LED
TV back light led voltage regulator working
There
are large arrays of LEDs located behind the LCD panel in a typical LCD TV LED
back-lighting system as shown in Figure below . In this array are a large number
of parallel channels of LEDs connected in series depending on the size of the
TV and the type of back-lighting, for example edge back-lighting (less LEDs but
more in series) or direct back-lighting (more LEDs in parallel) . The LED
voltage (VLED) is provided by the White LED Back-light Driver Board to each LED
channel and is regulated to a level needed by the highest voltage required to
maximize the light output of each LED string .
Depending
upon the power supply requirements determined by the number of LEDs in the
string or grouping of parallel LED strings, the up-stream power source for the
LED back-light driver board may be a DC/DC step-up boost converter, a DC/DC
step-down converter or more commonly an AC/DC converter . In the case where
supply voltage is lower than the required VLED, a step-up boost converter will
be used . As an example, a LED boost converter LED back-lighting system will be
described in detail in this paper for a direct back-lighting application,
however the theory of operation will also apply to both the step-down converter
and AC/DC converter situation.
LED
Forward Current (IF) vs. Forward Voltage (VF)
Simplified
LED Backlight LCD TV Block Diagram and Direct-type LED Array.
Typical
LED Forward Current (IF) vs. Forward Voltage (VF).
LED
String with a Current Sink
Figure
below shows an LED backlight string with a controller which sets the
brightness level with current sinks . VCS must be large enough in order for the
current sink to operate properly . The lowest VCS of all of the LED strings
will be the LED string with the largest composite forward voltage . The
regulated VLED must be greater than VCS plus the highest composite LED forward
voltage . If this condition is met, all the strings will have VCS greater than
the dropout voltage (see the Dropout Voltage section of this document) and thus
all current sinks will operate properly . In Figure 4, the LEDs in Ch . 1
(black) have a VF equal to 3 .5V and the LEDs in Ch . 2 have VF equal to 3 .2V
. So if the highest voltage condition is met, each LED current sink will have
sufficient headroom to provide accurate current to set the light output
correctly . If VLED is regulated too low, the highest voltage LED string will
be out of regulation and enter dropout, minimizing output current . On the
other hand, if VLED is regulated too high, each LED string will have more than
enough regulated voltage and good output current accuracy but unnecessary power
will be dissipated across the current sink, resulting in extra power and heat
dissipation.
Series
LED String with Current Sink Control.
LED
Array, 16 Channels of 10 LEDs Connected in Series.
The
LEDs in this example need 70mA to effectively backlight the LCD panel, but the
Ch .1 LEDs have a higher VF than Ch . 2, requiring 35V to get 70mA .
Boost
Converter Driving LED Array
In
a system where supply voltage (VIN) is lower than VLED, a step-up boost converter
is used to provide power to the LED strings . As shown in Figure bellow, the
output voltage of the boost converter drives all the LED strings . The feedback signal CSFBO connected to CSFB on the boost converter provides the lowest VCS
level from all the LED strings and also controls how the VLED voltage is
regulated . When the CSFB voltage is lower than the dropout voltage that is
necessary for the LEDs to operate correctly, the stepup converter will boost
the VLED level . However, when the CSFB voltage is higher than the dropout
voltage, the step-up converter will stop boosting VLED . During this time, the
LED current is provided by the boost output capacitor (C5) . This boost is set
as a forced PWM system, so the pass switch (T1) will turn on with a minimum
on-time (unless current limit or OVP is reached) to provide output current to
the LEDs as well . At some point when the required LED current is higher than
the current provided by the boost capacitor and the minimum on-time of T1, the
VLED will start to drop and CSFB will go below the dropout voltage . At that
time, the step-up converter will start boosting the VLED voltage level . Normal
operation of the AAT2404 and AAT2403 is shown in the scope shots in Figures.The
OVP setting on the boost is calculated to be at least 10% greater than the
total forward voltage of each LED string plus the minimum VCS .
Example
LED Back-light with Boost Controlled VLED.
Normal
Operation of AAT2404 and AAT2403 with all Current Sinks (CS) in
the AAT2403 set to 100mA, 80% DOT Correction (80mA) and 50% Gray Scale, No
Delay.
While
all CS are OFF, the VOUT level of the AAT2404 is at the OVP level . While all
CS are ON, the VOUT level of the AAT2404 is regulated to the voltage
required for the LEDs to sink 80mA
in
the AAT2403 set to 100mA, 50% DOT Correction (50mA) and 50% Gray Scale, No
Delay.
