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8-String WLED Driver with Integrated Step-Up
Regulator and SMBus/PWM Dimming Capability
V OUT(OVP) OVP_TH O (1 +
= V )
V OUT(OVP) = 1.25V O (1 +
) = 39.89V
R1
R2
In Figure 1, the output OVP voltage is set to:
2.21M I
71.5k I
Input Capacitor Selection
The input capacitor (C IN ) filters the current peaks drawn
from the input supply and reduces noise injection into
the IC. A 4.7 F F ceramic capacitor is used in the typical
operating circuit (Figure 1) because of the high source
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 . Alternatively, greater voltage varia-
tion 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 have a significant effect on
regulator performance. LED characteristics vary sig-
nificantly from manufacturer to manufacturer. Consult
the respective LED data sheets to determine the range
of output voltages for a given brightness and LED cur-
rent. In general, brightness increases as a function of
bias current. This suggests that the number of LEDs
could be decreased if higher bias current is chosen;
however, 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 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. Direct-PWM 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 the
maximum 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.
Application Information
LED V FB_ Variation
The forward voltage of each white LED may vary up to
25% from part to part, and the accumulated voltage
difference in each string equates to additional power
loss within the IC. For the best efficiency, the voltage
difference between strings should be minimized. The
difference between lowest voltage string and highest
voltage string should be less than 8V (typ). Otherwise,
the internal LED short-protection circuit disables the high
FB string.
FB_ Pin Maximum Voltage
The current through each FB_ pin is controlled only
during the step-up converter’s on-time. During the con-
verter’s off-time, the current sources are turned off. The
output voltage does not discharge and stays high. The
MAX17105 disables the FB current source from which
the string is shorted. In this case, the step-up converter’s
output voltage is always applied to the disabled FB_ pin.
FB_ pin can withstand 45V.
PCB Layout Guidelines
Careful PCB layout is important for proper operation. Use
the following guidelines for good PCB layout:
1) Minimize the area of the high-current switching loop of
rectifier diode, internal MOSFET, and output capacitor
to avoid excessive switching noise.
2) Connect high-current input and output components
with short and wide connections. The high-current
input loop goes from the positive terminal of the input
capacitor to the inductor, to the internal MOSFET, then
to the input capacitor’s negative terminal. The high-
current output loop is from the positive terminal of the
input capacitor to the inductor, to the rectifier diode,
and to the positive terminal of the output capacitors,
reconnecting between the output capacitor and input
capacitor ground terminals. Avoid using vias in the
high-current paths. If vias are unavoidable, use multiple
vias in parallel to reduce resistance and inductance.
24
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