LED TV BACK LIGHT LED VOLTAGE REGULATOR WORKING PRINCIPLE
LED TV BACK LIGHT LED VOLTAGE REGULATOR WORKING PRINCIPLE
There are large arrays of LEDs located behind
the LCD panel in a typical LCD TV LED backlighting 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 backlighting, for example edge
backlighting (less LEDs but more in series) or direct backlighting (more LEDs
in parallel) . The LED voltage (VLED) is provided by the White LED Backlight
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 backlight 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 backlighting system
will be described in detail in this paper for a direct backlighting application,
however the theory of operation will also apply to both the step-down converter
and AC/DC converter situation.
Simplified LED Backlight LCD TV
Block Diagram and Direct-type LED Array.
LED Forward Current (IF) vs.
Forward Voltage (VF)
High brightness LEDs used in LCD backlighting
require high LED current which also equates to higher LED forward voltage . A
typical relationship between TV backlight LED forward current (IF) and forward
voltage (VF) is illustrated in Figure below. For example, if a user wants to
set the LED current to 80mA maximum, a minimum of 3 .65V forward voltage must
be provided to each LED in the string . If the power supply can only provide 3
.6V to each LED, then the maximum LED current is limited to 74mA .
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.
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 .
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 Backlight 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
Normal Operation of AAT2404 and
AAT2403 with all Current Sinks (CS)
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 .
be regulated even when the current sinks are OFF, causing current slewing during turn on .
TV LED Backlight Drivers and
Related Devices
he 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.
