LTC3206 Datasheet, PDF (10/16 Pages) Linear Technology – I2C Multidisplay LED Controller

LTC3206
APPLICATIO S I FOR ATIO
White LED Brightness Control
The White LED displays (MAIN, SUB and AUX) have 16
individual brightness settings. The settings are exponen-
tially spaced to compensate for the nearly logarithmic
characteristic of human vision perception. The base of the
power settings is √2 . The off setting (0 power) is a special
case needed for shutdown.
The LTC3206 uses a subranging technique to control the
LED brightness with a combination of both DC level
control and pulse width modulation. Table 1 summarizes
the level control operation. The DC level of the LEDs will be
one of either three sub-range settings, 100%, 50% or 25%
of full scale. For example, if the full scale LED current is
programmed (via the IMS pin) to be 20mA, then the “on”
level of the LED will be either 20mA, 10mA or 5mA
respectively. The power to the LED will be the product of
the subrange (DC current) and the PWM setting. For
example, if an LED power of 2.25% is desired, then the
LTC3206 sets the sub range to 25% and the duty cycle to
8.8%. These settings are designed to optimize the effi-
ciency of the dual-mode LTC3206 power management
system while preserving LED color accuracy at low power
levels.
To achieve brightness control by purely DC means, only
the 100%, 50% or 25% power settings should be selected.
The DC current levels of the MAIN, SUB and AUX LEDs are
controlled by a precisely mirrored multiple of the current
at the I_MS pin. The I_MS pin servos to a fixed level of 0.6V so
the current is programmed simply by adding a resistor
from I_MS to ground.
The current that flows during the “on” time will follow the
relationship:
I_LED
=
400
•
S
•
0.6V
R_MS
where S is the subrange for the given power setting (it will
be either 25%, 50% or 100%, see Table 1) and R_MS is the
value of the resistor at the I_MS pin.
The average LED current (LED power level) will follow the
relationship:
10
AVG
(I_LED )
=
400
•
0.6V
R_MS
•
2^D−15
where D is the decimal equivalent of the 4-bit digital code
programmed for the given display (0 to 15).
The PWM frequency is 1/1024 of the frequency of the
charge pump oscillator (typically 938Hz). During PWM,
the LED currents are soft-switched to minimize noise.
AUX LEDs
The AUX1 and AUX2 LEDs can be arbitrarily assigned to
either the MAIN or SUB display. Table 2 summarizes the
assignment possibilities. When an AUX pin is assigned to
a display, it will follow the power level (both DC and PWM)
set for that display.
Unused White LED Pins
The LTC3206 can power up to eight white LEDs (four for
the MAIN display, two for the SUB display and the two
flexible AUX pins), however, it is not necessary to use all
eight in each application.
Any of these LED pins can cause the LTC3206 to switch
from 1x mode to 1.5x charge pump mode if they drop out.
In fact, if an unused LED pin is left unconnected or
grounded, it will drop out and force the LTC3206 into
charge pump mode.
To avoid this problem, unused MAIN, SUB or AUX LED
pins can be disabled by connecting them to CPO. Power is
not wasted in this configuration. When the LED pin voltage
is within approximately 1V of CPO, its LED current is
switched off and only a small 10µA test current remains.
Figure 5 shows a block diagram of each of the MAIN, SUB
and AUX LED pins.
CPO
1V
–+
MAIN1-MAIN4
SUB1, SUB2, AUX1, AUX2
ENABLE
I_LED
10µA
3206 F05
Figure 5. Internal MAIN, SUB and AUX LED Disable Circuit
3206f