Datasheet

16 Maxim Integrated
Integrated, 2-Channel, High-Brightness LED Driver
with High-Voltage Boost and SEPIC Controller
MAX16838
Output Capacitor Selection in
Boost Configuration
For the boost converter, the output capacitor
supplies the load current when the main switch is on.
The required output capacitance is high, especially at
higher duty cycles.
Calculate the output capacitor (C
OUT
) using the follow-
ing equation:
C
OUT
> (D
MAX
x I
LED
)/(V
LED_P-P
x f
SW
)
where V
LED_P-P
is the peak-to-peak ripple in the LED
supply voltage. Use a combination of low-ESR and high-
capacitance ceramic capacitors for lower output ripple
and noise.
Input Capacitor Selection in Boost Configuration
The input current for the boost converter is continuous
and the RMS ripple current at the input capacitor is low.
Calculate the minimum input capacitor C
IN
using the fol-
lowing equation:
C
IN
= IL
P-P
/(8 x f
SW
x V
IN_P-P
)
where V
IN_P-P
is the peak-to-peak input ripple voltage.
This equation assumes that input capacitors supply
most of the input ripple current.
Rectifier Diode Selection
Using a Schottky rectifier diode produces less forward drop
and puts the least burden on the MOSFET during reverse
recovery. A diode with considerable reverse-recovery time
increases the MOSFET switching loss. Select a Schottky
diode with a voltage rating 20% higher than the maximum
boost-converter output voltage and current rating greater
than that calculated in the following equation:
( ) ( )
-=
D AVG MAX
I IL 1 D A
Feedback Compensation
The voltage feedback loop needs proper compensa-
tion for stable operation. This is done by connecting
a resistor (R
COMP
) and capacitor (C
COMP
) in series
from COMP to SGND. R
COMP
is chosen to set the high-
frequency integrator gain for fast transient response,
while C
COMP
is chosen to set the integrator zero to main-
tain loop stability. For optimum performance, choose the
components using the following equations:
××
=
× × × ×−
ZRHP CS LED
COMP
COMP LED MAX
f RI
R
5 FP1 GM V (1 D )
where
−
=
××
2
LED MAX
ZRHP
LED
V (1 D )
f
2 LI
π
is the right-half plane zero for the boost regulator.
R
CS
is the current-sense resistor in series with the
source of the internal switching MOSFET. I
LED
is the total
LED current that is the sum of the LED currents in both
the channels. V
LED
is the output voltage of the boost
regulator. D
MAX
is the maximum duty cycle that occurs
at minimum input voltage. GM
COMP
is the transconduc-
tance of the error amplifier.
=
×× ×
LED
LED OUT
I
FP1
2V C
π
is the output pole formed by the boost regulator.
Set the zero formed by R
COMP
and C
COMP
a decade
below the crossover frequency. Using the value of
R
COMP
from above, the crossover frequency is at
f
ZRHP
/5.
=
××
COMP
COMP ZRHP
50
C
2R f
π