LED7707 datasheet(31/48 Pages) STMICROELECTRONICS

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LED7707

Application information

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6.3

Component selection

6.3.1

Inductor selection

Being the LED7707 mostly dedicated to backlighting, real-estate applications dictate severe

constrain in selecting the optimal inductor. The inductor choice must take into account

different parameters like conduction losses (DCR), core losses (ferrite or iron-powder),

saturation current and magnetic-flux shielding (core shape and technology).

The switching frequency of the LED7707 can be set in the 200 kHz-1 MHz range, allowing a

wide selecting room for the inductance value. Low switching frequencies takes to high

inductance value, resulting in significant DCR and size. On the other hand, high switching

frequencies result in significant core losses. The suggested range is 4.7-22 µH, even if the

best trade off between the different loss contributions varies from manufacturer to

manufacturer.

A 6.8 µH inductor has been experimentally found as the most suitable for applications

running at a 660 kHz switching frequency.

6.3.2

Capacitors selection

The input and output capacitors should have very low ESR (ceramic capacitors) in order to

minimize the ripple voltage. The boost converter of the LED7707 has been designed to

support ceramic capacitors. The required capacitance depends on the programmed LED

current and the minimum dimming frequency (the boost converter is off when the DIM pin is

low and the output capacitor is slowly discharged). Considering the worst case (i.e. 200 Hz

dimming frequency and 85 mA/channel), two 4.4 µF MLCCs are suitable for almost all

applications. Particular care must be taken when selecting the rated voltage and the

dielectric type of the output capacitors: 50 V rated MLCC may show a significant

capacitance drop when biased, especially in case of Y5V dielectric.

As in most of boost converters, the input capacitor is less critical, although it is necessary to

reduce the switching noise on the supply rail. The input capacitor is also important for the

internal LDO of the LED7707 and must be kept as close as possible to the chip. The rated

voltage of the input capacitor can be chosen according to the supply voltage range; a 10 µF

X5R MLCC is recommended.

Table 8.

Recommended inductors

Manufacturer

Part number

Description

Size

Coilcraft

LPS6235-682MLC

6.8

μH, 75 mΩ, 2.7 A

6x6 mm

Coilcraft

XPL7030-682ML

6.8

μH, 60 mΩ, 5.8 A

7x7 mm

Wurth

7440650068

6.8

μH, 33 mΩ, 3.6 A

10x10 mm

Table 9.

Recommended capacitors

Manufacturer

Part number

Description

Package

Notes

Taiyo Yuden

UMK325BJ106KM-T

Ceramic, 35V, X5R, 20 %

SMD 1210

CIN

Murata

GRM31CR71H225KA88B

Ceramic, 50V, X7R, 20 %

SMD 1206

COUT

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