SAMSUNG BN-00269A LED DRIVE AND POWER SUPPLY
SAMSUNG BN-00269A
PSLF171B01A
LED DRIVE AND POWER SUPPLY
USING ICS MC33067 – FAN7530 – HV9911
SMPS
D.C. to DC convertor and DC to AC converter belong to the category of switched mode power supply (SMPS). The
various types of voltage regulator used in linear power supplies (LPS),
fall in the category of dissipative regulator, as they have a voltage
control element usually transistor or zener diode which dissipates power
equal to the voltage difference between an unregulated input voltage
and a fixed supply voltage multiplied by the current flowing through
it. The switching regulator
acts as a continuously variable power converter and hence its efficiency
is negligibly affected by the voltage difference.
LED DRIVE WORKING
There
are large arrays of LEDs located behind the LCD panel in a typical LCD
TV LED. 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 .
High
brightness LEDs used in LCD backlighting require high LED current which
also equates to higher LED forward voltage . 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 .
Boost Converter Driving
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 ., 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 .
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 5) 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 .
POWER SUPPLY CIRCUIT
