Datasheet

The voltage rating and temperature characteristics of the
output capacitor must also be considered.
Rectier 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
sufficient 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 4.7µF ceramic capacitor is used in the
typical operating circuit (Figure 1) because of the high
source impedance seen in typical lab setups. Actual
applications usually have much lower source imped-
ance 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
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 have a significant effect on regulator
performance. LED characteristics vary significantly from
manufacturer to manufacturer. Consult the respective
LED data sheets to determine the range of output volt-
ages for a given brightness and LED current. 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-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 number of
LEDs in series should always be greater than maximum
input voltage. If the diode voltage drop is lower than maxi-
mum input voltage, the voltage drop cross 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.
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Maxim Integrated │ 19
MAX17127 Six-String WLED Driver with
Integrated Step-Up Converter