LTC3219_15 Datasheet, PDF (17/20 Pages) Linear Technology – 250mA Universal Nine Channel LED Driver

LTC3219
APPLICATIONS INFORMATION
Power Efficiency
To calculate the power efficiency (η) of an 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 cur-
rent sources. Stated mathematically, the power efficiency
is given by:
η = ^PLED
P_IN
(4)
The efficiency of the LTC3219 depends upon the mode in
which it is operating. Recall that the LTC3219 operates
as a pass switch, connecting V_BAT to CPO, until dropout
is detected at the I_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
(5)
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 LTC3219 is negligible and the expression above is
valid.
Once dropout is detected at any LED pin, the LTC3219
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 LTC3219.
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.
3219fa
17