Datasheet

15Maxim Integrated
High-Voltage HB LED Drivers with
Integrated High-Side Current Sense
MAX16833/MAX16833B–MAX16833D
Inductor Selection
Boost Configuration
In the boost converter (see the Typical Operating
Circuits), the average inductor current varies with the
line voltage. The maximum average current occurs at the
lowest line voltage. For the boost converter, the average
inductor current is equal to the input current. Calculate
maximum duty cycle using the following equation:
LED D INMIN
MAX
LED D FET
V V - V
D
V V -V
+
=
+
where V
LED
is the forward voltage of the LED string
in volts, V
D
is the forward drop of rectifier diode D1 in
volts (approximately 0.6V), V
INMIN
is the minimum input-
supply voltage in volts, and V
FET
is the average drain-to-
source voltage of the MOSFET Q1 in volts when it is on.
Use an approximate value of 0.2V initially to calculate
D
MAX
. A more accurate value of the maximum duty cycle
can be calculated once the power MOSFET is selected
based on the maximum inductor current.
Use the following equations to calculate the maximum
average inductor current IL
AVG
, peak-to-peak induc-
tor current ripple DI
L
, and peak inductor current IL
P
in
amperes:
LED
AVG
MAX
I
IL
1- D
=
Allowing the peak-to-peak inductor ripple to be DI
L,
the
peak inductor current is given by:
L
P AVG
I
IL IL
2
∆
= +
The inductance value (L) of inductor L1 in henries (H) is
calculated as:
( )
INMIN FET MAX
SW L
V -V D
L
fI
×
=
×∆
where f
SW
is the switching frequency in hertz, V
INMIN
and V
FET
are in volts, and DI
L
is in amperes.
Choose an inductor that has a minimum inductance
greater than the calculated value. The current rating of
the inductor should be higher than IL
P
at the operating
temperature.
Buck-Boost Configuration
In the buck-boost LED driver (see the Typical Operating
Circuits), the average inductor current is equal to the
input current plus the LED current. Calculate the maxi-
mum duty cycle using the following equation:
LED D
MAX
LED D INMIN FET
V V
D
V V V -V
+
=
++
where V
LED
is the forward voltage of the LED string
in volts, V
D
is the forward drop of rectifier diode D1
(approximately 0.6V) in volts, V
INMIN
is the minimum
input supply voltage in volts, and V
FET
is the average
drain-to-source voltage of the MOSFET Q1 in volts when
it is on. Use an approximate value of 0.2V initially to cal-
culate D
MAX
. A more accurate value of maximum duty
cycle can be calculated once the power MOSFET is
selected based on the maximum inductor current.
Use the equations below to calculate the maximum aver-
age inductor current IL
AVG
, peak-to-peak inductor cur-
rent ripple DI
L
, and peak inductor current IL
P
in amperes:
LED
AVG
MAX
I
IL
1- D
=
Allowing the peak-to-peak inductor ripple to be DI
L:
L
P AVG
I
IL IL
2
∆
= +
where IL
P
is the peak inductor current.
The inductance value (L) of inductor L1 in henries is
calculated as:
( )
INMIN FET MAX
SW L
V -V D
L
fI
×
=
×∆
where f
SW
is the switching frequency in hertz, V
INMIN
and V
FET
are in volts, and DI
L
is in amperes. Choose an
inductor that has a minimum inductance greater than the
calculated value.
Peak Current-Sense Resistor (R4)
The value of the switch current-sense resistor R4 for the
boost and buck-boost configurations is calculated as
follows:
SC
P
0.418V - V
R4
IL
= Ω
where IL
P
is the peak inductor current in amperes and
V
SC
is the peak slope compensation voltage.