MAX16814_15 Datasheet, PDF (16/25 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
Power-Circuit Design
First select a converter topology based on the previous
factors. Determine the required input-supply voltage
range, the maximum voltage needed to drive the LED
strings including the minimum 1V across the constant
LED current sink (VLED), and the total output current
needed to drive the LED strings (ILED) as follows:
= I_LED I_STRING × N_STRING
where ISTRING is the LED current per string in amperes
and NSTRING is the number of strings used.
Calculate the maximum duty cycle (DMAX) using the
following equations:
For boost configuration:
D_MAX
=
(V_LED + V_D1 − V_{IN_MIN})
(V_LED + V_D1 − V_DS − 0.3V)
For SEPIC and coupled-inductor boost-buck configura-
tions:
D_MAX
=
(V_{IN_MIN}
−
(V_LED + V_D1)
V_DS − 0.3V + V_LED
+
V_D1)
where VD1 is the forward drop of the rectifier diode in
volts (approximately 0.6V), VIN_MIN is the minimum input
supply voltage in volts, and VDS is the drain-to-source
voltage of the external MOSFET in volts when it is on,
and 0.3V is the peak current-sense voltage. Initially, use
an approximate value of 0.2V for VDS to calculate DMAX.
Calculate a more accurate value of DMAX after the power
MOSFET is selected based on the maximum inductor
current. Select the switching frequency (fSW) depending
on the space, noise, and efficiency constraints.
Inductor Selection
Boost and Coupled-Inductor Boost-Buck
Configurations
In all the three converter configurations, the average
inductor current varies with the line voltage and the
maximum average current occurs at the lowest line
voltage. For the boost converter, the average inductor
current is equal to the input current. Select the maxi-
mum peak-to-peak ripple on the inductor current (DIL).
The recommended peak-to-peak ripple is 60% of the
average inductor current.
Use the following equations to calculate the maximum
average inductor current (ILAVG) and peak inductor
current (ILP) in amperes:
IL
AVG
=
I_LED
1− D_MAX
Allowing the peak-to-peak inductor ripple DIL to be
+30% of the average inductor current:
and:
∆= IL IL _AVG × 0.3 × 2
= IL _P
IL
AVG
+
∆IL
2
Calculate the minimum inductance value, LMIN, in
henries with the inductor current ripple set to the maxi-
mum value:
L
MIN
=
(VIN_MIN
−
V_DS
f_SW
− 0.3V)
× ∆IL
×
D_MAX
where 0.3V is the peak current-sense voltage. Choose
an inductor that has a minimum inductance greater
than the calculated LMIN and current rating greater than
ILP. The recommended saturation current limit of the
selected inductor is 10% higher than the inductor peak
current for boost configuration. For the coupled-inductor
boost-buck, the saturation limit of the inductor with only
one winding conducting should be 10% higher than ILP.
SEPIC Configuration
Power circuit design for the SEPIC configuration is very
similar to a conventional boost-buck design with the
output voltage referenced to the input supply voltage.
For SEPIC, the output is referenced to ground and the
inductor is split into two parts (see Figure 5 for the SEPIC
configuration). One of the inductors (L2) takes LED
current as the average current and the other (L1) takes
input current as the average current.
Use the following equations to calculate the average
inductor currents (IL1AVG, IL2AVG) and peak inductor
currents (IL1P, IL2P) in amperes:
IL1_AVG
=
I_LED × D_MAX ×1.1
1− D_MAX
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