LTC3210 Datasheet, PDF (14/16 Pages) Linear Technology – MAIN/CAM LED Controller in 3mm × 3mm QFN

LTC3210
APPLICATIO S I FOR ATIO
Power Efficiency
To calculate the power efficiency (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
efficiency is given by:
η = ^PLED
P_IN
The efficiency of the LTC3210 depends upon the mode in
which it is operating. Recall that the LTC3210 operates
as a pass switch, connecting V_BAT to CPO, until dropout
is detected at the LED pin. This feature provides the op-
timum efficiency available for a given input voltage and
LED forward voltage. When it is operating as a switch, the
efficiency is approximated by:
η = ^PLED = ^{(VLED •ILED)} = ^VLED
P_IN (V_BAT •I_BAT ) V_BAT
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 LTC3210 is negligible and the expression above is
valid.
Once dropout is detected at any LED pin, the LTC3210
enables the charge pump in 1.5x mode.
In 1.5x boost mode, the efficiency 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
efficiency would be given by:
η_IDEAL
=
P_LED
P_IN
=
(V_LED •I_LED)
(V_BAT • (1.5)•I_LED)
=
V_LED
(1.5 • V_BAT )
Similarly, in 2x boost mode, the efficiency 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 efficiency would be given by:
η_IDEAL
=
P_LED
P_IN
=
(V_LED •I_LED)
(V_BAT • (2)•I_LED)
=
V_LED
(2 • V_BAT )
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3210.
If the junction temperature increases above approximately
150°C the thermal shut down 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.
3210f
14