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LTC3220/LTC3220-1
APPLICATIONS INFORMATION
Power Ef?ciency
To calculate the power ef?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 ef?ciency
In 1.5x boost mode, the ef?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
ef?ciency would be given by:
η=
η IDEAL =
=
=
is given by:
P LED
P IN
(4)
P LED V LED ?I LED V LED
P IN V IN ? 1.5 ?I LED 1.5 ? V IN
Similarly, in 2x boost mode, the ef?ciency is similar to
The ef?ciency of the LTC3220/LTC3220-1 depends upon
the mode in which it is operating. Recall that the LTC3220/
LTC3220-1 operate as pass switches, connecting V IN 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 ef?ciency would be given by:
CPO, until dropout is detected at the I LED pin. This feature
provides the optimum ef?ciency available for a given input
voltage and LED forward voltage. When it is operating as
η IDEAL =
P LED
P IN
=
V LED ?I LED
V IN ?2?I LED
=
V LED
2?V IN
P LED
η=
= LED LED = LED
a switch, the ef?ciency is approximated by:
V ?I V
(5)
P IN V IN ?I IN V IN
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 LTC3220/LTC3220-1 is negligible and the expres-
sion above is valid.
Once dropout is detected at any LED pin, the LTC3220/
LTC3220-1 enable the charge pump in 1.5x mode.
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3220/
LTC3220-1. 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 recom-
mended. 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.
32201fc
18
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