LTC3215EDD データシートの表示（PDF） - Linear Technology

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LTC3215EDD

700mA Low Noise High Current LED Charge Pump

Linear Technology 

LTC3215

APPLICATIONS INFORMATION

The efficiency of the LTC3215 depends upon the mode in

which it is operating. Recall that the LTC3215 operates

as a pass switch, connecting VIN to CPO, until dropout

is detected at the ILED pin. This feature provides the op-

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≡

PLED

PIN

=

VLED

VIN

• ILED

• IIN

≈

VLED

VIN

(5)

since the input current will be very close to the LED current.

At moderate to high output power, the quiescent current of

the LTC3215 is negligible and the expression above is valid.

Once dropout is detected at the ILED pin, the LTC3215

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:

ηIDEAL

≡

PLED

PIN

=

VLED •ILED

VIN • 1.5ILED

≈

VLED

1.5VIN

(6)

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

PIN

=

VLED •ILED

VIN • 2 •ILED

≈

VLED

2 • VIN

(7)

Thermal Management

For higher input voltages and maximum output current,

there can be substantial power dissipation in the LTC3215.

If the junction temperature increases above approximately

150°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.

3215fc

11

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