LTC3220-1_15 Datasheet, PDF (18/20 Pages) Linear Technology – 360mA Universal 18-Channel LED Driver

LTC3220/LTC3220-1
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
(4)
P_IN
The efficiency of the LTC3220/LTC3220-1 depends upon
the mode in which it is operating. Recall that the LTC3220/
LTC3220-1 operate as pass switches, 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 = ^PLED = ^{VLED •ILED} = ^VLED
(5)
P_IN
V_IN •I_IN
V_IN
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 LTC3220/LTC3220-1 is negligible and the expres-
sion above is valid.
Once dropout is detected at any LED pin, the LTC3220/
LTC3220-1 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_IN • 1.5 •I_LED
=
V_LED
1.5 • V_IN
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
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3220/
LTC3220-1. 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 recom-
mended. 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.
32201fc
18