3-Channel Constant Current LED Driver

with Programmable PWM Control

A6281

7

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

given in the following table:

Duty Cycle

0

0/1024 (0 %)

1

2/1024

2

3/1024

. . .

. . .

1023

1024/1024 (100 %)

When the brightness register is set to zero, the outputs remain off

for the duration of the PWM cycle for a 0% DC. When a bright-

ness register is set to 1023, the LED for that output remains on

(100% DC) when OEI is active and begins the PWM cycle. The

output remains on when the PWM counter rolls over and begins

a new count.

The PWM counter begins counting at zero and increments only

when the OEI pin is held low. When the PWM counter reaches

the count of 1024, the counter resets to zero and continues

incrementing. The counter resets to zero on a rising edge of OEI,

upon recovery from UVLO, or when powering up. Latching new

data into the brightness registers will not reset the PWM counter.

A free-running internal 800 kHz oscillator is the master clock

for the PWM counter. A programmable clock divider frequency

allows the PWM to be set at approximately at 200 kHz, 400 kHz,

or 800 kHz, or the PWM can be set to count on the rising edge

of the external CI signal. Bit assignments for the programmable

clock divider are shown in the following table:

Bits

Clock Mode

78

0

0

800 kHz

1

0

400 kHz

01

External (count on rising

edge of CI signal)

1

1

200 kHz

The total number of possible colors of an RGB pixel is over

1 billion. Refer to figure 4 for the mapping of shift register bits

to latches.

Output Current Selection

The overall maximum current is set by the external resistor,

being set, the maximum current remains constant regardless of

the LED voltage variation, supply voltage variation, temperature,

or other circuit parameters. The maximum output current can be

calculated using the following equation:

shown in figure 6.

Internal Linear Regulator

The A6281 has a built-in linear regulator. The regulator operates

from a supply voltage of 5.5 to 17 V. It allows the VIN pin of the

A6281 to connect to the same supply as the LEDs. This simpli-

fies board design by eliminating the need for a chip supply bus

and external voltage regulators. For 5 V supplies, connect VIN

to VREG externally. Note: When using 5 V supplies, ensure that

VIN does not exceed the absolute maximum rating of the VREG

pin (6 V).

The VREG pin is used by the internal linear regulator to connect

to a bypass capacitor. This pin is for internal use only and is not

Bits

0 1 2 3 4 5 6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

29

31

PWM Counter 0

PWM Counter 1

PWM Counter 2

Dot Correction

Register 0

Clock

Mode

X

Dot Correction

Register 1

XXX

Dot Correction

Register 2

Figure 5. Register Configuration

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

515

25

35

45

55

65

75