LTC3215 Datasheet, PDF (10/12 Pages) Linear Technology – 700mA Low Noise High Current LED Charge Pump

LTC3215
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 repre-
sents lost power whether it is in the charge pump or the
current sources. Stated mathematically, the power effi-
ciency is given by:
η
≡
P_LED
P_IN
(4)
The efficiency of the LTC3215 depends upon the mode in
which it is operating. Recall that the LTC3215 operates as
a pass switch, connecting V_IN to CPO, until dropout is
detected at the I_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
≡
P_LED
P_IN
=
V_LED • I_LED
V_IN • I_IN
≈
V_LED
V_IN
(5)
since the input current will be very close to the LED
current.
At moderate to high output power, the quiescent current
of the LTC3215 is negligible and the expression above is
valid.
Once dropout is detected at the I_LED pin, the LTC3215
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_IN • 1.5I_LED
≈
V_LED
1.5V_IN
(6)
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_IN • 2 • I_LED
≈
V_LED
2 • V_IN
(7)
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3215.
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.
3215f
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