LED7707TR datasheet(32/48 Pages) STMICROELECTRONICS

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Application information

LED7707

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6.3.3

Flywheel diode selection

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 75 mA each

(i.e. 450 mA output current) and the maximum output voltage (36 V), the STP1L40M

(If,ave = 1 A, Vr = 40 V) diode is a good choice. Smaller diodes can be used in applications

involving lower output voltage and/or lower output current.

6.4

Design example

In order to help the design of an application using the LED7707, in this section a simple

step-by-step design example is provided.

A possible application could be the LED backlight in a 17” LCD panel using the LED7707.

Here below the possible application conditions are listed:

●

VIN = 12 ± 10 %

●

4 strings of 42 white LEDs (60 mA) each (arranged in 6 rows, 7LEDs per row)

●

VF, LEDs = 3.5 V ± 200 mV

6.4.1

Switching frequency setting

To reduce the number of the external components, the default switching frequency is

selected (660 kHz typ.) by connecting the FSW pin to AVCC pin.

However, in case a different switching frequency is required, a resistor from FSW pin and

ground can be connected, according to the equation (5) in section 4.1.5.

6.4.2

Row current setting

Considering the equation 9 in Section 5.2.1, the RRILIM resistor can be calculated as:

Equation 24

The closest standard commercial value is 30 k

Ω. The actual value of the row current will be

a little lower (61.7 mA).

6.4.3

Inductor choice

The boost section, as all DC-DC converters, can work in CCM (continuous conduction

mode) or in DCM (discontinuous conduction mode) depending on load current, input and

output voltage and other parameters, among which the inductor value.

In a boost converter it is usually preferable to work in DCM.

Once the load, the input and output voltage, and the switching frequency are fixed, the

inductor value defining the boundary between DCM and CCM operation can be calculated

as:

1850

ROW

RILIM

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