Dimming options for LCD brightness control
The environment of the application
often determines an LCD panel's dimming technique
JIM MORONSKI
Endicott Research Group
Endicott, NY
http://www.ergpower.com
The two most common techniques used to
control the brightness of the cold-cathode fluorescent tubes
(CCFLs) that backlight an LCD panel are analog dimming and digital
(or PWM) dimming. In analog dimming, a dc voltage directly controls
the output current of the inverter powering the CCFL, which in
turn, controls the display's brightness.
The difference
between the dimming ratios offered by analog and digital/PWM
techniques show most clearly at lower voltages.
The pulse width modulation (PWM)
dimming technique fixes the output current of the inverter and
modulates the time that the inverter is on. Each technique has its
own set of advantages and limitations�important design
considerations that must be taken into account during system design
and integration.
Analog dimming
Analog dimming is the simplest technique to implement. An applied
dc voltage directly controls the output current of the CCFL
inverter. Generally, this voltage is electrically summed into the
on-board voltage control loop or is used to supply drive current to
the inverter's primary switches. In either case, the inverter
operates continuously. In this mode, power supply requirements are
reduced since there is little input ripple current and additional
filtering of the power supply is not required.
However, the dimming range available
when using analog dimming is quite poor, particularly for
applications such as daylight- and nighttime-readable displays that
require a wide dimming range. The display's minimum brightness
would occur when the specified CCFL is operating at its specified
minimum operating current, often 30% to 50% of the rated typical
current.
Since light output is relatively
linear, a dimming ratio of roughly 2 or 3:1 can be accomplished.
This is further complicated by large displays, typically larger
than 17 in., with high electrical losses in the CCFL assembly that
could effectively reduce the dimming ratio to as low as 1.5:1.
The dimming ratio is the ratio between
the highest achievable brightness of a display and the lowest
attainable brightness level. Lower dimming ratios are perfectly
acceptable for many applications.
Typically, office environments�where
the ambient light levels are fixed�require a relatively limited
dimming range. However, if the ambient light levels change
significantly, such as in the automotive or aeronautical industry,
very bright backlights are needed to make the display bright and
readable in direct sunlight and the display must also not blind the
user at night.
PWM dimming
Digital or PWM dimming is rapidly becoming the dimming technique of
choice since it is less display-sensitive and offers more
flexibility in choosing brightness levels. To make PWM dimming
possible, an inverter needs to be specifically designed for it.
On many generic closed loop inverters,
the control loop is often too slow for an effective dimming range.
When PWM dimming is used, the time that the inverter is on is
modulated and the applied duty cycle roughly equates to display
brightness, with 100% being the maximum.
Since the inverter is actually being
turned on and off at the PWM frequency, care should be taken in the
design of the inverter's power supply to take into account the
low-frequency pulses of current produced by the inverter.
Furthermore, magnetic structures may sing as a result of
the windings compressing and expanding (magnetostriction) as the
current through them changes rapidly at the PWM frequency, so
acoustics are also a consideration.
One of the greatest advantages of
using PWM dimming is a wide dimming range. Since the minimum tube
current requirement is always met (the inverter is either fully on
or fully off), the minimum brightness is a function of controllable
duty cycle, which can be below 1%. With a minimum duty cycle of 1%,
a dimming ratio of 100:1 is easily accomplished.
Some inverters offer on-board
analog-to-PWM converters that allow for the increase in dimming
range while interfacing to legacy circuits or potentiometers.
However, when possible, the source of the dimming signal should be
digital and at the desired PWM frequency.
If it is not, undergoing the
conversion process from analog to digital or digital to analog to
digital can be plagued by noise and tolerance problems. These are
manifested in minimum brightness stability (flicker or brightness
drift) and lack of repeatability (consistent brightness levels from
assembly to assembly).
Soft starting and the signal
interface
If an extreme dimming ratio is not required, the disadvantage of
acoustics and power supply ripple requirements can be reduced by
choosing an inverter with soft-start functionality. Soft
start gradually increases and reduces the CCFL current at each PWM
cycle, which in turn softens the edge on startup and turnoff. The
performance hit in dimming ratio isn't a killer either, as dimming
ratios of greater than 50:1 can still be accomplished, which is
sufficient for applications like portable tracking stations and POS
terminals.
Interfacing signals to inverters is
not always a straightforward task. It is important to note that
many of the interface signals on the inverter side are not
digital. Some interface to voltage levels outside of the
digital range, are not compatible with TTL-level signals or require
current sourcing or sinking.
In some cases, it can be as simple as
a mismatch in the PWM analog control range voltage or swapped
polarity on enable or control signals. Instead of developing
circuitry to accommodate the inverter, the knowledgeable inverter
manufacturer will configure the inverter to accommodate the
interface signals or offer interface solutions.
