Rainbow Electronics MAX16834 - Manual (Page 12)

MAX16834
High-Power LED Driver with Integrated High-Side LED
Current Sense and PWM Dimming MOSFET Driver
12 ______________________________________________________________________________________
Allowing the peak-to-peak inductor ripple (
∆I
L ) to be
±30% of the average inductor current:
and
The inductance value (L) of the inductor L1 in henries
(H) is calculated as:
where f
SW
is the switching frequency in hertz, V
INMIN
and V FET are in volts, and
∆I
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 (Figure 3), the average
inductor current is equal to the input current plus the
LED current.
Calculate maximum duty cycle using the below equation:
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
∆I
L
, and the peak inductor current IL
P
in
amperes:
Allowing the peak-to-peak inductor ripple
∆I
L to be
±30% of the average inductor current:
The inductance value (L) of the inductor L1 in henries is
calculated as:
where f SW is the switching frequency in hertz, V INMIN
and V FET are in volts, and
∆I
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 buck-boost configurations is calculated as
follows:
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 buck-boost configurations: The calcula-
tion of the output capacitance is the same for both
boost and buck-boost 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
R
IL
P
8
0 25
1 25
=
×
.
( . )
Ω
L
V V D
f I
INMIN FET MAX
SW L
=
− Ч
Ч
( )
∆
∆
∆
I IL
IL IL
I
L AVG
P AVG
L
= Ч Ч
= +
0 3 2
2
.
IL
I
D
AVG
LED
MAX
=
− 1
D
V V
V V V V
MAX
LED D
LED D INMIN FET
=
+
+ + −
L
V V D
f I
INMIN FET MAX
SW L
=
− Ч
Ч
( )
∆
IL IL
I
P AVG
L = +
∆
2
∆I IL
L AVG
= Ч Ч 0 3 2 .