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Save power in LCD TVs with LED driving techniques

Posted: 29 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:direct backlit? LED backlighting? HD plasma? OLED?

This solution offers the best overall power efficiency because it combines the advantage of local dimming in direct backlit systems with good DC-DC output voltage regulation. It also offers a substantial BOM saving over the efficient multi-string, multi DC-DC converter architecture.

Regulating current to match the characteristics of LEDs
The LED manufacturing process causes wide variations in brightness and colour temperature from one LED to the next. As a guide to users, white LED manufacturers allocate each manufactured unit to groups or 'bins' of LEDs with comparable performance in terms of colour, brightness and forward voltage. But the manufacturer's specification for each brightness and colour temperature bin is only valid under specific nominal operating conditions. This means that the LED current must be set to the nominal current stated in the datasheet in order to generate the specified brightness and colour.

Consequently, dimming and brightness control can only be implemented by switching the current to any single LED either to ON (nominal current) or OFF (zero current) through a digital PWM control signal. In analogue dimming, the LED would be operating outside its specified nominal current, leading to unacceptable changes in colour temperature and poor LED-to-LED brightness matching (figure 6).

Figure 6: Brightness of LEDs from the same bin is guaranteed to match only at nominal current (in this case, 20 mA).

Current sink characteristics
Since LEDs require a perfectly regulated constant-current power supply, it follows that the primary role of the LED driver is to set the current to the nominal value when ON and to 0 A when OFF. Therefore, the feedback loop controlling the accuracy of regulation requires an extremely precise current sink (figure 7).

While there are a variety of current sink designs, the precision requirements of TV backlighting (current regulation better than 0.5 per cent) require an accurate op amp to set the ILED current independent of the ILED voltage. But in backlighting driver applications, the task is more challenging because the accuracy of current regulation must be maintained even when the voltage at the current sink falls to very low levels.

This is a difficult requirement to meet but 4 generations of very accurate current sink LED drivers from ams C AS369x, AS381x, AS382x, AS385xhave been designed specifically for such applications. These devices also incorporate an offset-compensated op amp. Current sink drivers require a minimum voltage at the drain (VDS(sat)) to ensure the full accuracy and proper operation of the sink transistor inside the saturation region. For the saturated region the gate-source voltage primarily controls the output current.

If the current sink is to operate at high efficiency, it is important that the voltage drop between VSET and VDS is low. LED drivers with op amps that include built-in offset cancellation can maintain VSET at levels as low as 125-250 mV. Allowing an additional margin for VDS above VDS(sat) of 150 mV, a total voltage drop at the current sink of approximately 400 mV is necessary. For a string of eight LEDs (where Vf = 8 x 3.2 = 25.6 V) this results in a power loss of around 1.5 per cent in ISINK. Without the offset cancellation included in ams' backlight LED drivers, the value of VSET would be higher, leading to higher power losses at the current sink.

Figure 7: Current sink designs; a precision current sink requires an accurate op amp with offset compensation.

Feedback regulation for power optimisation
As has been shown above, a feedback path from the LED driver to the SMPS sets the drain voltage to the minimum required value. The output current sink can be implemented either with a simple, defined current output driver and an external capacitor (figure 9, left-hand diagram) or with a digital control circuit which sets attack/release times and controls the current output with a digital-to-analogue converter (IDAC) (figure 8, right- hand diagram).

Figure 8: Two different methods for implementing a feedback loop to the SMPS.

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