MAX16833 Datasheet, PDF (15/22 Pages) Maxim Integrated Products – High-Voltage HB LED Drivers with Integrated High-Side Current Sense

High-Voltage HB LED Drivers with
Integrated High-Side Current Sense
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:
D_MAX
=
V_LED + V_D - V_INMIN
V_LED + V_D - V_FET
where VLED is the forward voltage of the LED string
in volts, VD is the forward drop of rectifier diode D1 in
volts (approximately 0.6V), VINMIN is the minimum input-
supply voltage in volts, and VFET 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
DMAX. 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 ILAVG, peak-to-peak induc-
tor current ripple DIL, and peak inductor current ILP in
amperes:
IL
AVG
=
I_LED
1- D_MAX
Allowing the peak-to-peak inductor ripple to be DIL, the
peak inductor current is given by:
IL _P
=
IL _AVG
+
∆I_L
2
The inductance value (L) of inductor L1 in henries (H) is
calculated as:
L = ^{(VINMIN - VFET ) × DMAX}
f_SW × ∆I_L
where fSW is the switching frequency in hertz, VINMIN
and VFET are in volts, and DIL 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 ILP 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:
D _MAX
=
V_LED + V_D
V_LED + V_D + V_INMIN - V_FET
where VLED is the forward voltage of the LED string
in volts, VD is the forward drop of rectifier diode D1
(approximately 0.6V) in volts, VINMIN is the minimum
input supply voltage in volts, and VFET 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 DMAX. 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 ILAVG, peak-to-peak inductor cur-
rent ripple DIL, and peak inductor current ILP in amperes:
IL
AVG
=
I_LED
1- D_MAX
Allowing the peak-to-peak inductor ripple to be DIL:
IL _P
=
IL _AVG
+
∆I_L
2
where ILP is the peak inductor current.
The inductance value (L) of inductor L1 in henries is
calculated as:
L = ^{(VINMIN - VFET ) × DMAX}
f_SW × ∆I_L
where f_SW is the switching frequency in hertz, V_INMIN
and VFET are in volts, and DIL 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:
R4 = ^{0.418V - VSC} Ω
IL _P
where ILP is the peak inductor current in amperes and
VSC is the peak slope compensation voltage.
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