Datasheet

LTC3219
17
3219fa
Power Effi ciency
To calculate the power effi ciency (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 effi ciency
is given by:
η=
P
P
LED
IN
(4)
The effi ciency of the LTC3219 depends upon the mode in
which it is operating. Recall that the LTC3219 operates
as a pass switch, connecting V
BAT
to CPO, until dropout
is detected at the I
LED
pin. This feature provides the op-
timum effi ciency available for a given input voltage and
LED forward voltage. When it is operating as a switch, the
effi ciency is approximated by:
η= = =
P
P
VI
VI
V
V
LED
IN
LED LED
BAT BAT
LED
BAT
•
•
(5)
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 LTC3219 is negligible and the expression above is
valid.
Once dropout is detected at any LED pin, the LTC3219
enables the charge pump in 1.5x mode.
In 1.5x boost mode, the effi ciency 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
effi ciency would be given by:
η
IDEAL
LED
IN
LED LED
BAT LED
LED
P
P
VI
VI
V
== =
•
•.•15 1
.. •5V
BAT
Similarly, in 2x boost mode, the effi ciency 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 effi ciency would be given by:
η
IDEAL
LED
IN
LED LED
BAT LED
LED
P
P
VI
VI
V
V
== =
•
•• •22
BBAT
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3219.
If the junction temperature increases above approximately
150°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.
APPLICATIONS INFORMATION