LTC3210 datasheet(13/16 Pages) LINER

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LTC3210

3210fb

• The flying capacitors must be placed close to the part.

The traces from the pins to the capacitor pad should

be as wide as possible.

• VBAT, CPO traces must be wide to minimize inductance

and handle high currents.

• LED pads must be large and connected to other layers

of metal to ensure proper heat sinking.

• RM and RC pins are sensitive to noise and capacitance.

The resistors should be placed near the part with mini-

mum line width.

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 =

LED

The efficiency of the LTC3210 depends upon the mode in

which it is operating. Recall that the LTC3210 operates

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

is detected at the LED 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 =

LED

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 LTC3210 is negligible and the expression above is

valid.

Once dropout is detected at any LED pin, the LTC3210

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

LED

LED LED

BAT

LED

BAT

(

•

)

(

• ( . )•

) ( . •

)

1 5

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

LED

LED LED

BAT

LED

BAT

(

•

)

(

• ( )•

) ( •

)

Thermal Management

For higher input voltages and maximum output current,

there can be substantial power dissipation in the LTC3210.

Ifthejunctiontemperatureincreasesaboveapproximately

150°C the thermal shut down 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

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