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Six-String WLED Driver with Integrated
Step-Up Converter
V IN(MIN) × ( V OUT(MAX) ? V IN(MIN) )
I RIPPLE =
I PEAK IN(DC,MAX) + RIPPLE
= I
120mA × 32V 7V × ( 32V ? 7V )
I PEAK = + = 0.95A
Calculate the ripple current at that operating point and
the peak current required for the inductor:
L × V OUT(MAX) × f SW
I
2
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:
A 10 F H 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:
7V × 0.85 2 × 10 μ H × 32V × 0.9MHz
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
L DCM(MAX) = ? 1 ?
?
V IN(MIN)
V OUT(MAX) + V DIODE ? ?
?
?
?
?
current to prevent excessive switch current from causing
current limit:
V IN(MIN) 2 × η
L DCM(MAX) = ? 1 ?
(7V) 2 × 0.85
×
2 × f SW(MAX) × V OUT(MAX) × I OUT(MAX)
The peak inductor current in DCM is calculated with the
following equation:
? 7V ?
?
? 32V + 0.4V ?
× = 3.9 μ H
2 × 1.1MHz × 32V × 120mA
A 3.3 F H inductor is chosen. The peak inductor current at
120mA × 2 × 32V × ( 32V + 0.4V ? 7V )
3.3 μ H × 1.1MHz × 0.85 × ( 32V + 0.4V )
I PEAK =
× ( V OUT(MAX) + V DIODE ? V IN(MIN) )
I OUT(MAX) × 2 × V OUT(MAX)
L × f SW(MIN) × η× ( V OUT(MAX) + V DIODE
)
minimum input voltage is calculated as follows:
I PEAK = = 1.40A
I OUT(MAX) ? V OUT(MAX) ? V IN(MIN) ?
V RIPPLE(C) ≈ ? ?
V OUT(MAX) × f SW
C OUT
?
?
32V ? 7V
? 7V ? ?
?? 0.85 ?
L = ? ? ? ?? ? = 12.1 μ H
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 0.1 I 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
? 32V ? ? 120mA × 1MHz ?? 0.7 ?
In CCM, the inductor has to be higher than L CCM(MIN) :
Output Capacitor Selection
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):
V RIPPLE = V RIPPLE(C) + V RIPPLE(ESR)
? ?
and:
V RIPPLE(ESR) ≈ I PEAK R ESR(COUT)
where I PEAK is the peak inductor current (see the
Inductor Selection section).
The output voltage ripple should be low enough for the
L CCM(MIN) =
( 32V + 0.4V ? 2 × 7V ) × 13.7m ? = 5.5 μ H
2 × 25.5mV × 0.9MHz
FB_ current-source regulation. The ripple voltage should
be less than 200mV P-P . For ceramic capacitors, the out-
put voltage ripple is typically dominated by V RIPPLE(C) .
18
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