Datasheet

LED Driver with Integrated MOSFET
for MR16 and Other 12V AC Input Lamps
Applications information
Boost Configuration
In MR16 and other 12V AC input applications, the boost
configuration is used when the LED string voltage
exceeds 18V. This means that, in general, if the number
of LEDs in series in the application is greater than or
equal to 6 LEDs, the IC can be used in the boost con-
figuration to provide the best efficiency. Figure 2 shows a
boost MR16 application. The number of LEDs can range
from 6 to 10 LEDs. The maximum voltage across the LED
string should not exceed 40V.
In the boost configuration, the current in the inductor is
the same as the current in the current-sense resistor R1.
Resistor (R1)
The current in resistor R1 is the same as the current in
inductor L2. If the desired maximum output power with
12V AC at 50Hz or 60Hz is P
OUT
, the input power P
IN
is
given:
P
IN
= P
OUT
/n
where n is the efficiency. The input current is given by:
I
IN
= P
IN
/10.8V
Resistor R1 is then given by:
R1 = 0.2V/I
IN
where R1 is in ohms and I
IN
is in amps.
Boost Inductor (L2)
For optimum efficiency, the boost inductor must be oper-
ating in continuous-conduction mode. The maximum
peak current in the inductor occurs at the peak of the
highest input voltage. The
P-P
ripple at this input voltage
is ∆I
L
. The highest input voltage is 13.2V AC, which is
10% higher than the typical value.
The peak current in the inductor is:
I
PK
= I
IN
+ 0.5∆I
L
The output voltage of the LED string is given by V
LED
. As
a first-order approximation, the LED voltage is assumed
constant. The duty cycle at the peak of the input voltage
is therefore:
×
= − = −
MIN
LED LED
13.2V 2 18.66V
D 1 1
V V
The percentage
P-P
ripple is typically selected in the 30%
to 60% range of the maximum input current. Assuming
a 60%
P-P
inductor-current ripple, the maximum inductor
current is given by:
I
PK
= 1.3I
INMAX
The minimum inductor value is given by:
×
=
×
MIN
MIN
INMAX
18.66V D
L
0.6I 300kHz
where the switching frequency is 300kHz. The saturation
current of the inductor should be higher than
P-P
and the
minimum value of the inductor at I
P-P
should exceed L
MIN
.
The temperature of the inductor can be in the +90NC to
+100NC range in the application and the losses should
be estimated at these elevated temperatures.
Figure 2. Boost LED Driver
L1 BD1
C1
C4
R3
L2 D1
LED-
LED+
EXT
COMP SOURCE
IN
GND
DRAIN
C3
12V AC
REFI
FB
R1
R2
MAX16840
MAX16840
Maxim Integrated
9