Design example. STMicroelectronics LED7706

Application information LED7706

6.4.1

In order to help the design of an application using the LED7706, in this section a simple step-by-step design example is provided.

A possible application could be the LED backlight in a 15” LCD panel using the LED7706.

Here below the possible application conditions are listed:

●

●

●

V

IN

= 12 ± 20 %

2 strings of 48 white LEDs (20 mA) each (arranged in 6 rows, 8 LEDs per row)

V

F, LEDs

= 3.5 V ± 200 mV

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 5.1.5

.

Considering the equation 10 in

Section 5.2.1

, the R

RILIM

resistor can be calculated as:

Equation 25

R

RILIM

=

K

R

I

ROW

=

987

20

V mA

=

49 .

35 k

Ω

The closest standard commercial value is 51 k

Ω. The actual value of the row current will be a little lower (19.3 mA).

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:

Equation 26

L

B

=

R

0

⋅

D

⋅

(

1

−

D

)

2

2

⋅

F

SW where D is the duty-cycle defined as:

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