LED7708 datasheet(26/53 Pages) STMICROELECTRONICS

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Recommended operating conditions

LED7708

26/53

Doc ID 022698 Rev 1

Figure 6.

Effect of slope compensation on small inductor current perturbation

(D>0.5)

The SLOPE pin allows to properly set the amount of slope compensation connecting a

simple resistor RSLOPE between the SLOPE pin and the output. The compensation ramp

starts at 35% (typ.) of each switching period and its slope is given by the following equation:

Equation 7

where KS = 4.0 s

-1 and S

E is the slope ramp in [A/s].

To avoid sub-harmonic instability, the compensating slope should be at least half the slope

of the inductor current during the off-phase when the duty cycle is greater than 50%. The

value of RSLOPE can be calculated according to Equation 8.

Equation 8

8.3.4

External MOSFET control

The LED7708 employs an external MOSFET as the power switch for the boost converter.

The internal gate driver is capable of sourcing and sinking up to 1 A and it is supplied

through the VDR pin, internally connected to the +5 V LDO. A 1 µF decoupling capacitor,

connected between the VDR pin and PGND, is highly recommended. When an external +5

V supply is available, the whole device can be supplied by connecting both the VDR and VIN

pins to this rail.

SLOPE

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

SLOPE

OUT

–

0.7V

–

()

⋅⋅

⋅

OUT

–

()

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

≤

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