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LTC3220/LTC3220-1

32201fb

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)

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 VIN to

CPO, until dropout is detected at the ILED 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

VIN •IIN

VLED

VIN

(5)

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.

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:

ηIDEAL =

PLED

VLED •ILED

VIN •1.5 •ILED

VLED

1.5 • VIN

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:

ηIDEAL =

PLED

VLED •ILED

VIN •2•ILED

VLED

2• VIN

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.

LTC3220 Datasheet(PDF) 18 Page - Linear Technology

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