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LTC3208 Datasheet, PDF (19/24 Pages) Linear Technology – High Current Software Confi gurable Multidisplay LED Controller | |||
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LTC3208
APPLICATIO S I FOR ATIO
Power Efï¬ciency
To calculate the power efï¬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 current sources. Stated mathematically, the power
efï¬ciency is given by:
η = PLED
PIN
(4)
The efï¬ciency of the LTC3208 depends upon the mode in
which it is operating. Recall that the LTC3208 operates
as a pass switch, connecting VBAT to CPO, until dropout
is detected at the ILED pin. This feature provides the op-
timum efï¬ciency available for a given input voltage and
LED forward voltage. When it is operating as a switch, the
efï¬ciency is approximated by:
η = PLED = VLED â¢ILED = VLED
PIN VBAT â¢IBAT VBAT
(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 LTC3208 is negligible and the expression above is
valid.
Once dropout is detected at any LED pin, the LTC3208
switches the charge pump to 1.5x mode.
In 1.5x boost mode, the efï¬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
efï¬ciency would be given by:
ηIDEAL
=
PLED
PIN
=
VLED â¢ILED
VBAT â¢1.5 â¢ILED
=
VLED
1.5 ⢠VBAT
Similarly, in 2x boost mode, the efï¬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 efï¬ciency would be given by:
ηIDEAL
=
PLED
PIN
=
VLED â¢ILED
VBAT ⢠2 â¢ILED
=
VLED
2 ⢠VBAT
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3208.
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.
3208fa
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