ltc3210-2 Linear Technology Corporation, ltc3210-2 Datasheet - Page 14

• Manufacturer Part Number: ltc3210-2
• Description: Main/cam Led Controllers With 32-step Brightness Control In 3mm ? 3mm Qfn
• Manufacturer: Linear Technology Corporation

LTC3210-2/LTC3210-3
APPLICATIONS INFORMATION
Power Effi ciency
To calculate the power effi ciency (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
effi ciency is given by:
The effi ciency of the LTC3210-2/LTC3210-3 depends upon
the mode in which it is operating. Recall that the LTC3210-2/
LTC3210-3 operates as a pass switch, connecting V
CPO, until dropout is detected at the LED pin. This feature
provides the optimum effi ciency available for a given input
voltage and LED forward voltage. When it is operating as
a switch, the effi ciency is approximated by:
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 LTC3210-2/LTC3210-3 is negligible and the expres-
sion above is valid.
Once dropout is detected at any LED pin, the LTC3210-2/
LTC3210-3 enable the charge pump in 1.5x mode.
14
η =
η =
P
P
P
P
LED
LED
IN
IN
=
(
(
V
V
BAT
LED
•
•
I
I
LED
BAT
)
)
=
V
V
LED
BAT
BAT
to
In 1.5x boost mode, the effi ciency 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
effi ciency would be given by:
Similarly, in 2x boost mode, the effi ciency 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 effi ciency would be given by:
Thermal Management
For higher input voltages and maximum output cur-
rent, there can be substantial power dissipation in the
LTC3210-2/LTC3210-3. If the junction temperature
increases above approximately 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.
η
η
IDEAL
IDEAL
=
=
P
P
P
P
LED
LED
IN
IN
=
=
(
(
V
V
BAT
(
BAT
V
(
LED
V
LED
• ( )•
• ( . )•
2
•
1 5
I
•
LED
I
LED
I
LED
)
I
LED
)
)
=
)
( •
=
2
V
( . •
1 5
LED
V
BAT
V
LED
V
)
BAT
321023fa
)