The choice of architecture for an LED backlit driver system is the decision with the greatest potential to produce power savings and significantly enhanced picture quality. The designer looks for the best balance between local control of strings of LEDs and the lowest possible bill of materials (BOM).
Single string and single DC-DC converter
A switched-mode power supply (SMPS) is used to provide the voltage for backlit LEDs arranged in strings. A current sink is included to regulate the current through the LED string. To minimize power dissipation, the voltage at the ILED sink needs to be a fraction above the voltage necessary to guarantee that the LEDs receive their specified current (see figure 2).
Figure 2: Single-string, single DC-DC converter backlight system architecture
A common design approach is to use a feedback path from the ILED sink to the SMPS to regulate the SMPS’ output voltage. This feedback path is required to allow for variations in forward voltage (Vf) from one LED to another. The typical Vf of a white LED is around 3.2 V, and may vary as much as ±200 mV per LED. So, for example, in a string of 10 LEDs, the total for VLED may range from 30 V to 34 V.
The voltage required at the DC-DC converter can be expressed as:
VSINK is assumed to be 0.5 V, so the ILED sink must regulate VDC-DC in the range of 30.5 V to 34.5 V, depending on the actual LED forward voltages. Multi-string and multi DC-DC converter
A single string of LEDs is rarely adequate because as the number of LEDs in the string rises the required output voltage also rises. Above a certain VOUT/VIN ratio, the SMPS’s efficiency falls dramatically. LED backlight designers can therefore use several strings in order to avoid an excessively high output voltage required of the SMPS.
The simplest approach is to duplicate the single-string single DC-DC converter topology at each string (see Figure 3). The advantage is efficiency, because each string’s voltage is regulated separately. The disadvantage is the high cost, since each string requires its own DC-DC converter, MOSFET, coil, diode, and output capacitor. In order to save bill of material (BOM) cost, the designer could reduce the number of LED channels, using long strings with many LEDs in each string. But this compromises the system’s ability to implement local dimming, which is another important power saving technique. Therefore, none of the trade-offs of this topology is particularly attractive.
Figure 3: A separate DC-DC converter with each LED string is an expensive option Click on image to enlarge