LTC3205_15 Datasheet, PDF (13/16 Pages) Linear Technology – Multidisplay LED Controller

LTC3205
APPLICATIO S I FOR ATIO
If the desired input voltage range is below the data shown
in Figure 9, and a precise control of the LED current is
desired, then a precision current source may be added to
either the I_MS or I_RGB pins as shown in Figure 10.
1.8V
80k
+
LTC3205
14
I_MS
13
I_RGB
–
10k
24.3k
800Ω
3205 F10
Figure 10. Precision Reference Current
Brightness Control
Although the LTC3205 has three exponentially spaced
brightness settings for the main and sub displays, it is
possible to control the brightness by alternative means.
Figure 11 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
50k, then the LED current would range from 20mA to 0mA
as V_CNTRL is swept from 0V to 2.5V.
Alternatively, if only digital outputs are available, the
number of settings can be doubled from 3 to 6 by simply
connecting V_CNTRL to a digital signal. With a 1.8V logic
supply, the circuit shown in Figure 12 has LED current
settings of 2.5mA, 5mA, 7.5mA, 10mA, 15mA and 20mA.
This topology can be extended to any number of bits and
can also be applied to the RGB panel.
Finally, PWM brightness control can be achieved by
applying a PWM signal to the I_MS programming resistor as
shown in Figure 13. The signal should range from 0V (full
on) to any voltage above 1.3V (full off).
Thermal Management
For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3205.
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.
LTC3205
I_MS
( ) ^ILED
=
400
1.223V
R1^||R2
–
V_CNTRL
R2
14
R2
V_CNTRL
13
I_RGB
R1
24.9k
3205 F11
Figure 11. Alternative Linear Brightness Control
LTC3205
14
I_MS
13
I_RGB
24.9k
71.5k
38.3k
V_DIG
0V TO 1.3V
OR HIGHER
3205 F12
Figure 12. Alternative Digital Brightness Control
LTC3205 _{14 24.9k}
I_MS
13 ^24.9k
I_RGB
PWM SIGNAL
0V TO 1.3V OR HIGHER
BRIGHTNESS = 1 – D
3205 F13
Figure 13. PWM Brightness Control of the MAIN and SUB Displays
3205f
13