Cutting LCD Panel Power With LED Driver ICs

The widespread use of notebook computers and other similar portable devices have brought significant changes to the technology associated with the use of LCD panels. The most significant of these changes has been the use of brighter, lower-power LEDs to provide backlighting. Taking advantage of these developments, driver IC manufacturers offer devices that provide the necessary LED control.

To understand the details of LED use, you first have to look at an LCD panel that operates by selectively controlling the light that reaches its screen. When an electric current passes through the liquid crystal material it behaves like a shutter; either allowing some light to pass through or blocking it to form the displayed image. This shutter-like effect requires a light source behind the screen. In the past, cold cathode fluorescent lamps (CCFL) provided backlighting for small- and medium-size LCDs.

THE LED ADVANTAGE

LEDs offer advantages over CCFLs, including lower operational voltage, lower power, longer life, accurate wide-range dimming control and advanced architectures for improved image quality. LEDs also avoid hazardous materials such as the mercury used in CCFLs.

Other LED backlighting advantages include relatively low cost, long life, vibration immunity, and precise control over light intensity. Factors driving the adoption of LED backlighting include energy-efficient operation leading to extended battery life, and the ability to produce slimmer displays for thinner, lighter notebook computers.

A past drawback to LED backlighting was that it required more power than other methods, particularly for larger LCD panels. However, solid-state optoelectronics technologies have addressed the power consumption problem, enabling brighter LEDs with lower power consumption. This, coupled with higher-efficiency power-management LED drivers enable an effective solution for backlighting LCD panels.

LED backlighting architecture depends on the size of the display and features required. For displays in the 7- to 17-inch range edge-lit LEDs with diffused lighting offer very thin designs down to 2 mm or less. In contrast to CCFLs, the higher efficiency of LED backlighting also extends battery life in portable equipment. For larger panels, LED backlighting supports advanced architectures such as local dimming, in which power consumption and contrast ratio are dramatically improved.

A common backlighting technique is “array backlighting.” It creates a matrix of LEDs behind the LCD surface using diffraction and diffused layers to produce a homogenous and even light on the LCD surface. Each row or column is formed by a number of LEDs in series, forcing a single current flow through all LEDs in a string.

Using one current-control driver per row or column helps the system maintain a constant current flow through each line. This maintains a uniform light level even in the presence of line or load variations. You can also control the light intensity by increasing or decreasing the current flowing through each LED string. Reducing the current extends battery life in portable equipment, but it can also also affect the white-point (color) of the output. It is better to use the pulse-width modulator (PWM) to vary the brightness.

A NEW SOLUTION

Freescale's MC34845 series devices (Fig. 1) are high-efficiency LED drivers for use with edge-lit backlighting for LCD displays up to 17 inches. The IC supports LED currents from 3.0 to 30 mA and supports up to six strings of LEDs. The associated current drivers match the current between LEDs to provide good uniformity across the display surface.

MC34845 LED drivers feature high-speed PWM capability. They provide accurate output waveforms even at high frequencies, enabling accurate square output pulses down to 200 ns widths. The ability to operate at high frequencies with low jitter eliminates display waterfall noise and flicker. Operation at frequencies greater than 20 kHz eliminates audible noise that may be caused by MLCC vibration.

Panel dimming involves the application of a PWM input signal to the PWM pin, which modulates the LED channels directly. An Enable Pin (EN) provides low-power standby, which supports low-power shutdown with 1-µA maximum supply current. A single wire scheme selects power down when the PWM pin is connected to the Wake Pin and held low.

The MC34845 also features a boost converter to produce up to 60 V to drive each LED string. The boost converter can deliver the required LED voltage from either a two- or three-cell Li-ion battery, or a direct 12-V-input supply. There are two device versions for boost frequency, the MC34845 (600 kHz) and the MC34845A, (1.2 MHz). External compensation allows the use of different inductor/capacitor combinations. The boost also includes a soft start circuit. Each time the IC comes out of shutdown mode, the soft start period controls start-up.

The boost converter uses a dynamic headroom control (DHC) loop to automatically set the output voltage required to drive the LED strings. DHC operates for pulse widths higher than 400 ns. If the pulse widths are shorter than specified, the DHC circuit will not operate and the voltage across the LED drivers will increase to a value given by the overvoltage protection (OVP) minus the total LED voltage in the LED string. Therefore, designers should select the proper OVP level to avoid exceeding the maximum off-state voltage of the LED drivers (45 V). The boost operates in current mode and is compensated externally through a Type 2 network on the COMP pin.

