LED7706 Application information 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.
Table 7.
Recommended inductors
Manufacturer Part number
Coilcraft
Wurth
LPS6235-682MLC
7440650068
Description
6.8
μH, 75 mΩ, 2.7 A
6.8
μH, 33 mΩ, 3.6 A
Size
6x6 mm
10x10 mm
The input and output capacitors should have very low ESR (ceramic capacitors) in order to minimize the ripple voltage. The boost converter of the LED7706 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 30 mA/channel), two 2.2 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 LED7706 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 capacitors
Manufacturer
Taiyo Yuden
Murata
Murata
Part number Description Package Notes
UMK325BJ106KM-T Ceramic, 35V, X5R, 20 % SMD 1210 C
IN
GRM31CR71H225KA88B Ceramic, 50V, X7R, 20 % SMD 1206 C
OUT
GRM31CR71H475KA88B Ceramic, 50V, X7R, 20 % SMD 1206 C
OUT
The flywheel diode must be a Schottky type to minimize the losses. This component is subject to an average current equal to the output one and must sustain a reverse voltage equal to the maximum output rail voltage. Considering all the channels sinking 30mA each
(i.e. 180 mA output current) and the maximum output voltage (36 V), the STP1L40M
(I f,ave
= 1 A, V r
= 40 V) diode is a good choice. Smaller diodes can be used in applications involving lower output voltage and/or lower output current.
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