While
all CS are OFF, the VOUT level of the AAT2404 is at the OVP level . While all
CS are ON, the VOUT level of the AAT2404 is regulated to the voltage
required for the LEDs to sink 50mA .
Normal
Operation of AAT2404 and AAT2403 while AAT2404 is being set to 100mA,
100%
PWM Duty Ratio (fully turned on at 100mA).
Dropout
Voltage
The
dropout voltage is the voltage level when the current sinks in the backlight
controller start to go out of current regulation . This is also the voltage
level used as the reference for the power supply converter . For a current sink
to operate with good accuracy, VCS has to be greater than the dropout voltage .
As shown in Figure 9 for the AAT2403, the dropout voltage will be different
depending upon the maximum current requirement and is proportional to the
current sink current (ILED) and RSET value (R2 in Figure 5) . When using the
AAT2404 boost converter, the same values of RSET should be used for both
AAT2403 and AAT2404 . However for a stand alone power supply working with
AAT2403, the dropout voltage is the reference voltage for the converter which
should be determined by the RSET value .
Dropout
Voltage is the VCS Voltage Level Point where ILED starts to Roll Off
(100%
PWM Duty Ratio and 100% DOT).
Startup
During
startup, since LED strings are not yet turned on, the CSFB voltage is at 0V, so
the boost converter will start stepping up VLED . In the case of the
AAT2404, the startup voltage soft starts to 90% of the OVP voltage . This
provides enough voltage level for VLED when the first LED string turns on
. The boost converter will not step up full VLED until the CSFB voltage is
below the reference (dropout) voltage .
Sample
and Hold During ON/OFF
In
some systems it is desirable to maintain the operating LED voltage when the LED
current sinks are OFF . When the LEDs are OFF, the voltage across the LED
string decreases . When the LED string turns off, the current sink voltage
will rise . Without a sample and hold technique, the LED voltage will
regulate down in order to drive the current sink voltage to the regulation
point even though the LED string is OFF . Since there is no power consumed when
the LED string is off, regulating the current sink voltage during the OFF
time of the LED string is unnecessary . A potentially unwanted effect of
regulating the LED voltage during the OFF time is that additional time is
necessary to establish the proper LED voltage when the current sink is turned
back on as the LED voltage slews to the required voltage level . During this
time, the current in the LED string will not be regulated and will tend to
be less than the final desired LED current level . For the AAT2404, when
the external current sinks are ON, the CSFB is regulated to the internal
reference (dropout) voltage . When the external current sinks are OFF or
CSFB voltage is greater than an internal set voltage (2 .5V), the LED
voltage is determined by the voltage level left on the on the compensation
capacitor (C3 in Figure ) which has been disconnected from the feedback
loop . During this OFF time, since the inductor current is proportional to the
compensation capacitor’s voltage, VLED will not decrease and will be either
held or increase slightly until required to turn on the LEDs . In the
extreme case when no LED current is drawn, VLED could be charged as high as
OVP, which will provide enough voltage level headroom for the next current
sink to turn back on . In this case, VLED will operate between OVP and
the minimum VLED . This will eliminate the unwanted effect of converter
response time and current slewing when the LED is turned back on again .
To
disable the sample and hold feature on the AAT2404, a 10k resistor can be
inserted from CSFB to ground . VLED will be regulated even when the current sinks are OFF, causing current slewing
during turn on .
TV
LED Backlight Drivers and Related Devices
The
following devices have a Voltage mode CSFBO output (and input for cascading
ICs):
AAT2401
AAT2402S
AAT2403
The voltage mode feedback of each of these ICs is intended for direct interfacing to the AAT2404 boost converter or in master/slave operation with the AAT2400 or AAT2402M .
When using these devices as a single driver or if the device is the first IC in a cascaded chain of ICs, then the CSFBI input pin should be terminated to VCC to remove it from the system . The reference voltage (dropout voltage) is unique to each external power supply and can be determined as described in the Dropout Voltage.
AAT2401
AAT2402S
AAT2403
The voltage mode feedback of each of these ICs is intended for direct interfacing to the AAT2404 boost converter or in master/slave operation with the AAT2400 or AAT2402M .
When using these devices as a single driver or if the device is the first IC in a cascaded chain of ICs, then the CSFBI input pin should be terminated to VCC to remove it from the system . The reference voltage (dropout voltage) is unique to each external power supply and can be determined as described in the Dropout Voltage.
