LTC3214

10

3214fb

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

represents lost power whether it is in the charge pump

or the current sources. Stated mathematically, the power

efficiency is given by:

η≡

P

P

LED

IN

The efficiency of the LTC3214 depends upon the mode in

which it is operating. Recall that the LTC3214 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

VI

VI

V

V

LED

IN

LED

LED

IN

IN

LED

IN

•

•

since the input current will be very close to the LED

current.

At moderate to high output power, the quiescent current

of the LTC3214 is negligible and the expression above is

valid.

enables the charge pump in 1.5x mode.

APPLICATIONS INFORMATION

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

LED

IN

LED

LED

IN

LED

LED

IN

P

P

VI

VI

V

V

≡=

≈

•

•.

.

15

15

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

LED

IN

LED

LED

IN

LED

LED

IN

P

P

VI

VI

V

V

≡=

≈

•

••

22

Thermal Management

For higher input voltages and maximum output current,

there can be substantial power dissipation in the LTC3214.

If the junction temperature increases above approximately

165°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.