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LTC3209-1/LTC3209-2
APPLICATIO S I FOR ATIO
η =
P LED ( V LED ? I LED ) V LED
) (
)
(
P LED ( V LED ? I LED ) = V LED
(
I BAT )
P LED ( V LED ? I LED ) V LED
) (
)
(
Power Efficiency
To calculate the power efficiency ( η ) of a white LED driver
chip, the LED power should be compared to the input
power. The difference between these two numbers repre-
sents lost power whether it is in the charge pump or the
current sources. Stated mathematically, the power effi-
ciency is given by:
P LED
P IN
The efficiency of the LTC3209-1/LTC3209-2 depends upon
the mode in which it is operating. Recall that the
LTC3209-1/LTC3209-2 operates as a pass switch,
connecting V BAT to CPO, until dropout is detected at the
I LED pin. This feature provides the optimum efficiency
available for a given input voltage and LED forward
voltage. When it is operating as a switch, the efficiency is
approximated by:
η = = (1x Mode)
P IN V BAT ? V BAT
since the input current will be very close to the sum of the
LED currents.
At moderate to high output power, the quiescent current
of the LTC3209-1/LTC3209-2 is negligible and the expres-
sion above is valid.
Once dropout is detected at the LED pin, the LTC3209-1/
LTC3209-2 enables the charge pump in 1.5x mode.
In 1.5x boost mode, the efficiency is similar to that of a
linear regulator with an effective input voltage of 1.5 times
the actual input voltage. This is because the input current
for a 1.5x charge pump is approximately 1.5 times the load
current. In an ideal 1.5x charge pump, the power efficiency
would be given by:
η IDEAL = = =
P IN V BAT ? ( 1 . 5 ) ? I LED 1 . 5 ? V BAT
(1.5x Mode)
Similarly, in 2x boost mode, the efficiency is similar to that
of a linear regulator with an effective input voltage of 2
times the actual input voltage. In an ideal 2x charge pump,
the power efficiency would be given by:
η IDEAL = = =
P IN V BAT ? ( 2 ) ? I LED 2 ? V BAT
(2x Mode)
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the
LTC3209-1/LTC3209-2. If the junction temperature in-
creases above approximately 150 ° C the thermal shut-
down circuitry will automatically deactivate the output
current sources and charge pump. To reduce maximum
junction temperature, a good thermal connection to the PC
board is recommended. Connecting the Exposed Pad to a
ground plane and maintaining a solid ground plane under
the device will reduce the thermal resistance of the pack-
age and PC board considerably.
320912fa
18
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