LTC3216
10
3216fc
APPLICATIONS INFORMATION
LED forward voltage. When it is operating as a switch, the
effi ciency is approximated by:
(5)
P
LED
P
IN
=
V
LED
•I
LED
V
IN
•I
IN
V
LED
V
IN
since the input current will be very close to the LED current.
At moderate to high output power, the quiescent current
of the LTC3216 is negligible and the expression above is
valid.
Once dropout is detected at the I
LED
pin, the LTC3216
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:
(6)
IDEAL
P
LED
P
IN
=
V
LED
•I
LED
V
IN
• 1.5I
LED
V
LED
1.5V
IN
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:
(7)
IDEAL
P
LED
P
IN
=
V
LED
•I
LED
V
IN
•2•I
LED
V
LED
2•V
IN
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3216.
If the junction temperature increases above approximately
150°C, the thermal shutdown circuitry will automatically
deactivate the output. To reduce maximum junction tem-
perature, 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 can reduce the thermal resistance of the package
and PC board considerably.
