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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 above
factors. Determine the required input supply voltage
range, the maximum voltage needed to drive the LED
Use the following equations to calculate the maximum
average inductor current (IL AVG ) and peak inductor cur-
rent (IL P ) in amperes:
strings including the minimum 1V across the constant
LED current sink (V LED ), and the total output current
needed to drive the LED strings (I LED ) as follows:
IL AVG =
I LED
1 ? D MAX
D MAX =
= IL AVG +
IL P
I LED
= I STRING × N STRING
where I STRING is the LED current per string in amperes
and N STRING is the number of strings used.
Calculate the maximum duty cycle (D MAX ) using the fol-
lowing equations:
For boost configuration:
(V LED + V D1 ? V IN_MIN )
(V LED + V D1 ? V DS ? 0.3V)
For SEPIC and coupled-inductor boost-buck-configura-
Allowing the peak-to-peak inductor ripple D IL to be
+30% of the average inductor current:
IL
?= IL AVG × 0.3 × 2
and:
? IL
2
Calculate the minimum inductance value, L MIN , in hen-
ries with the inductor current ripple set to the maximum
value:
L MIN =
tions:
D MAX =
(V LED + V D1 )
(V IN_MIN ? V DS ? 0.3V + V LED + V D1 )
(VIN MIN ? V DS ? 0.3V) × D MAX
f SW × ? IL
where 0.3V is the peak current-sense voltage. Choose
where V D1 is the forward drop of the rectifier diode in
volts (approximately 0.6V), V IN_MIN is the minimum input
supply voltage in volts, and V DS 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 V DS to calculate D MAX .
Calculate a more accurate value of D MAX after the power
MOSFET is selected based on the maximum inductor
current. Select the switching frequency (f SW ) 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 maximum
peak-to-peak ripple on the inductor current ( D IL). The
an inductor that has a minimum inductance greater
than the calculated L MIN and current rating greater than
IL P . 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 IL P .
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 cur-
rent 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 (IL1 AVG , IL2 AVG ) and peak inductor
currents (IL1 P, IL2 P ) in amperes:
IL1 AVG = LED MAX
recommended peak-to-peak ripple is 60% of the aver-
age inductor current.
16
I × D
1 ? D MAX
× 1.1
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