STLA02PUR Datasheet, PDF (15/23 Pages) STMicroelectronics – White LED driver for display backlight

STLA02
Introduction
For nominal operation, the peak inductor current can be calculated by the formula:
Equation 2
[ ] ( ) _IPEAK
=
⎪⎨⎧⎜⎛
⎪⎩⎝
I_OUT
ν
⎟⎞ +
⎠
V_OUT − V_IN ∗ V_IN2
2 ∗ L ∗ f ∗ V_OUT2
⎪⎫
⎬
⎪⎭
∗
V_OUT
V_IN
where:
I_PEAK
I_OUT
ν
Peak inductor current
Current sourced at the V_OUT pin
Efficiency of the STLA02
V_OUT
V_IN
L
Output voltage at the V_OUT pin
Input voltage at the V_IN pin
Inductance value of the inductor
f
Switching frequency
For the optimal function of the STLA02 device, it is recommended to use the inductor value
10 µH and higher with low serial resistance and relevant saturation current calculated from
the equation above.
R_FB value
R_FB = V_FB / I_LED
In the case of a typical setting I_LED = 20 mA, V_FB = 200 mV, and R_SENSE = 10 Ω. The
resistor must be rated for a power dissipation of 10 x 0.02² W = 0.004 W.
PCB layout
STLA02 is a powerful switched device, the PCB must be designed in line with rules for
designing switched supplies. It is recommended to use a two layer PCB. The power wirings
must be as short as possible and wide. Place all external components close to the STLA02.
High-energy switched loops should be as small as possible to reduce EMI. Most LEDs need
cooling, which may be done by a defined area of copper on the PCB. Use the reference
guide of each LED to design the heatsink. Place the R_FB resistor as close as possible to pin
4 and 5. When a change of PCB layer is needed, use enough vias. During routing the PCB
must be focused on the minimum area of the application ground - the smaller the ground
area of the DC-DC converter ^(a), the better stability and lower noise issues are achieved. It is
recommended to place the copper plate, connected through the vias to the Exposed pad, on
the bottom layer to create the heatsink of the device.
a. The application ground area is represented by the area which is created by the ground pins of the C_IN, C_OUT,
ground of the DEVICE and GND connection of the load.
Doc ID 010005 Rev 1
15/23