LTC3212

0

3212fb

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 =

P

P

LED

IN

The efficiency of the LTC3212 depends upon the mode in

which it is operating. Recall that the LTC3212 operates

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 =

=

=

P

P

V

I

V

I

V

V

LED

IN

LED LED

BAT BAT

LED

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 LTC3212 is negligible and the expression above is

valid.

Once dropout is detected at any LED pin, the LTC3212

enables the charge pump in 2x mode.

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:

LED

IN

LED LED

BAT

LED

LED

P

P

V

I

V

I

V

V

=

=

=

•

• •

•

2

In some applications it may be possible to increase the

efficiency of the LTC3212. If any of the LED’s maximum

not needed to drive that LED. This is often the case with

the red LED due to its lower forward voltage. Its anode

pump’s losses in 2x mode.

Thermal Management

Ifthejunctiontemperatureincreasesaboveapproximately

140°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.

applicaTions inForMaTion