MAX17127 Maxim Integrated Products, MAX17127 Datasheet - Page 18

• Manufacturer Part Number: MAX17127
• Description: Six-String WLED Driver
• Manufacturer: Maxim Integrated Products

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 is calculated with the following equation:
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 = 0.7 at 1MHz
and estimating efficiency of 85% at this operating point:
In CCM, the inductor has to be higher than L
Six-String WLED Driver with Integrated
Step-Up Converter
18
PEAK,
OUT(MAX)
L
I
PEAK
CCM(MIN)
_____________________________________________________________________________________
I
L
RIPPLE
L
×
and the inductor’s DC current rating should
=
DCM(MAX)
IN(DC,MAX)
=
2 f



) is 120mA with a 32V output and a minimal
32V
×
7V
L f
I
SW(MAX)
=
PEAK
=
×
I
×
 
 
 
(
OUT(MAX)
32V 0.4V 2 7V
V
2
SW(MIN)
(
IN(MIN)
V
120mA 1MHz
OUT(MAX)
=
=
. For good efficiency, choose an
2 25.5mV 0.9MHz
+
I




32V 7V
×
IN(DC,MAX)
1
V
L V
×
−
IN(MIN)
×
V
×
× η ×
V
OUT(MAX)
−
× ×
×
(
OUT(MAX)
V
OUT(MAX)
− ×
2 V
OUT(MAX)
+
(
2
V
×
V
OUT(MAX)
V
OUT(MAX)
× η
DIODE



+
IN(MIN)
)
I
0.85
RIPPLE
0.7
×
×
+
I
13.7m
×
OUT(MAX)
2
−
V
f
−



SW
DIODE
V
=
V
IN(MIN)
+
IN(MIN)
12.1 H
Ω
V
CCM(MIN)
DIODE
=
µ




5.5 H
)
)
µ
)
:
A 10FH inductor is chosen, which is higher than the mini-
mum L that guarantees stability in CCM.
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:
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing on the output capacitor, and the ohmic ripple due to
the capacitor’s equivalent series resistance (ESR):
and:
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
I
PEAK
I
PEAK
V
RIPPLE(C)
=
=
PEAK
120mA 32V
V
L
×
7V 0.85
RIPPLE
120mA 2 32V
3.3 H 1.1MHz 0.85
V
DCM(MAX)
2 1.1MHz 32V 120mA
RIPPLE(ESR)
×
×
µ ×
is the peak inductor current (see the
≈
×
I
OUT(MAX)
=
(7V)
× ×
C
P-P
V
Output Capacitor Selection
OUT
RIPPLE(C)
+
=
. For ceramic capacitors, the out-
×
2
2 10 H 32V 0.9MHz



×
1
≈
×
−
0.85
×
I
PEAK ESR(COUT)
×




32V 0.4V
7V
×
V
µ ×
(
32V 0.4V 7V
OUT(MAX)
×
V
+
7V
R
+
×
OUT(MAX)
(
V
32V 7V
(
+
RIPPLE(ESR)
32V 0.4V
−
=
×



+
3.9 H
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−
−
)
V
µ
×
IN(MIN)
f
SW
)
)
RIPPLE(C)
=
=
1.40A
0.95A




.