The IC has an internally fixed OVP value of 60 V (typical) that serves as a secondary fault protection mechanism if the externally programmed OVP fails (i.e. resistor divider opens up). You could use the internal 60-V OVP detector without the external OVP network, but this is only recommended for applications where the LED string voltage approaches 55 V or more. You can set the OVP level with an external resistor divider connected between the output voltage and ground with its output connected to the OVP pin. The OVP can be set up to 60 V by varying the resistor divider to match the OVP internal reference of 6.9 V (typical).

An integrated 2.0-A (minimum) power MOSFET supplies the required output current. An overcurrent protection (OCP) circuit limits the output current cycle-by-cycle. The device shuts down if the condition exists longer than 10 ms.

Fig. 2 shows a typical MC34845 application. The MC34845 supports 5.0 V to 21 V at the VIN input pin. Two internal regulators generate internal rails for internal operation. Both rails are de-coupled using capacitors on the VDC1 and VDC2 pins.

The VIN, VDC1, and VDC2 supplies each have their own undervoltage lock-out (UVLO) mechanisms. When any voltage is below the UVLO threshold, the device stops operating. All UVLO comparators have hysteresis to ensure constant on/off cycling does not occur.

The power-up sequence for applying VIN respective to the ENABLE and PWM signals is important since the MC34845 device will behave differently depending on how the application sequence of these signals is accomplished. For the case where VIN is applied before the ENABLE and PWM signals, the device has no limitation as to how fast the VIN ramp should be. However, when the PWM and ENABLE signals are applied before VIN, the ramp-up time of VIN between 0 V and 5 V should be no longer than 2 ms.

The current in the LED strings is set by tying the RSET resistor to GND from the ISET pin. The LED current level is: RSET = 153/ILED. For best performance, the accuracy of the RSET resistor should be 0.1%. The driver is also very accurate, supporting ±2% current matching (max) across the operating range of -40° to 85°C, which enhances display uniformity. The current-drive outputs rely on a single direct drive PWM input.

Among the other fault-protection modes are those for guarding against LED short and LED open. If an LED is open, the output voltage ramps to the OVP level. If there is still no current in the LED string, the LED channel turns off and the output voltage ramps back down to normal operating levels.

If LEDs are shorted and the voltage in any of the channels is greater than the short fault detection voltage (SFDV) threshold (7.0 V typical), the IC turns off that channel. However, if the on-time of the channels is less than 10 µs, the SFDV circuit will not disable any of the channels regardless of the voltage across them. LED errors can be cleared by recycling the EN pin or applying a complete power-on-reset (POR).

The MC34845 includes overtemperature protection. If the internal temperature exceeds the overtemperature threshold, the device shuts down all functions. The device is re-enabled once the temperature falls below the low-level threshold.

The FAIL pin is at its low-impedance state when no error is detected. However, if an error such as an LED channel open or boost overcurrent is detected, the FAIL pin goes into a high-impedance state. The FAIL pin can be cleared by recycling the EN pin or applying a POR. If the detected failure is an overcurrent time out, the EN pin or a POR must be used to restart.

OPERATING LED DRIVERS FOR BACKLIGHTING LCD PANELS

ONE OF THE MANUFACTURERS of LEDs for backlighting LCD panels is Nichia Corporation of Japan (www.nichia.com). The company makes white LEDs by combining blue LEDs and special phosphors. Therefore, the LED color exhibits very little change due to variation in operating current. The table lists the electrical and illumination characteristics of some of the company's LEDs intended for backlighting LCD panels.

Nichia notes that the LED driver must not exceed the absolute maximum rating specified for each device. The recommended circuit is the one that regulates the current through each LED. If operating the LEDs with a constant voltage, the current through the LEDs may vary due to the variations in their forward voltage (VF). And, in some cases, LEDs may be subjected to stresses in excess of their absolute maximum rating.

LEDs should always be operated in the forward-bias mode. Design the driving circuit so that the LED is not subjected to either forward or reverse voltage while it is off. Plus, continuously applying reverse voltage can damage the LED.

Thermal design is important, so consider the LED's heat generation when undertaking the system design. The coefficient of temperature increase is affected by the thermal resistance of the circuit board and density of LED placement on the board, as well as other components. Avoid intense heat generation and operate within the maximum ratings listed in the product specifications. Determine the operating current after checking the LED's ambient temperature.