LTC3206 Datasheet, PDF (14/16 Pages) Linear Technology – I2C Multidisplay LED Controller

LTC3206
APPLICATIO S I FOR ATIO
Once an LED pin drops out, the LTC3206 switches into
step-up mode. Employing the fractional ratio 1.5x charge
pump, the LTC3206 provides more efficiency than would
be achieved with a voltage doubling charge pump.
In 1.5x boost mode, the efficiency 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 fractional charge pump is approximately 1.5
times the load current. In an ideal 1.5x charge pump, the
power efficiency would be given by:
η_IDEAL
≡
P_LED
P_IN
=
V_LED •I_LED
V_IN •1.5I_LED
≅
V_LED
1.5V_IN
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3206.
If the junction temperature increases above approximately
160°C the thermal shutdown circuitry will automatically
deactivate the output. To reduce the maximum junction
temperature, a good thermal connection to the PC board
is recommended. Connecting the PGND pin (exposed
center pad) to a ground plane and maintaining a solid
ground plane under the device can reduce the thermal
resistance of the package and PC board considerably.
Brightness Control
Although the LTC3206 has many exponentially spaced
brightness settings for the main and sub displays, it is
possible to control the brightness by alternative means.
Figure 8 shows an example of how an external voltage
source can be use to inject a current into the I_MS or I_RGB
pins to control brightness. For example, if R1 and R2 are
24k, then the LED current would range from 20mA to 0mA
as V_CNTRL is swept from 0V to 1.2V.
Alternatively, if only digital outputs are available, the
number of settings can be doubled from 15 to 30 by simply
LTC3206
I_MS
( ) ^{ILED = 400}
0.6V
R1||R2
–
V_CNTRL
R2
11
R2
V_CNTRL
12
I_RGB
R1
12k
3206 F08
Figure 8. Alternative Linear Brightness Control
connecting V_CNTRL to a digital signal. This topology can be
extended to any number of bits and can also be applied to
the RGB display.
Finally, PWM brightness control can be achieved by apply-
LTC3206
11
I_MS
12
I_RGB
12k
34k
18.2k
V_DIG
0V TO 0.7V
OR HIGHER
3206 F09
Figure 9. Alternative Digital Brightness Control
ing a PWM signal to the I_MS programming resistor as
shown in Figure 10. The signal should range from 0V (full
on) to any voltage above 0.7V (full off).
LTC3206 _{11 12k}
I_MS
12 ^12k
I_RGB
PWM SIGNAL
0V TO 0.7V OR HIGHER
BRIGHTNESS = 1 – D
3206 F10
Figure 10. PWM Brightness Control of the MAIN and SUB Displays
3206f
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