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Low-Cost, 6-String WLED Drivers with
Quick-PWM Step-Up Converter
120mA × 2 × 32V × ( 32V ? 7V )
3.3uH × 1MHz × 0.85 × ( 32V )
I OUT(MAX) ? V OUT(MAX) ? V IN(MIN) ?
V RIPPLE(C) ≈ ? ?
C OUT
V OUT(MAX) SW
?
?
A 3.3FH inductor is chosen. The peak inductor current at
minimum input voltage is calculated as follows:
I PEAK_DCM = = 1.46A
Output Capacitor Selection
The total output-voltage ripple has two components: the
capacitive ripple caused by the charging and discharging
on the output capacitor, and the ohmic ripple due to the
capacitor’s equivalent series resistance (ESR):
V RIPPLE = V RIPPLE(C) + V RIPPLE(ESR)
? × f ?
and:
V RIPPLE(ESR) ≈ I PEAK x R ESR(COUT)
where I PEAK is the peak inductor current (see the
Inductor Selection section).
The output-voltage ripple voltage should be low enough
for the FB_ current-source regulation. The ripple voltage
should be less than 200mV P-P . For ceramic capaci-
tors, the output-voltage ripple is typically dominated by
V RIPPLE(C) . The voltage rating and temperature charac-
teristics of the output capacitor must also be considered.
Rectifier Diode Selection
The devices’ high switching frequency demands a high-
speed rectifier. Schottky diodes are recommended for
most applications because of their fast recovery time
and low forward voltage. The diode should be rated to
handle the output voltage and the peak switch current.
Make sure that the diode’s peak current rating is at least
I PEAK calculated in the Inductor Selection section and
that its breakdown voltage exceeds the output voltage.
Input Capacitor Selection
The input capacitor (C IN ) filters the current peaks drawn
from the input supply and reduces noise injection into
the ICs. A 4.7 F F ceramic capacitor is used in the Typical
Operating Circuit (Figure 1) because of the high source
18
impedance seen in typical lab setups. Actual applica-
tions usually have much lower source impedance since
the step-up regulator often runs directly from the output
of another regulated supply. In some applications, C IN
can be reduced below the values used in the Typical
Operating Circuit . Ensure a low-noise supply at IN by
using adequate C IN , especially when running at low IN
voltage. Alternatively, greater voltage variation can be
tolerated on C IN if IN is decoupled from C IN using an RC
lowpass filter.
LED Selection and Bias
The series/parallel configuration of the LED load and the
full-scale bias current has a significant effect or regula-
tor performance. LED characteristics vary significantly
from manufacturer to manufacturer. Consult the respec-
tive LED data sheets to determine the range of output
voltages for a given brightness and LED current. In
general, brightness increases as a function of bias cur-
rent. This suggests that the number of LEDs could be
decreased if higher bias current is chosen; however, a
high current increases LED temperature and reduces
operating life. Improvements in LED technology are
resulting in devices with lower forward voltage while
increasing the bias current and light output.
LED manufacturers specify the LED color at a given LED
current. With lower LED current, the color of the emitted
light tends to shift toward the blue range of the spectrum.
A blue bias is often acceptable for business applications
but not for high-image-quality applications such as DVD
players. DPWM dimming is a viable solution for reducing
power dissipation while maintaining LED color integrity.
Careful attention should be paid to switching noise to
avoid other display quality problems.
Using fewer LEDs in a string improves step-up converter
efficiency, and lowers breakdown voltage requirements
of the external MOSFET and diode. The minimum num-
ber of LEDs in series should always be greater than max-
imum input voltage. If the diode voltage drop is lower
than maximum input voltage, the voltage drop across the
current-sense inputs (FB_) increases and causes excess
heating in the IC. Between 8 and 12 LEDs in series are
ideal for input voltages up to 20V.
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