Choosing LED driver ICs for backlighting LCDs
The key to matching driver ICs with their
intended apps lies in understanding their fundamental
characteristics and features
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.
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
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
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.
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
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
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 (I2C) 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
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.
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.