STLA02PUR Datasheet, PDF (14/23 Pages) STMicroelectronics – White LED driver for display backlight

Introduction
7
Introduction
STLA02
The STLA02 is a boost converter dedicated to powering and controlling the current of white
LEDs in an LCD backlight. The device operates at a typical constant switching frequency of
2.3 MHz. It steps an input voltage ranging from 2.5 V to 18 V, up to 27 V. The output current
is adjustable by the resistor R_FB connected between the V_OUT and FB pins. The STLA02
device contains high side sensing to simplify the PCB layout in terms of connection of the
LEDs.
The output current is dimmable by the PWM signal applied to the EN pin with minimum
PWM frequency equal 100 Hz.
7.1
Note:
7.2
14/23
PWM input (EN)
Light intensity can be dimmed by a signal applied to the PWM (EN) input.
The PWM signal is directly connected to the enable pin of the STLA02. It is recommended
to use the frequency of the PWM signal in the range of 100 Hz to 1 kHz and amplitude of the
signal 1.8 V min. The result of the direct PWM dimming method (300 Hz PWM and V_EN =
1.8 V) is shown in Figure 17.
When the device is required to operate in a constant current mode with the EN pin
connected to the voltage higher than 1.8 V, then the delay between rise times of V_IN voltage
of the device and the EN voltage is mandatory to guarantee the proper internal reset of the
logic of the device during ramping of the V_IN. It is recommended to delay the EN voltage rise
time by 2 ms after the rise time on V_IN appears.
Selection of the external components
C_IN selection
It is recommended to use 1 µF as the input capacitor to achieve good stability of the device
and low noise on the V_IN track.
C_OUT selection
It is recommended to use 1 µF as the optimal value of output capacitor to get the best
compromise between output voltage ripple and load transients response. The output ripple
can be checked according to the equation for step-up architecture:
Equation 1
( ) _{[ ] VPK−PK}
= ^{IOUT(MAX) ∗ VOUT − VIN(MIN)}
C_OUT ∗ V_OUT2 ∗ f
* 100
V; A, V,F,Hz
Inductor selection
A thin shielded inductor with a low DC series winding resistance is recommended for this
application. To achieve a good efficiency in step-up mode, it is recommended to use an
inductor with a DC series resistance R_DCL = R_D/10 [Ω; Ω; 1], where R_D is the dynamic
resistance of the LED [Ω; Ω; 1].
Doc ID 010005 Rev 1