LTC3220/LTC3220-1
9
32201fd
For more information www.linear.com/LTC3220
operaTion
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:
OUT
=
1.5V
IN
− V
CPO
R
(1)
For 2x mode, the available current is given by:
OUT
=
2V
IN
− V
CPO
R
(2)
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.
Mode Switching
The LTC3220/LTC3220-1 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 a current source voltage becomes
too low for the programmed current to be supplied. The
mode change will not occur unless dropout exists for
approximately 400µs.
The mode will automatically switch back to 1x whenever
a register is updated via the I
2
C port, when gradation
completes ramping down and after each blink period.
The parts 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 operating loads powered by CPO.
Non-programmed current sources do not affect dropout.
Universal Current Sources (ULED1 to ULED18)
There are eighteen universal 20mA current sources. Each
current source has a 6-bit linear DAC for current control.
The output current range is 0mA to 20mA 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. Unused outputs should be connected to GND.
GPO Mode
ULED1 to ULED18 can be used as general purpose outputs
(GPO). Current sources in the GPO mode can be used as
I
2
C controlled open-drain drivers. A ULED output can be
selected to operate in GPO mode by programming both Bit 6
and Bit 7 of its data register (REG1 to REG18) to a logic
high. In the GPO mode, dropout detection is disabled and
output swings to ground will not cause mode switching.
The GPOs can be programmed to either act as a switch
(strong pull-down mode) in which the part will only con
-
sume approximately 3µA
of quiescent current, or they can
be programmed to have a regulated current of up to 20mA
(current limit mode), which would require several hundred
microamps of additional quiescent current.
When a ULED output is used in GPO mode during shut
-
down, CPO should not be used as a power source since
the current available from the CPO pin would be limited
by the weak pull-up current sour
ce
. This weak pull-up is
only meant to keep the output capacitor charged to V
IN
during shutdown and is unable to supply large amounts
of current. CPO can, however, be used as a power source
when the part is enabled.
Conversely, when a ULED output is used in GPO strong
pull-down mode, a current limiting resistor should be used
in series with the ULED output so that the current does
not exceed the Absolute Maximum rated current.
