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LTC3218
APPLICATIONS INFORMATION
η ≡ LED
η ≡ LED = LED
≈ LED
Thefollowingguidelinesshouldbefollowedwhendesign-
ing a PCB layout for the LTC3218.
? The Exposed Pad should be soldered to a large copper
plane that is connected to a solid, low impedance ground
plane using plated, through-hole vias for proper heat
sinking and noise protection.
? Input and output capacitors (C IN and C CPO ) must also
be placed as close to the part as possible.
? The ?ying capacitor must also be placed as close to
the part as possible. The traces running from the pins
to the capacitor pads should be as wide as possible.
? V IN , CPO and I LED traces must be made as wide as pos-
sible. This is necessary to minimize inductance, as well
as provide suf?cient area for high current applications.
? LED pads must be large and should be connected to
as much solid metal as possible to ensure proper heat
sinking.
Power Ef?ciency
To calculate the power ef?ciency ( η ) of a white LED
driver chip, the LED power should be compared to the
input power. The difference between these two numbers
represents lost power whether it is in the charge pump
or the sense resistor. Stated mathematically, the power
ef?ciency is given by:
P
P IN
The ef?ciency of the LTC3218 depends on the mode in
which it is operating. In 1x mode, the LTC3218 regulates
the output down to the LED forward voltage required to
achieve the desired current by varying the strength of the
series switches. This mode provides the optimum ef?ciency
available for a given input voltage and LED forward voltage.
The ef?ciency is approximated by:
P V   ? I LED V
P IN V IN ? I IN V IN
since the input current will be very close to the LED current.
C FLY
C CPO
PIN 1
C IN
R SETT
R SETF
3218 F03
Figure 3. Example Board Layout
3218fb
10
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