external PWM signal directly to corresponding LED intensity-level variations, without interfering with its internal PWM function, which is normally set via the serial interface . For example, an external PWM signal duty cycle of 50% causes no intensity change to that set by the internal PWM block. Thus, a PWM signal of less than 50% duty cycle dims the intensity, while more than 50% brightens the intensity.

Pump Up The PWM Intensity

Many LED drivers (e.g., the MAX6948) don’t have a particular input pin to accept this external PWM signal. Nevertheless, the PWM intensity control for dynamic backlighting can still be enhanced by adding a couple of components to their typical application circuits.

Adding external PWM intensity control to supplement a device’s internal PWM for dynamic backlight control is a rather straightforward process. In one scenario, an I/O pin from a microcontroller may generate the extra backlight control signal. However, the signal could represent the output of a moving light sensor or circuitry that analyzes screen content.

Here, the MAX6948 white-LED (WLED) driver will serve as the example device. The driver is designed to operate in mobile phones, but the concept can be applied to any system with an LCD screen.

The MAX6948 accepts standard input voltages between +2.7 and +5.5 V, and it boosts the output voltage up to +28 V to drive the backlight. Although this chip was designed to drive a cell phone’s backlight, this technique of adding external PWM can be used with LCD TVs, PDAs, laptops, or just about any LED backlit display.

An external feedback resistor, R_B, sets the peak backlight intensity (Fig. 1). Note that using a larger resistor will lower the peak current, and, in turn, lower the backlight intensity. Adding a transistor (Q1) and a second resistor (RB2), though, will modulate the resistance to change the backlight brightness without changing the internal PWM.

The chip internally generates the PWM signal that determines the LED intensity, based on commands sent from the host over an I²C port. The boost output of the WLED driver can be fully on, fully off, or pulse-width modulated with 10 bits of resolution (1024 steps).

R_B decides the maximum current. If R_B is 3.3 Ω, the maximum current through the LEDs is approximately 30 mA (V_FB/RB = 100 mV/3.3 Ω ≈ 30 mA). If R_B is 30 Ω, the maximum current is approximately 3.3 mA. The regulation voltage (V_FB) is stable at around 100 mV and controls the maximum current driven through the WLEDs. Modulating the feedback resistance enables additional control of the WLED intensity.

In this example, a PWM control signal is generated by a MAXQ2000 microcontroller, shown here on an evaluation board (Fig. 2). The PWM control signal ranges from 0 to +3.3 V. The frequency measures 5 kHz, and the duty cycle can be adjusted from 0% to 100%.

The MAX6948 is also mounted on an evaluation board, and a Vishay SI4800BD n-FET transistor (Q1) modulates the feedback resistance. For cell-phone applications, a smaller size n-FET transistor with a low drain-to-source resistance (R_DS(ON)) can be used. Otherwise, reducing the resistance of R_B will compensate for the larger R_DS(ON).

Due to the low 5-kHz PWM switching frequency, the gate charge has a negligible effect when using the MAXQ2000’s drivers. The transistor’s power consumption is negligible because both the switching losses and the average current passing through are low. 

Test Results

Waveforms captured with a current probe show the currents passing through the series WLEDs (Fig. 3). The MAX6948’s internal PWM function was on and its duty cycle was set to 50%. Depicted is the LED current with an external PWM duty cycle of 15% (Fig. 3a); the current with an external duty cycle of 85% (Fig. 3b); and the external PWM’s effect on the current through the WLEDs in series (Fig. 3c).

According to the data, the LED current level isn’t switched between the lowest current level determined by the 30-Ω resistor and the highest level determined by the 3.3-Ω resistor. It’s a direct result of the time constant and feedback behavior of the MAX6948. The average amplitude and the PWM swing change according to the external PWM duty-cycle settings.

In this case, changing the instant and average resistance of the n-FET transistor will deliver the external PWM control. By doing so, it will change the current going through the series LEDs.

There are two important facts to note about this configuration. First, the external PWM frequency of 5 kHz is much higher than the internal frequency of 125 Hz. Second, the external PWM control also regulates the dc portion of the LED current. Because of these two features, the solution avoids the common “beating” problem associated with dual-PWM intensity control. External PWM control of varied duty cycles was applied with internal PWMs from 0% to 100%, and the external control is effective. The beating issue wasn’t observed in various duty-cycle settings.

Luminous Intensity

The luminescence of an LED varies linearly with the forward current over a limited section. Shown are test results for WLED luminous intensity versus forward current for the Kingbright WLED used on the MAX6948 EV board (Fig. 4). Modulating R_B resistance between 3.3 and 30 Ω produces a forward current between 30 and 3.3 mA.

The current-to-luminescence relationship in the region between 3 to 30 mA is close to linear (Fig. 4, again). A 0% external PWM duty cycle produces the luminous intensity at 3 mA, and a 100% duty cycle at 30 mA. These results assume that the internal PWM intensity is fully on. To lower the intensity level, use the device’s internal PWM control via I²C PWM commands.