Design Article
LED driving techniques reduce power in LCD TVs
Peter Rust, Werner Schögler, Manfred Pauritsch, Herbert Truppe, ams
1/15/2013 2:19 PM EST
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 (see figure 7).
While there are a variety of current sink designs, the precision requirements of TV backlighting (current regulation better than +/- 0.5 percent) 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 – AS369x, AS381x, AS382x, AS385x -- have 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 percent 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
Click on image to enlarge
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 (see figure 7).
While there are a variety of current sink designs, the precision requirements of TV backlighting (current regulation better than +/- 0.5 percent) 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 – AS369x, AS381x, AS382x, AS385x -- have 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 percent 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
Click on image to enlarge
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docdivakar
1/16/2013 10:23 PM EST
Good article! The authors claim that the 'edge-lit LED backlighting TV's provide good optical uniformity in screen sizes up to 40"...' but there are many brands that go higher, up to 60" that display reasonably good pictures. Perhaps these larger ones employ more sophisticated light guides?
MP Divakar
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William Miller
4/1/2013 10:09 AM EDT
This technology is growing so fast) 15 years ago I had a lamp TV. 5 years ago I bought a plasma. A year ago I replaced it for a LED Tv. Now what? I will have to buy this new direct backlit system TV? I know it's more efficient. But at the moment I have doubts..
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William - http://www.carid.com/
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anne-francoise.pele
4/12/2013 8:55 AM EDT
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