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MAX6974

24-Output PWM LED Drivers for Message Boards

Maxim Integrated 

24-Output PWM LED Drivers

for Message Boards

LED-Intensity Control

The MAX6974/MAX6975 provide three levels of output

current control for LED drive: calibration DACs

(CALDACs), global-intensity control, and individual-

intensity control. The CALDACs set the port output cur-

rent levels, while the global-intensity and individual-

intensity controls modulate the output current on/off

times, providing a fine-resolution control of average

output currents (see Figure 2). The individual-intensity

control operates on each output independently to set

each individual LED intensity level. The global-intensity

controls modulate MAX6974/MAX6975 outputs simulta-

neously for a uniform brightness control without affect-

ing color. Using a fixed output current level that is

modulated only by on/off control leaves the LED color

unaffected while precisely controlling intensity. Finally,

all outputs can be turned on and off simultaneously by

setting or clearing configuration bit D3 (PWM-ON).

Calibration DACs

The 8-bit R, G, and B CALDACs set the output current

level for all eight outputs in the R, G, and B ports,

respectively (see the MAX6974 Block Diagram and

MAX6975 Block Diagram). The R CALDAC, G

CALDAC, and B CALDAC range from a low of 6mA

(0x00) to a maximum of 30mA (0xFF), providing

94µA/step of current trimming. The CALDACs are

loaded by the serial interface using command 01 (see

Table 4). The B CALDAC data is loaded first, followed

by the G CALDAC data, and then the R CALDAC data

(see the Serial Interface section). The loaded data takes

effect immediately.

Global-Intensity Control

The MAX6974/MAX6975 adjust global and individual

intensities over a time period called a frame. One frame

requires 219 (524,288) periods of CLKI and corre-

sponds to one video-frame time. Video frames generally

contain consecutive images displayed rapidly to yield

a motion picture display. Running the MAX6974/

MAX6975 at fCLKI = 31.5MHz allows a video-frame

update rate of 60fps for full-motion video (see the

MAX6974 Video-Frame Timing and MAX6975 Video-

Frame Timing sections).

The MAX6974/MAX6975 further divide frames into sub-

frames to allow a unique combination of global- and

individual-intensity controls. The number of subframes

is equal to the number of global-intensity control steps.

The MAX6974 uses 128 subframes per frame in

nonmultiplexed mode (corresponding to 7-bit global-

intensity PDM control) and 64 subframes in multiplexed

mode (corresponding to 6-bit global-intensity PDM

control). The MAX6975 features 5-, 4-, 3-, and 2-bit

global-intensity control to yield 32, 16, 8, and 4 sub-

frames per frame, respectively.

The MAX6974/MAX6975 control global intensity by

driving subframes on and off. When a subframe is on, it

allows the individual PWM intensity control to be driven

on the outputs. Subframes that are off do not have any

PWM modulation on the outputs.

Individual PWM Control

The MAX6974/MAX6975 further modulate the time that

each subframe is ON by a pulse-width modulation

(mA)

100%

30

CALDAC

CURRENT

30mA MAX

GLOBAL-INTENSITY

PDM

INDIVIDUAL-INTENSITY

PWM

25

20

50%

15

10

0%

5 6mA MIN

0

21.8mA 100%

50%

CALDAC

= 169

0%

255 0

100%

Rn, Gn, OR Bn IAVE = 10.22mA

50%

0%

GLOBAL 127 0

= 96

Rn, Gn, OR Bn PWM 4095

= 2560

Figure 2. Relationship Among the CALDACs, Global-Intensity, and Individual-Intensity PWM Controls

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