LTC3210-2 Datasheet, PDF (14/16 Pages) Linear Technology – MAIN/CAM LED Controllers with 32-Step Brightness Control in 3mm × 3mm QFN

LTC3210-2/LTC3210-3
APPLICATIONS INFORMATION
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 LTC3210-2/LTC3210-3 depends upon
the mode in which it is operating. Recall that the LTC3210-2/
LTC3210-3 operates as a pass switch, connecting V_BAT to
CPO, until dropout is detected at the LED pin. This feature
provides the optimum 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 LTC3210-2/LTC3210-3 is negligible and the expres-
sion above is valid.
Once dropout is detected at any LED pin, the LTC3210-2/
LTC3210-3 enable 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 cur-
rent, there can be substantial power dissipation in the
LTC3210-2/LTC3210-3. 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.
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