MAX16814_15 Datasheet, PDF (17/25 Pages) Maxim Integrated Products – Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller

MAX16814
Integrated, 4-Channel, High-Brightness LED
Driver with High-Voltage DC-DC Controller
The factor 1.1 provides a 10% margin to account for the
converter losses:
IL2 _AVG = I_LED
Assuming the peak-to-peak inductor ripple DIL is Q30%
of the average inductor current:
and:
∆= IL1 IL1_AVG × 0.3 × 2
= IL1_P
IL1_AVG
+
∆IL1
2
and:
∆I= L2 IL2 _AVG × 0.3 × 2
= IL2_P
IL2
AVG
+
∆IL2
2
Calculate the minimum inductance values L1MIN and
L2MIN in henries with the inductor current ripples set to
the maximum value as follows:
L1_MIN
=
(VIN_MIN
− V_DS − 0.3V)
f_SW × ∆IL1
×
D_MAX
L2 _MIN
=
(VIN_MIN
− V_DS − 0.3V)
f_SW × ∆IL2
×
D_MAX
where 0.3V is the peak current-sense voltage. Choose
inductors that have a minimum inductance greater than
the calculated L1MIN and L2MIN and current rating
greater than IL1P and IL2P, 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:
and:
L _MIN
=
L1_MIN
L1_MIN
×
+
L2 _MIN
L2_MIN
I= L _AVG IL1_AVG + IL2 _AVG
where ILAVG 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 CS so that the peak-to-
peak ripple on it is less than 2% of the minimum input
supply voltage. This ensures that the second-order
effects created by the series resonant circuit comprising
L1, CS, and L2 does not affect the normal operation of
the converter. Use the following equation to calculate the
minimum value of CS:
C_S
≥
I_LED
V_{IN_MIN}
× D_MAX
× 0.02 ×
f_SW
where CS is the minimum value of the coupling capacitor
in farads, ILED 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
compensation. This ramp current is in sync with
the switching 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, RSCOMP, is connected between
the CS input and the source of the external MOSFET.
This adds a programmable ramp voltage to the CS input
voltage to provide slope compensation.
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