Rainbow Electronics MAX17127 - Manual (Page 19)

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MAX17127
Six-String WLED Driver with Integrated
Step-Up Converter
The voltage rating and temperature characteristics of the
output capacitor must also be considered.
Rectifier Diode Selection
The MAX17127’s 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.
Overvoltage-Protection Determination
The overvoltage-protection circuit ensures the circuit
safe operation; therefore, the controller should limit the
output voltage within the ratings of all MOSFET, diode,
and output capacitor components, while providing suf-
ficient output voltage for LED current regulation. The
OVP pin is connected to the center tap of a resistive
voltage-divider (R1 and R2 in Figure 1) from the high-
voltage output. When the controller detects the OVP pin
voltage reaching the threshold V
OVP_TH , typically 1.25V,
overvoltage protection is activated. Hence, the step-up
converter output overvoltage-protection point is:
OUT(OVP) OVP_TH
R1
V V (1 )
R2
= × +
V OUT(OVP) depends on how many LEDs are used for
each string and V OUT(OVP) = 1.25V x V OUT , generally
and where V OUT is the LED’s operating voltage for each
string.
In Figure 1, the output OVP voltage is set to:
OUT(OVP)
2.21M
V 1.25V (1 ) 39.71V
71.5k
Ω
= × + =
Ω
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 4.7FF 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 emit-
ted 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-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
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.
Applications 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_ voltage string.