MAX17129EVKIT+ Maxim Integrated Products, MAX17129EVKIT+ Datasheet - Page 16

• Manufacturer Part Number: MAX17129EVKIT+
• Description: Power Management Modules & Development Tools MAX17129 EVAL KIT MAX17129 EVAL KIT
• Manufacturer: Maxim Integrated Products

Specifications of MAX17129EVKIT+

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Low-Cost, 6-String WLED Drivers with
Quick-PWM Step-Up Converter
inductor resistance is relatively high, more ripples can
be accepted to reduce the number of required turns and
increase the wire diameter. If the inductor resistance is
relatively low, increasing inductance to lower the peak
current can reduce losses throughout the power path. If
extremely thin high-resistance inductors are used, as is
common for LCD panel applications, an LIR higher than
2.0 can be chosen for DCM operating mode.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions. The detailed
design procedure for CCM can be described as:
Calculate the approximate inductor value using the
typical input voltage (V
rent (I
from an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current
at the minimum input voltage V
tion of energy and the expected efficiency at that operat-
ing point (E
Typical Operating Characteristics:
Calculate the ripple current at that operating point and
the peak current required for the inductor:
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 (or the minimum inductor value for CCM
mode) is calculated with the following equation:
16
OUT(MAX)
I
RIPPLE
I
PEAK_CCM
L
MIN
2 f
I
IN(DC,MAX)
DCM(MAX)
×
=
) taken from an appropriate curve in the
SW
), the expected efficiency (E
V
IN(MIN)
×
L
V
CCM
V
OUT(MAX)
=
IN(MIN)
=
=
I
IN
×
IN(DC,MAX)
I




OUT(MAX)
1
(
), the maximum output cur-
×
V
V
−
IN(MIN)
V
OUT(MAX)
V
OUT(MAX)
2
OUT(MAX)
V
× η
IN(MIN),
IN(MIN)
×
I
OUT(MAX)
× η
+
×
I
V
RIPPLE
MIN
−
OUT
×
using conserva-
V
2




f
IN(MIN)
SW
×
TYP
)
) taken
The peak inductor current in DCM is calculated with the
following equation:
The inductor’s saturation current rating should exceed
I
exceed I
inductor with less than 0.1I series resistance.
Considering the circuit with six 10-LED strings and
20mA LED full-scale current, the maximum load current
(I
input voltage of 7V.
Choosing a CCM operating mode with LIR = 0.8 at 1MHz
and estimating efficiency of 85% at this operating point:
A 10FH inductor is chosen and the peak inductor current
at minimum input voltage is calculated as follows:
Alternatively, choose a DCM operating mode by using
lower inductance and estimating efficiency of 85% at this
operating point. Since DCM has higher peak inductor
current at lower input, it causes current limit when the
parameters are not chosen properly. Considering the
case with six 10-LED strings and 20mA LED full-scale
current to prevent excessive switch current from causing
current limit:
A 3.3FH inductor is chosen. The peak inductor current at
minimum input voltage is calculated as follows:
PEAK
I
OUT(MAX)
PEAK_DCM
I
I
PEAK_DCM
PEAK_CCM
L
L
CCM
DCM(MAX)
and the inductor’s DC current rating should
IN(DC,MAX)
=
) is 120mA with a 32V output and a minimal



=
32V
7V
=
=
=
I
120mA 32V
OUT(MAX)
 
 
 
120mA 2 32V



2
3.3uH 1MHz 0.85
1
7V 0.85
−
. For good efficiency, choose an
120mA 1MHz
×
32V
7V
32V 7V
=
×
=
×
× ×
4.24 H



0.92A
× ×
×
−
L
×
2
2 1MHz 32V 120mA
DCM
µ
+
×
×
(
2 10 H 32V 1MHz
V
×
OUT(MAX)
×
×
(7V)



7V
(
f
32V 7V
SW
0.85
µ ×
0.8
×
×
×
2
(
(
32V
× η
32V 7V
×
−



0.85
=
×
)
−
10.59 H
)
−
V
×
=
IN(MIN)
1.46A
)
µ
)