LTC3209-1 Datasheet, PDF (10/20 Pages) Linear Technology – 600mA Main/Camera LED Controller

LTC3209-1/LTC3209-2
OPERATIO
Consider the example of driving white LEDs from a 3.1V
supply. If the LED forward voltage is 3.8V and the current
sources require 100mV, the advantage voltage for 1.5x
mode is 3.1V • 1.5 – 3.8V – 0.1V or 750mV. Notice that if
the input voltage is raised to 3.2V, the advantage voltage
jumps to 900mV—a 20% improvement in available
strength.
From Figure 1, for 1.5x mode the available current is given
by:
I_OUT
=
1.5V_BAT –
R_OL
V_CPO
For 2x mode, the available current is given by:
I_OUT
=
2V_BAT – V_CPO
R_OL
3.2
V_BAT = 3V
V_CPO = 4.2V
3.0 C2 = C3 = C4 = 2.2µF
Notice that the advantage voltage in this case is
3.1V • 2 – 3.8V – 0.1V = 2.3V. R_OL is higher in 2x mode but
a significant overall increase in available current is achieved.
Typical values of R_OL as a function of temperature are
shown in Figures 2 and 3.
Shutdown Current
Shutdown occurs when all the current source data bits
have been written to zero or when DV_CC is below the DV_CC
UVLO threshold.
Although the LTC3209-1/LTC3209-2 is designed to have
very low shutdown current, it will draw about 3µA from
V_BAT when in shutdown. Internal logic ensures that the
LTC3209-1/LTC3209-2 is in shutdown when DV_CC is
grounded. Note, however that all of the logic signals that
are referenced to DV_CC (SCL, SDA, CAMHL) will need to be
at DV_CC or below (i.e., ground) to avoid violation of the
absolute maximum specifications on these pins.
3.8
V_BAT = 3V
V_CPO = 4.8V
3.6 C2 = C3 = C4 = 2.2µF
2.8
3.4
2.6
3.2
2.4
3.0
2.2
2.8
2.0
–40
–15 10
35
60
85
TEMPERATURE (°C)
3209 G06
2.6
–40
–15 10
35
60
85
TEMPERATURE (°C)
3209 G08
Figure 2. Typical 1.5x R_OL vs Temperature
Figure 3. Typical 2x R_OL vs Temperature
10
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