Choosing LED driver ICs for backlighting LCDs and TVs

Choosing LED driver ICs for backlighting LCDs and TVs

The key to matching driver ICs with their intended apps lies in understanding their fundamental characteristics and features

BY MICHAEL JENNINGS
Freescale Semiconductor
Tempe, AZ
http://www.freescale.com/LED

The inherent strengths of the light-emitting diode (LED) make it an excellent choice to succeed the cold-cathode fluorescent lamp (CCFL) as the backlighting solution for coming generations of televisions, desktops, and laptop displays. LEDs consume much less power than CCFLs, last five times longer, are more efficient, reduce display thickness, are dimmable in fine steps, use low-voltage drivers, and are inherently “greener,” since unlike the CCFLs they don’t contain mercury or other hazardous substances.

However, all these features can only be fully exploited when the LED backlighting array is well matched to the IC that drives it. So designers need to understand key driver IC characteristics and features before choosing the one that best matches the application’s requirements. One look at a driver IC data sheet shows that there are many of these to consider, but the parameters and features described in this article are the most important.

Parameters

The first of these specifications is the input voltage that the driver IC can accept. If the voltage range is narrow, so too will be the breadth of applications it can serve. In addition, it may not be able to withstand the broad input voltage swings and other transient conditions that invariably occur in service.

The driver’s maximum output voltage is also key because each LED produces a voltage drop of 1 to 4 V. The driver must have sufficient output to accommodate the voltage drop over the number of LEDs in the array. The maximum output voltage and number of channels determine how many LEDs it can support.

The same is true for the maximum current the driver can deliver per channel. Its available current must be matched to each design with emphasis on the type of LEDs employed.

LEDs specified in most portable applications draw 20 to 30 mA, and monitors and TVs typically use LEDs that consume 40 to 120 mA (although some applications use LEDs that draw up to 350 mA). For output voltage and output current, higher values are generally better, but realize that high-output drivers cost more than their less-powerful siblings, so closely matching driver and application can shave design cost.

The number of channels delivered by the driver IC ranges from a few to 16 or more. Selecting a driver with the “right” number is driven entirely by system requirements. The goal is to meet the system’s needs with as few drivers as possible to reduce cost and complexity.

However, the number of LEDs in series a driver can support depends not just on its number of channels but on its maximum output voltage as well. For example, a lower-output, 16-channel driver may support only five LEDS in series for a total of 80 LEDs, while a 10-channel driver with higher output voltage may drive up to 16 in series for a total of 160 LEDs.

LEDs

Depending on the display’s size, the number of LEDs can vary anywhere from 30 for a 10-in. display to more than 1,000 in a large-panel TV. As the light output of these LEDs depends on the current, it is vital that the current for all LEDs be matched tightly, even though as with all electronic components LED characteristics vary from device to device.

If these variations are not minimized, perceptible differences in lighting uniformity will occur across the display. The driver IC controls this by maintaining a tight tolerance on current variation as stated by its current-matching specification. A good target is ±2% or better for notebooks and monitors, and ±1% for TVs.

As a number of the LED’s properties can vary with the LED current, the dimming function should be performed with pulse width modulation (PWM) control, keeping constant current in the on-state. Though some systems use an external PWM signal (some notebook PCs, for example, use direct PWM control), a driver with an onboard PWM generator is often a desirable choice. Such devices need no external PWM generators and can simplify system design. Some driver ICs support both.

Many of the existing backlighting applications employ PWM frequencies below 1 kHz that in some cases can result in audible noise when low-cost ceramic capacitors are used. This problem can be avoided by choosing a device that supports a wide range of PWM frequencies including those above the audible range.

Such a device, for example, is Freescale’s MC34844 10-channel LED backlight driver. Some drivers also offer the ability to synchronize themselves with other devices or an external source to reduce the possibility of noise caused by beat frequencies and harmonics caused by interaction of devices, and to remove certain visual artifacts.

The granularity or precision with which a driver can dim an LED depends on its number of bits. The greater the number, the finer the increments into which the PWM signal can be sliced, which provides greater dimming control. For example, the MC34844 has 8 bits, which allows LEDs to be dimmed to any of 256 levels.

The driver’s specification for transition times between low and high PWM states should be as fast as possible to ensure a precise square wave output pulse even with very low duty cycles. This is necessary to ensure tighter current matching and a more linear dimming range.

Drivers that provide linear dimming down to one least-significant bit (LSB) at a high PWM rate such as 25 kHz provide the best performance. However, transition speed should not be too fast because higher frequencies can cause ringing and other forms of electromagnetic interference (EMI). Speeds of about 50 ns satisfy this requirement while also optimizing efficiency.

Although LED driver ICs are available without a communications interface, it is desirable that devices used in many backlight applications have this feature, and it is essential for devices with on-board PWM generators. The interface facilitates programming, fault monitoring, and other functions, and is most commonly of the inter-integrated circuit (I2 C) type. For systems requiring high-speed update, interfaces such as the low-voltage differential signal (LVDS) are becoming popular.

Drivers with onboard boost converters eliminate the need to externally implement this function. In addition, the use of an integrated switch is preferred as it eliminates the potential for EMI caused by board interconnects, reduces the bill-of-materials (BOM), saves PCB space, and removes the need for the designer to specify a transistor that is well matched to the driver. The boost frequencies used vary widely and are sometimes programmable.

Higher frequencies such as 1.2 MHz have the advantage of using smaller inductors and capacitors. A dynamic headroom control (DHC) scheme is another essential feature. It measures the forward voltage of all LED strings connected to the boost converter and automatically adjusts the output voltage to the minimum required to drive them. The result is less voltage drop across the linear drivers in the current mirrors and lower driver power dissipation, which increases overall efficiency.

Drivers with optical loop control allow designers to use an optical sensor to compensate for the temperature and lifetime variations that occur with LEDs. Thermal sensors may also be used for compensation of the thermal effects. Optical sensors can also be used to adjust the backlight brightness in response to changes in ambient conditions, dimming the display in dark environments.

A desirable LED driver IC includes many features that provide protection for both the driver and the LEDs. LED short/open protection allows backlighting to continue when an LED or LED string fails. In addition, overvoltage, overcurrent, and overtemperature protection provide the necessary assurance for both the driver and the LEDs. Undervoltage lockout is used to ensure the driver does not operate outside of the specified range, which may lead to incorrect operation of the device.

Final thoughts

Remember that drivers with a variety of on-chip features reduce the circuit’s required PCB “real estate” and bill of materials. They also lessen design complexity by eliminating the need for designers to create external circuits and select the best components for them. Of course, the importance of each feature varies with the application, and the winning driver IC will be chosen based on its cost versus system performance. ■

For more on LED driver ICs, visit http://www2.electronicproducts.com/AnalogMixICs.aspx.