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MAX16840
LED Driver with Integrated MOSFET
for MR16 and Other 12V AC Input Lamps
Boost Diode (D1)
A Schottky diode must be used as rectifier diode D1 to
reduce power dissipation. The voltage rating of diode
D1 must be greater than the maximum output voltage.
Choose a diode that minimizes dissipation at tempera-
tures in the +90 N to +100 N C range. The diode should be
selected to minimize reverse leakage at the maximum
output voltage while minimizing forward losses when the
diode is conducting.
Loop Compensation (R2, C4)
The crossover frequency of the loop must be maximized
to get the optimum performance. The maximum value of
Buck-Boost Configuration
In MR16 applications, the buck-boost configuration is
used when the number of LEDs is in the 3-to-5 LED
range. Figure 3 shows the schematic of a buck-boost
LED driver. The maximum voltage on LED+ should not
exceed 40V.
Resistor (R1)
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:
R2 is given by:
R2 MAX =
2 × 300kHz × L2
R1 × V LED × g m
in k ?
I IN = P IN /12V
The resistor R1 is then given by:
R1 = 0.2V/I IN
where R1 is in ohms, V LED is in volts, g m is the transcon-
ductance of the error amplifier in F S, and the value of
the inductor L2 is in F H. Choose a value of R2 20% lower
than the above value.
The zero from C4, R2 should be placed at:
where R1 is in ohms and I IN is in amps.
Buck-Boost Inductor (L2)
For optimum efficiency, the inductor must be operating
in continuous-conduction mode. The maximum peak
current in the inductor occurs at the peak of the highest
fz =
R1 × V LED × g m × R2
12V × π × L
input voltage. The P-P ripple at this input voltage is D I L .
The maximum input current occurs at the highest input
voltage. Typically, the highest input voltage is 13.2V AC,
Therefore, C4 is given by:
which is 10% higher than the typical value.
C4 =
1
2 × π × fz × R2
The peak current in the inductor is:
I PK = I INDMIN + 0.5?I L
LED-
L2
D1
C2
LED+
L1
BD1
R3
REFI
DRAIN
IN
12V AC
C1
EXT
COMP
MAX16840
SOURCE
C4
GND
FB
R1
R2
Figure 3. Buck-Boost LED Driver
10
Maxim Integrated
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