LTC3205

9

3205f

OPERATIO

APPLICATIO S I FOR ATIO

Interfacing to a Microcontroller

The serial port of the LTC3205 can be connected directly

to an MC68HC11 style microcontroller’s serial port. The

microcontroller should be configured as the master device

and its clock’s idle state should be set to high (MSTR = 1,

CPOL = 1 and CPHA = 1 for the MC68HC11 family).

Figure 5 shows the recommended configuration and

directon of data flow. Note that an additional I/O line is

Figure 5. Microcontroller Interface

µCONTROLLER

MOSI

SCK

GPIO

3205 F05

LTC3205

SCLK

LD

Unused MAIN or SUB Display LED Pins

Any of the six white LED pins (MAIN1-MAIN4, SUB1 and

SUB2) can cause the LTC3205 to switch from 1:1 mode to

2:3 charge pump mode if they drop out. If an unused LED

pin is left unconnected or grounded, it will automatically

drop out and force the LTC3205 into charge pump mode.

To avoid this problem, unused LED pins on the MAIN and

SUB displays should be connected to CPO. However, 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 10

µA test current remains.

Figure 4 shows a block diagram of each of the MAIN and

SUB LED pins. The RED, GREEN and BLUE pins do not affect

the state of the charge pump so they can be left floating or

grounded if unused.

total of 4096 colors. Table 3 indicates the decoding of the

red, green and blue LEDs.

The full-scale currents in the red, green and blue LEDs are

regulated at 1.223V and the LED current is a precise

The RGB display LED currents will follow the relationship:

I

V

R

RED GREENBLUE

RGB

,,

.

= 400

1 223

Figure 4. Internal MAIN and SUB Panel LED Disable Circuit

–

+

+

–

~1V

CPO

MAIN1-MAIN4

SUB1, SUB2

10

µA

3205 F04

ENABLE

Table 3. RGB Duty Cycles

D11

D10

D9

D8

D7

D6

D5

D4

HEX

DUTY

D3

D2

D1

D0

CODE

CYCLE (%)

0

0

0

0

0

0/15

0

0

0

1

1

1/15

0

0

1

0

2

2/15

0

0

1

1

3

3/15

0

1

0

0

4

4/15

0

1

0

1

5

5/15

0

1

1

0

6

6/15

0

1

1

1

7

7/15

1

0

0

0

8

8/15

1

0

0

1

9

9/15

1

0

1

0

A

10/15

1

0

1

1

B

11/15

1

1

0

0

C

12/15

1

1

0

1

D

13/15

1

1

1

0

E

14/15

1

1

1

1

F

15/15

RGB Illuminator Drivers

The red, green and blue LEDs can be individually set to

have a duty cycle ranging from 0/15 (off) to 15/15 (full on)

with 1/15 increments in between. The combination of 16

possible brightness levels gives the RGB indicator LED a