MAX16840_15 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – LED Driver with Integrated MOSFET

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 +90N to +100NC 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
R2 is given by:
= R2_MAX
2 × 300kHz × L2 _{in kΩ}
R1× V_LED × g_m
where R1 is in ohms, VLED is in volts, gm is the transcon-
ductance of the error amplifier in FS, and the value of the
inductor L2 is in FH. Choose a value of R2 20% lower
than the above value.
The zero from C4, R2 should be placed at:
fz = ^{R1× VLED × gm × R2}
12V × π × L
Therefore, C4 is given by:
C4 =
1
2 × π × fz × R2
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 POUT, the input power PIN is given:
PIN = POUT/n
where n is the efficiency. The input current is given by:
IIN = PIN/12V
The resistor R1 is then given by:
R1 = 0.2V/IIN
where R1 is in ohms and IIN 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
input voltage. The P-P ripple at this input voltage is DIL.
The maximum input current occurs at the highest input
voltage. Typically, the highest input voltage is 13.2V AC,
which is 10% higher than the typical value.
The peak current in the inductor is:
IPK = IINDMIN + 0.5∆IL
L1
BD1
12V AC
L2
D1
R3
C1
DRAIN
REFI
IN
EXT ^MAX16840
COMP
C4
SOURCE
FB
GND
R2
LED-
C2
LED+
R1
Figure 3. Buck-Boost LED Driver
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