MAX17061A Maxim Integrated Products, MAX17061A Datasheet - Page 22

• Manufacturer Part Number: MAX17061A
• Description: 8-String White LED Driver
• Manufacturer: Maxim Integrated Products

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where I
Inductor Selection section).
The output voltage ripple should be low enough for the
FB_ current-source regulation. The ripple voltage should
be less than 200mV
put voltage ripple is typically dominated by V
The voltage rating and temperature characteristics of the
output capacitor must also be considered. The actual
capacitance of a ceramic capacitor is reduced by DC
voltage biasing. Ensure the selected capacitor has
enough capacitance at actual DC biasing.
The MAX17061A’s high switching frequency demands
a high-speed rectifier. Schottky diodes are recom-
mended 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
Inductor Selection section and that its breakdown volt-
age exceeds the output voltage.
The OV protection circuit should ensure the circuit safe
operation; therefore, the controller should limit the out-
put voltage within the ratings of all MOSFET, diode, and
output capacitor components, while providing sufficient
output voltage for LED current regulation. The OV pin is
tied 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 OV pin voltage reaching the
threshold V
vated. Hence, the step-up converter output overvoltage
protection point is:
In Figure 1, the output OVP voltage is set to:
The input capacitor (C
drawn from the input supply and reduces noise injec-
tion into the IC. A 10µF 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
impedance since the step-up regulator often runs
8-String White LED Driver with
SMBus for LCD Panel Applications
22
______________________________________________________________________________________
Overvoltage Protection Determination
PEAK
V
OUT OVP
OV_TH
V
V
RIPPLE ESR
(
OUT OVP
is the peak inductor current (see the
, typically 1.23V, OV protection is acti-
)
(
P-P
(
1 236
.
Input Capacitor Selection
)
. For ceramic capacitors, the out-
Rectifier Diode Selection
)
IN
V
V
I
OV TH
PEAK ESR COUT
) filters the current peaks
(
_
PEAK
1
R
2 21
61 9
.
.
calculated in the
1
(
M
k
R
R
1
2
)
)
45
RIPPLE(C)
V
.
directly from the output of another regulated supply. In
some applications, C
ues used in the Typical Operating Circuit (Figure 1).
Ensure a low-noise supply at IN by using adequate C
Alternatively, greater voltage variation can be tolerated
on C
pass filter.
The series/parallel configuration of the LED load and the
full-scale bias current have 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 gen-
eral, 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 and
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 spec-
trum. A blue bias is often acceptable for business appli-
cations but not for high-image-quality applications such
as DVD players. Direct DPWM dimming is a viable solu-
tion 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.
The MAX17061A has accurate (±1.5%) matching for
each current source. However, the forward voltage of
each white LED can vary up to 25% from part to part.
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 4.8V (typ). Otherwise, the internal LED short-
protection circuit disables the high FB string.
IN
if IN is decoupled from C
Applications Information
IN
can be reduced below the val-
LED Selection and Bias
LED V
IN
using an RC low-
FB_
Variation
IN
.