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Landmark Technology Backlight Inverters Landmark Technology Introduction
An Inverter is an electronic circuit that transforms a DC
voltage to an AC voltage. Cold cathode fluorescent lamps
(CCFLs) in LCD backlights are most efficient to operate in
AC. Consequently, all CCFL powered LCD backlights
require inverters.
The typical inverter circuitry for operating a backlight
consists of a Royer oscillator and a control circuit. The
Royer oscillator converts the DC to AC and the control
circuit provides the dimming, On/Off control, and possibly
some other functions.
The Royer oscillator circuit in an inverter has a transformer
(T) to step up the AC voltage to well beyond 1000 V rms
(root mean square value) for lamp ignition. The output
voltage is connected to the lamp through a high voltage
capacitor (C
2
). Typically, the impedance of this capacitor at
the inverter oscillating frequency is several times higher
than the lamp impedance.
Before ignition, the lamp does not conduct electricity and
behaves as an open circuit. As a result, 100% of the output
voltage (1000+ V) is applied to the lamp for ignition. After
ignition, the lamp current causes a voltage drop across the
capacitor C
2
. This voltage drop increases as the lamp
current increases until an equilibrium is established where
the vector sum of this voltage drop and the lamp voltage
equals to the output voltage of the transformer. That is, if I
is the lamp current, V is the output voltage of the
transformer, V
lamp
is the CCFL operating voltage at the
lamp current I, then:
V = V
lamp
+ I x 1/(j2 fC
2
)
where f is the oscillating frequency, and j = (-1)
½
which is
the base of imaginary numbers.
In the above equation, symbols in boldface are vectors (or
complex numbers). Typically, a CCFL behaves very close
to a resistance. Therefore, V
lamp
is nearly in phase with V
where the second term I x 1/(j2 fC) is -90
o
out of phase with
V.
Thus, by selecting the value of the capacitor C
2
, we can adjust
the final equilibrium condition to provide the recommended
operating lamp current.
Backlight Dimming
One of the main functions provided by the control circuit is
the lamp dimming. In general, there are two types of dimming
methods:
1. Linear dimming (or dimming by reducing lamp current)
2. PWM (pulse width modulation) dimming
In linear dimming, the lamp luminance is adjusted down by
reducing the lamp current. Consequently, the lamp stays
ON all the time but is operating at a lower current. Linear
dimming is commonly used in inverters for notebook PCs and
desktop monitors where a very limited dimming range of 2 -
5 (i.e., brightness varying from 100% to 50% or 20%
respectively) is adequate. With carefully designed inverters
and backlights, linear dimming can provide a luminance
adjustment range up to about 40 (100% down to 2.5%).
Beyond that, linear dimming does not perform too well due
to the following reason:
The voltage - current relationship of a CCFL does not follow
the Ohms law. For the first order approximation, the voltage
across a CCFL is nearly independent to the lamp current (in
fact, the lamp voltage increases slightly as the lamp current
reduces). Consequently, the lamp impedance increases almost
inversely with the lamp current. Therefore at low lamp
current, the lamp impedance becomes very high. For example,
when a

lamp is operated at 5% of its normal current to achieve
a 20:1 dimming, the lamp impedance increases about 20 times,
reaching 1,000K to 3,000K for most of the lamps in
Landmark backlights. At this level, the effects due to the stray
capacitance (i.e., from the lead wires and the lamps to the
Landmark Technology, 172 Component Drive, San Jose, CA 95131
(408) 434-9302 Fax: (408) 434-0954,

