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LTC3207/LTC3207-1
OPERATION
Mode Switching
The LTC3207/LTC3207-1 will automatically switch from
1x mode to 1.5x mode and subsequently to 2x mode
whenever a dropout condition is detected at an LED pin.
Dropout occurs when an active current source voltage
becomes too low for the programmed current to be sup-
plied. The mode change will not occur unless dropout
has existed for approximately 400μs. This delay will allow
the LEDs to warm up and achieve the ?nal LED forward
voltage value.
The mode will automatically switch back to 1x whenever
a register is updated via the I 2 C port, when gradation
completes ramping down, on the falling edge of either
ENU or CAMHL and after each blink period.
The part can be forced to operate in 1x, 1.5x or 2x mode
by writing the appropriate bits into REG0. This feature
may be used for powering loads from CPO.
Nonprogrammed current sources do not affect dropout.
In addition, ENU controlled current sources do not affect
dropout when ENU is low.
Universal Current Sources (ULED1 to ULED12)
There are twelve universal 27.5mA current sources. Each
current source has a 6-bit linear DAC for current control.
The output current range is 0 to full-scale in 64 steps.
Each current source is disabled when an all zero data word
is written. The supply current for that source is reduced
to zero. Connect unused outputs to ground.
ULED1 to ULED12 can also be used as general purpose
outputs (GPO). Current sources in the GPO mode can be
used as I 2 C controlled open-drain drivers. The GPO mode
is selected by programming REG1 to REG12, Bit 6 and
Bit 7 to a logic one. In the GPO mode dropout detection
is disabled, output swings to ground will not cause mode
switching.
Camera Current Source
There is one CAM current source. This current source has
a 4-bit linear DAC for current control. The output current
range is 0mA to 425mA in 16 steps.
The current source is disabled when this section receives
an all zero data word. The supply current for the current
source is reduced to zero.
CAMHL
The CAMHL pin quickly selects the camera high register
for ?ash applications without re-accessing the I 2 C port.
When low, the CAM current range will be controlled by
the camera low 4-bit register. When CAMHL is asserted
high, the current range will be set by the camera high 4-bit
register. The charge pump mode will be reset to 1x on the
falling edge of CAMHL.
A safety timer will disable the CAM output after two sec-
onds whenever the high current mode is selected. To reset
the safety timer, the CAMHL signal from the external pin
or through the I 2 C port is set low. Alternatively the CAM
register can be written to all zeroes. If unused, the pin
should be connected to ground.
Blinking
Each universal output (ULED1 to ULED12) can be set to
blink 0.156s or 0.625s with a period of 1.25s or 2.5s via
the I 2 C port. The blinking rate is selected via REG15 and
ULED outputs are selected via REG1 to REG12. Blinking
and gradation rates are independent. Blink resets the charge
pump to 1x mode after each period. Please refer to Ap-
plication Note 108 for detailed information and examples
on programming blinking.
Gradation
Universal LED outputs ULED1 to ULED12 can be set to
have the current ramp up and down at 0.24s, 0.48s and
0.96s rates via the I 2 C port. The total gradation period is
longer than the ramp time since there is a region at the
start and end where the ULED current does not change.
A new gradation period cannot be started until after the
previous gradation period has ended. Each of these outputs
can have either blinking or gradation enabled. The grada-
tion time is set via REG15 and ULED outputs are selected
via REG1 to REG12. The ramp direction is controlled via
REG0. Setting the UP bit high causes gradation to ramp
up; setting this bit low causes gradation to ramp down.
Please refer to Application Note 108 for detailed informa-
tion and examples on programming gradation.
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