Datasheet

Calculate the ripple current at that operating point and the
peak current required for the inductor:
( )
IN(MIN) OUT(MAX) IN(MIN)
RIPPLE
OUT(MAX) SW
RIPPLE
PEAK IN(DC,MAX)
VV V
I
LV f
I
II
2
×−
=
××
= +
When DCM operating mode is chosen to minimize
the inductor value, the calculations are different from
those above in CCM operating mode. The maximum
inductor value for DCM mode is calculated with the
following equation:
IN(MIN)
DCM(MAX)
OUT(MAX) DIODE
2
IN(MIN)
SW(MAX) OUT(MAX) OUT(MAX)
V
L1
VV
V
2f V I


= −

+

×η
×
×× ×
The peak inductor current in DCM is calculated with the
following equation:
( )
( )
OUT(MAX) OUT(MAX)
OUT(MAX) DIODE IN(MIN)
PEAK
SW(MIN) OUT(MAX) DIODE
I 2V
V VV
I
Lf V V
××
× +−
=
× ×η× +
The inductor’s saturation current rating should exceed
I
PEAK,
and the inductor’s DC current rating should exceed
I
IN(DC,MAX)
. For good efficiency, choose an inductor with
less than 0.1Ω series resistance.
Considering the circuit with six 10-LED strings and
20mA LED full-scale current, the maximum load current
(I
OUT(MAX)
) is 120mA with a 32V output and a minimal
input voltage of 7V.
Choosing a CCM operating mode with LIR = 0.7 at 1MHz
and estimating efficiency of 85% at this operating point:
2
7V 32V 7V 0.85
L 12.1 H
32V 120mA 1MHz 0.7
−
    
= = µ
    
×
    
In CCM, the inductor has to be higher than L
CCM(MIN)
:
( )
CCM(MIN)
32V 0.4V 2 7V 13.7m
L 5.5 H
2 25.5mV 0.9MHz
+ −× × Ω
= = µ
××
A 10µH 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:
( )
PEAK
7V 32V 7V
120mA 32V
I 0.95A
7V 0.85 2 10 H 32V 0.9MHz
×−
×
=+=
× × µ× ×
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:
DCM(MAX)
2
7V
L1
32V 0.4V
(7V) 0.85
3.9 H
2 1.1MHz 32V 120mA

= −

+

×
×=µ
× ××
A 3.3µH inductor is chosen. The peak inductor current at
minimum input voltage is calculated as follows:
( )
( )
PEAK
120mA 2 32V 32V 0.4V 7V
I 1.40A
3.3 H 1.1MHz 0.85 32V 0.4V
×× × + −
= =
µ× × × +
Output Capacitor Selection
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):
RIPPLE RIPPLE(C) RIPPLE(ESR)
OUT(MAX) OUT(MAX) IN(MIN)
RIPPLE(C)
OUT OUT(MAX) SW
VV V
IV V
V
CV f
= +

−

≈

×

and:
RIPPLE(ESR) PEAK ESR(COUT)
V IR≈
where I
PEAK
is the peak inductor current (see the 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
P-P
. For ceramic capacitors, the
output-voltage ripple is typically dominated by V
RIPPLE(C)
.
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Maxim Integrated │ 18
MAX17127 Six-String WLED Driver with
Integrated Step-Up Converter