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 2
19
(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 f
CLKI
= 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
MAX6974/MAX6975
24-Output PWM LED Drivers
for Message Boards
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