t =

F

C

OUT

I

LED

´ (V

V

)

INIT

HVM

-

V

(V)

LED

V

(V)

LED

V

(V)

LED

V

INIT

V

OPT

V

INIT

V

OPT

V

INIT

V

OPT

(c)FasterLEDSwitchingwithLargeC

: TightLEDSelectionNeeded

OUT

(b) FasterLEDSwitching:NotEnoughSettlingTime

(a) TypicalSettingwithAppropriateLEDSwitching

t

F

t

R

Time(s)

Time(s)

Time(s)

TLC5960

SBVS147 – SEPTEMBER 2010

www.ti.com

Figure 30(a) shows a typical switching of the on-off sequence in this conventional system. Here, VLED (the output

voltage of the dc/dc converter) starts from a higher point (VINIT). As the drain node information feedback loops,

the VLED is driven lower to the appropriate voltage level (VOPT) for lower power consumption while maintaining

the constant current regulation. After the signal feedback, the VLED transient speed is limited by the primary loop

response speed and the slowest timing in this sequence is normally the falling edge of VLED. The maximum

speed of the falling time (tF) is calculated in Equation 18, and only the constant current of ILED is available to

lower the VLED in an LED backlight system.

(18)

Figure 30. VLED Voltage Regulation Sequence on Conventional System

In this case, with COUT = 100µF, ILED = 100mA, VINIT = 180V and VOPT = 155V, the maximum speed of tF is

calculated as 25ms. This slower transient speed absolutely limits the LED switching speed. If maximum tF is

25ms, the fastest dimming cycle with a full dual-loop system advantage is limited to only 40Hz. If the LED

switching speed is faster than that value, the VLED regulation voltage would appear as shown in Figure 30(b).

HVM power saving is only achievable with one-fourth of the typical case (a).

To solve this situation on a conventional system, COUT can sometimes be increased to keep the voltage stable.

Figure 30(c) shows that switching waveform. These possible power-savings are achieved by lowering VINIT using

tightly selected LEDs in relation to its forward voltage specification.

26

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