03/2000, Rev 02/2002
Landmark Technology Backlight Inverters
Landmark Technology
Technote TK0300
P1 backlight case and to the ground) become significant. As a
result, a portion of the current normally going through the
lamp leaks away through the stray capacitance. To make it
worse, since the stray capacitance associated to each lamp
is different, the amount of current leaking away also differs
from lamp to lamp. The net effect is that some of the lamps
may be operated at a lower current making them dimmer
than the others. At the extreme case, certain lamps may even
quit where the others are still brightly lit.
In the case of PWM dimming, the lamp current is pulse width
modulated at a repetition rate high enough to prevent LCD
screen flicker. Within each PWM cycle, the lamps are turned
fully ON for a fraction of the cycle time. The human eyes,
being very slow with respect to the PWM rate, response to
the average light produced over the PWM cycle.
Consequently, the luminance of the backlight and/or the
LCD screen is approximately proportional to the duty cycle
of the PWM waveform (Fig. 1).
In general, inverters with PWM dimming have a very wide
luminance adjustment range for the following reason. In
PWM dimming, the lamp is operated at its full current during
the time when the lamp is ON. So,the lamp impedance is
typically in the 50 to 150 K region. Since the lamp
impedance is quite low, the effects due to the stray
capacitance are not significant.
With the PWM dimming, some inverters designed for
military use can achieve a dimming ratio higher than 2,000:1.
That is, the luminance of the backlight can be adjusted from
100% down to less than 0.05%!
Pulse width modulating the lamp current at a high repetition
rate can generate a significant amount of noise. Thus, PWM
inverters are in general noisier than the linear inverters.
However, when the backlight is running at its maximum
brightness, the PWM duty cycle is usually at 100%.
Consequently, the lamp

current waveform is a continuous sine
wave which does not generate too much EM interference.
Inverter Open Circuit Output Voltage
The transformer in the inverter steps up the voltage to beyond
1,000 Vrms for lamp ignition. Before ignition, the lamp does
not conduct electricity and thus behaves as an open circuit.
So, this ignition voltage is frequently called the inverter open
circuit output voltage.
The voltage required to ignite a lamp depends on the length
of the lamp, the temperature, and the aging of the lamp. For
example, the CCFL used in Landmark 10.4" backlight is 220
mm long. Its ignition voltage at 25
o
C temperature is about
500 V. However, the lamp used in our 20.1" backlight is 420
mm long and its ignition voltage at the same condition is 830
V. When the temperature drops to 0
o
C, the ignition voltage
increases by about 40%. Moreover, when the lamp ages, its
ignition voltage also increases. For example, after 10,000
hours of operation, the ignition voltage increases by about
20%.
In order to ensure that the lamp will be turned on under the
worst condition (i.e. at low temperature and after the lamp is
well aged), it is common to design the inverter open circuit
output voltage more than twice higher than the minimum
ignition voltage of the lamp at 25
o
C.

Landmark Technology
Technote TK0300
P2
Fig. 1
Landmark Technology, 172 Component Drive, San Jose, CA 95131
(408) 434-9302 Fax: (408) 434-0954,

03/2000, Rev 02/2002 The BI200A PWM Inverter

In early 1998, Landmark Technology introduced the
BI200A inverter with 200:1 (100% down to 0.5%) PWM
dimming capability. This inverter can drive all Landmark
10.4" and 12.1" backlights and LCD modules. Examples
such as the C053G, BK103A, BK106A (10.4") backlights,
LM073A, LM121A (10.4"), LM110A, LM114A, LM123
(12.1"). LCD modules.
The BI200A inverter can deliver up to 24 Watts lamp driving
power. It is operated at 12V and its efficiency (defined as
output lamp power divided by the DC input power at the
peak conditions) is about 75%.
The open circuit output voltage of BI200A is about 1,300 V
and therefore is suitable for lamps up to about 320 mm long,
which is enough to cover backlights and LCD modules up
to about 15" size. It is possible to use BI200A to drive longer
lamps. However, the low temperature (for example, 0
o
C or
below) performance will not be very satisfactory.
When the BI200A is operated at a dimming ratio near 200:1
or higher, the PWM circuit is running at a very low duty
cycle. The Royer oscillator generates a burst oscillation of
one or a few cycles and is then forced to stop by the PWM
circuit. During this burst, the inverter draws a large transient
current that in some cases can pull down the 12 V supply
and potentially causes instability of the burst oscillation.
When this happens, the backlight flickers. On the other hand,
if the power supply has an exceptionally