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LTC3212
APPLICATIONS INFORMATION
η = LED
η IDEAL =
P LED V LED ? I LED V LED
P IN
V BAT ? 2 ? I LED
2 ? V B AT
= = LED
P LED V LED ? I LED V
P IN
V BAT ? I BAT
V BAT
PowerEfficiency
To calculate the power efficiency ( η ) of an 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 cur-
rent sources. Stated mathematically, the power efficiency
is given by:
P
P IN
The efficiency of the LTC3212 depends upon the mode in
which it is operating. Recall that the LTC3212 operates
as a pass switch, connecting V IN to CPO, until dropout
is detected at an I LED pin. This feature provides the op-
timum efficiency available for a given input voltage and
LED forward voltage. When it is operating as a switch, the
efficiency is approximated by:
η=
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 LTC3212 is negligible and the expression above is
valid.
Once dropout is detected at any LED pin, the LTC3212
enables the charge pump in 2x mode.
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:
= =
In some applications it may be possible to increase the
efficiency of the LTC3212. If any of the LED’s maximum
forward voltage is less than the minimum V IN supply volt-
age minus I LED dropout voltage then the charge pump is
not needed to drive that LED. This is often the case with
the red LED due to its lower forward voltage. Its anode
may be connected directly to V IN , bypassing the charge
pump’s losses in 2x mode.
Thermal Management
If the junction temperature increases above approximately
140°C the thermal shutdown 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 package and PC board considerably.
3212fb
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