LTC3207-1_15 Datasheet, PDF (17/20 Pages) Linear Technology – 600mA Universal Multi-Output LED/CAM Driver

LTC3207/LTC3207-1
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
P_IN
The efficiency of the LTC3207/LTC3207-1 depends upon
the mode in which they are operating. Recall that the
LTC3207/LTC3207-1 operate as pass switches, connect-
ing V_BAT 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
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 LTC3207/LTC3207-1 is negligible and the expres-
sion above is valid.
Once dropout is detected at any the LED pin, the LTC3207/
LTC3207-1 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_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
2x 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 LTC3207/
LTC3207-1. 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.
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