LTC3208EUH Linear Technology, LTC3208EUH Datasheet - Page 19

• Manufacturer Part Number: LTC3208EUH
• Description: IC,Laser Diode/LED Driver,CMOS,LLCC,32PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Power Effi ciency
To calculate the power effi 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
effi ciency is given by:
The effi ciency of the LTC3208 depends upon the mode in
which it is operating. Recall that the LTC3208 operates
as a pass switch, connecting V
is detected at the I
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:
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.
η =
η =
P
P
P
P
LED
LED
IN
IN
=
V
V
BAT
LED
LED
U
•
•
I
I
pin. This feature provides the op-
LED
BAT
U
=
V
V
LED
BAT
BAT
to CPO, until dropout
W
U
(4)
(5)
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:
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:
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.
η
η
IDEAL
IDEAL
=
=
P
P
P
P
LED
LED
IN
IN
=
=
V
V
BAT
BAT
V
V
LED
LED
• . •
• •
1 5
2
•
•
I
LED
I
LED
I
LED
I
LED
=
=
2
LTC3208
V
•
1 5
LED
V
. •
BAT
V
LED
V
BAT
19
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