LED7707 STMicroelectronics, LED7707 Datasheet - Page 31

• Manufacturer Part Number: LED7707
• Description: LED Drivers 6-ROW 85mA LED DRIV BOOST REG LCD PANEL
• Manufacturer: STMicroelectronics

Specifications of LED7707

• High Level Output Current: 85 mA
• Operating Supply Voltage: 4.5 V to 36 V
• Maximum Supply Current: 1 mA
• Maximum Power Dissipation: 2.3 W
• Maximum Operating Temperature: + 150 C
• Mounting Style: SMD/SMT
• Package / Case: VFQFPN-24
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

LED7707
6.3.3
6.4
6.4.1
6.4.2
6.4.3
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
(I
involving lower output voltage and/or lower output current.
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:
●
●
●
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.
Row current setting
Considering the equation 9 in
Equation 24
The closest standard commercial value is 30 kΩ. The actual value of the row current will be
a little lower (61.7 mA).
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:
f,ave
V
4 strings of 42 white LEDs (60 mA) each (arranged in 6 rows, 7LEDs per row)
V
= 1 A, V
IN
F, LEDs
= 12 ± 10 %
= 3.5 V ± 200 mV
r
= 40 V) diode is a good choice. Smaller diodes can be used in applications
R
RILIM
Section 5.2.1
=
I
ROW
K
R
=
, the R
1850
60
mA
RILIM
V
=
resistor can be calculated as:
30
.
83
k
Ω
Application information
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