LED7708TR datasheet(24/53 Pages) STMICROELECTRONICS

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Recommended operating conditions

LED7708

24/53

Doc ID 022698 Rev 1

in order to guarantee enough steering room for the optimization. Because of the significant

dependence of the LED forward voltage with temperature, this must be taken into account

when calculating the minimum expected output voltage.

The best way to proceed when designing the external components is the following:

Determine the output voltage range required by the LED strings, considering VF spread

and temperature variation:

Calculate the middle-point of the output voltage range VBOOST,middle.

Calculate the output divider through Equation 4 and 5.

Calculate the resulting four output voltage ranges (VMIN setting options).

Select the output voltage range closest to the one calculated at point 1.

Equation 4

Equation 5

8.3.2

Boost converter loop

The voltage drop of each current generator is sent to the minimum voltage selector: the

lowest one is used as feedback and sent to the window comparator, whose outputs are used

to increase or decrease the loop-counter. The digital outputs of the counter are connected to

the DAC and the target reference is generated. As previously mentioned, this reference

voltage is compared to a partition of the output voltage: the difference is translated into an

error current at the output of the trans-conductance amplifier (see Figure 4, COMP pin).

At this point the compensation network, externally connected to the COMP pin, provides the

programmed trip level for the inductor current. An additional slope compensation ramp is

mixed, as usual, to avoid possible sub-harmonic instability when the boost converter

operates in continuous conduction mode (CCM). The SLOPE pin allows the proper setting

of the amount of slope compensation connecting a simple resistor RSLOPE between the

SLOPE pin and ground. The compensation ramp of a master device starts at 35% (typ.) of

each switching period.

The boost converter switching frequency can be set in the 200 kHz-1 MHz range by

connecting the FSW pin to ground through a resistor, calculated according to Equation 6:

Equation 6

)

(

max

LED

LE DS

RO WS

BOOST middle

VFB

–

()

VF B

---------------------------------------------------------------

⋅

BOOST MAX

---------------------------------- 100

μA

≤

FSW

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