Datasheet
17Maxim Integrated
Integrated, 4-Channel, High-Brightness LED
Driver with High-Voltage DC-DC Controller
MAX16814
The factor 1.1 provides a 10% margin to account for the
converter losses:
AVG LED
IL2 I=
Assuming the peak-to-peak inductor ripple DIL is Q30%
of the average inductor current:
AVG
IL1 IL1 0.3 2∆= × ×
and:
P AVG
IL1
IL1 IL1
2
∆
= +
AVG
IL2 IL2 0.3 2∆= ××
and:
P AVG
IL2
IL2 IL2
2
∆
= +
Calculate the minimum inductance values L1
MIN
and
L2
MIN
in henries with the inductor current ripples set to
the maximum value as follows:
MIN DS MAX
MIN
SW
MIN DS MAX
MIN
SW
(VIN V 0.3V) D
L1
f IL1
(VIN V 0.3V) D
L2
f IL2
−− ×
=
×∆
−− ×
=
×∆
where 0.3V is the peak current-sense voltage. Choose
inductors that have a minimum inductance greater than
the calculated L1
MIN
and L2
MIN
and current rating
greater than IL1
P
and IL2
P
, respectively. The recom-
mended saturation current limit of the selected inductor
is 10% higher than the inductor peak current:
For simplifying further calculations, consider L1 and
L2 as a single inductor with L1 and L2 connected in
parallel. The combined inductance value and current is
calculated as follows:
MIN MIN
MIN
MIN MIN
L1 L2
L
L1 L2
×
=
+
and:
AVG AVG
AVG
IL IL1 IL2= +
where IL
AVG
represents the total average current through
both the inductors together for SEPIC configuration. Use
these values in the calculations for SEPIC configuration
in the following sections.
Select coupling capacitor C
S
so that the peak-to-peak
ripple on it is less than 2% of the minimum input sup-
ply voltage. This ensures that the second-order effects
created by the series resonant circuit comprising L1,
C
S
, and L2 does not affect the normal operation of the
converter. Use the following equation to calculate the
minimum value of C
S
:
LED MAX
S
IN_MIN SW
ID
C
V 0.02 f
×
≥
××
where C
S
is the minimum value of the coupling capacitor
in farads, I
LED
is the LED current in amperes, and the
factor 0.02 accounts for 2% ripple.
Slope Compensation
The MAX16814 generates a current ramp for slope com-
pensation. This ramp current is in sync with the switch-
ing frequency and starts from zero at the beginning of
every clock cycle and rises linearly to reach 50FA at the
end of the clock cycle. The slope-compensating resistor,
R
SCOMP
, is connected between the CS input and the
source of the external MOSFET. This adds a program-
mable ramp voltage to the CS input voltage to provide
slope compensation.