MAX16834_15 Datasheet, PDF (12/23 Pages) Maxim Integrated Products – High-Power LED Driver with Integrated High-Side LED Current Sense and PWM Dimming MOSFET Driver

MAX16834
High-Power LED Driver with Integrated
High-Side LED Current Sense and
PWM Dimming MOSFET Driver
IL _AVG
=
I_LED
1− D_MAX
Allowing the peak-to-peak inductor ripple (ΔI_L) to be
±30% of the average inductor current:
ΔI_L = IL _AVG × 0.3 × 2
and
IL _P
=
IL _AVG
+
ΔI_L
2
The inductance value (L) of the inductor L1 in henries
(H) is calculated as:
L = ^{(VINMIN − VFET ) × DMAX}
f_SW × ΔI_L
where f_SW is the switching frequency in hertz, V_INMIN
and V_FET are in volts, and ΔI_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.
Boost-Buck Configuration
In the boost-buck LED driver (Figure 3), the average
inductor current is equal to the input current plus the
LED current.
Calculate maximum duty cycle using the following
equation:
D_MAX
=
V_LED
+
V_LED + V_D
V_D + V_INMIN
−
V_FET
where V_LED is the forward voltage of the LED string in
volts, V_D is the forward drop of the 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 below equations to calculate the maximum
average inductor current IL_AVG, peak-to-peak inductor
current ripple ΔI_L, and the peak inductor current IL_P in
amperes:
IL
AVG
=
I_LED
1− D_MAX
Allowing the peak-to-peak inductor ripple ΔI_L to be
±30% of the average inductor current:
ΔI_L = IL _AVG × 0.3 × 2
IL _P
= IL _AVG
+
ΔI_L
2
The inductance value (L) of the inductor L1 in henries is
calculated as:
L = ^{(VINMIN − VFET ) × DMAX}
f_SW × ΔI_L
where f_SW is the switching frequency in hertz, V_INMIN
and V_FET are in volts, and ΔI_L is in amperes. Choose an
inductor that has a minimum inductance greater than
the calculated value.
Peak Current-Sense Resistor (R8)
The value of the switch current-sense resistor R8 for the
boost and boost-buck configurations is calculated as
follows:
R8 = ^0.25 Ω
(IL_P × 1.25)
where 0.25V is the minimum peak current-sense thresh-
old, IL_P is the peak inductor current in amperes, and
the factor 1.25 provides a 25% margin to account for
tolerances. The worst cycle-by-cycle current limiter trig-
gers at 350mV (max). The I_SAT of the inductor should
be higher than 0.35V/R8.
Output Capacitor
The function of the output capacitor is to reduce the
output ripple to acceptable levels. The ESR, ESL, and
the bulk capacitance of the output capacitor contribute
to the output ripple. In most applications, the output
ESR and ESL effects can be dramatically reduced by
using low-ESR ceramic capacitors. To reduce the ESL
and ESR effects, connect multiple ceramic capacitors
in parallel to achieve the required bulk capacitance. To
minimize audible noise generated by the ceramic
capacitors during PWM dimming, it may be necessary
to minimize the number of ceramic capacitors on the
output. In these cases an additional electrolytic or tan-
talum capacitor provides most of the bulk capacitance.
Boost and boost-buck configurations: The calcula-
tion of the output capacitance is the same for both
boost and boost-buck configurations. The output ripple
is caused by the ESR and the bulk capacitance of the
output capacitor if the ESL effect is considered negligi-
ble. For simplicity, assume that the contributions from
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