Friday, November 20, 2015






USING ICS BD9204F – STRA6000 – NTMD4820 –NCP33262 –NCP1396

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 .

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. hence the switching regulator is also known as “non-dissipative regulator” in a SMPS, The input DC supply is chopped at a higher frequency around 15 to 50KHz using an active device like the BJT power MOSFET or SCR and the convertor transformer .