MAX17061A Maxim Integrated Products, MAX17061A Datasheet - Page 21

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

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The MAX17061A has a minimum inductor value limita-
tion for a stable operation in CCM mode at low input
voltage because of the internal fixed-slope compensa-
tion. The minimum inductor value for stability is calcu-
lated with the following equation:
where 24.7mV is a scale factor from slope compensa-
tion, and the R
factor (12mΩ typ):
1) Choose an available inductor value from an appropri-
2) Calculate the ripple current at that operating point
When DCM operating mode is chosen to minimize the
inductor value, the calculations are different from those
above in CCM mode. The maximum inductor value for
DCM mode is calculated with the following equation:
The peak-inductor current in DCM is calculated with fol-
lowing equation:
The inductor’s saturation current rating should exceed
I
exceed I
inductor with less than 0.1Ω series resistance.
PEAK
L
L
CCM MIN
ate inductor family. Calculate the maximum DC input
current at the minimum input voltage V
conservation of energy and the expected efficiency
at that operating point (
ate curve in the Typical Operating Characteristics :
and the peak current required for the inductor:
DCM MAX
I
and the inductor’s DC current rating should
(
RIPPLE
(
IN(DC,MAX)
)
I
IN DCMAX
)
I
PEAK
(
S
V
2
OUT MAX
V
Inductor Selection in DCM Operation
is the equivalent current-sensing scale
1
IN MIN
,
______________________________________________________________________________________
f
(
OSC MAX
V
(
. For good efficiency, choose an
OUT MAX
I
IN DCMAX
L V
)
2 24 7
(
)
(
(
)
I
OUT MAX
V
OUT MAX
,
V
V
IN MIN
MIN
IN MIN
V
OUT MAX
)
V
.
(
DIODE
(
(
)
IN MIN
(
mV f
V
)
) taken from an appropri-
(
(
OUT MAX
V
SMBus for LCD Panel Applications
)
)
DIODE
I
)
RIPPLE
)
)
(
OSC MIN
2
)
2
V
2
MIN
f
OSC
OUT
V
(
V
IN MIN
)
IN MIN
IN(MIN),
8-String White LED Driver with
(
(
I
OUT MAX
)
)
)
(
using
R
S
)
Examples:
Considering the Typical Operating Circuit with four
10-LED strings and 25mA LED full-scale current, the
maximum load current (I
35.9V output and a minimal input voltage of 7V.
Choosing a CCM operating mode with LIR = 1 at 1MHz
and estimating efficiency of 85% at this operating point:
In CCM, the inductor has to be higher than L
A10µH inductor is chosen, which is higher than the
minimum L that guarantees stability in CCM.
The peak-inductor current at minimum input voltage is
calculated as follows:
Alternatively, choosing a DCM operating mode at 750kHz
and estimating efficiency of 85% at this operating point:
A 4.7µH inductor is chosen. The peak inductor current
at minimum input voltage is calculated as follows:
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):
and:
I
PEAK
L
L
DCM MAX
CCM MIN
L
V
RIPPLE C
I
PEAK
(
(
35 9
4 7
V
7
100
.
.
RIPPLE
)
V
( )
)
uH
V
mA
2 10
100
2 0 825
35 9
1
2
0 675
I
7
OUT MAX
mA
.
.
100
V
2 35 9
7
35 9
C
2 24 7
.
V
Output Capacitor Selection
V
V
35 9
OUT
H
(
.
RIPPLE C
MHz
0 85
mA
.
.
0 4
35 9
V
MHz
.
7
35 9
(
35 9
.
V
.
7
V
V
.
OUT(MAX)
V
)
V
mV
.
0 4
.
1
)
Inductor Selection Design
V
0 85
( )
.
MHz
2
7
V
V
.
V
35 9
V
2 7
V
OUT MAX
35 9
.
0 85
V
7
0 9
0 9
.
.
V
OUT MAX OSC
V
V
.
.
RIPPLE ESR
35 9
V
(
V
MHz
MHz
0 85
) is 100mA with a
0 4
.
.
(
1
.
100
V
12
V
)
m
(
0 4
mA
)
V
7
.
0 92
f
IN MIN
V
9 44
.
V
CCM(MIN):
.
)
(
6 0
A
5 6
.
1 47
.
)
H
.
H
H
A
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