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LTC3205

3205f

APPLICATIO S I FOR ATIO

LTC3205

IMS

24.9k

3205 F11

VCNTRL

IRGB

1.223V

R1||R2

VCNTRL

–

ILED = 400

()

LTC3205

IMS

24.9k

38.3k

3205 F12

VDIG

0V TO 1.3V

OR HIGHER

71.5k

IRGB

Figure 11. Alternative Linear Brightness Control

Figure 12. Alternative Digital Brightness Control

LTC3205

IMS

3205 F13

PWM SIGNAL

0V TO 1.3V OR HIGHER

BRIGHTNESS = 1 – D

24.9k

IRGB

Figure 13. PWM Brightness Control of the MAIN and SUB Displays

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 IMS or IRGB pins as shown in Figure 10.

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.

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 IMS or IRGB

pins to control brightness. For example, if R1 and R2 are

50k, then the LED current would range from 20mA to 0mA

as VCNTRL 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 VCNTRL 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 IMS programming resistor as

shown in Figure 13. The signal should range from 0V (full

on) to any voltage above 1.3V (full off).

LTC3205

IMS

800

Ω

80k

10k

24.3k

1.8V

3205 F10

IRGB

–

Figure 10. Precision Reference Current

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