LTC3214

10

3214fa

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 repre-

sents lost power whether it is in the charge pump or the

current sources. Stated mathematically, the power effi-

ciency 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 as

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.

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.

APPLICATIO S I FOR ATIO