Rainbow Electronics MAX6975 - Manual (Page 9)

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/MAX6975 Block
Diagrams). The R CALDAC, G CALDAC, and B CAL-
DAC 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.
MAX6974/MAX6975
24-Output PWM LED Drivers
for Message Boards
_______________________________________________________________________________________ 9
5
10
15
25
30
(mA)
CALDAC
CURRENT
20
30mA MAX
6mA MIN
127 0 GLOBAL
= 96
GLOBAL-INTENSITY
PDM
INDIVIDUAL-INTENSITY
PWM
4095 0 Rn, Gn, OR Bn PWM
= 2560
Rn, Gn, OR Bn I AVE = 10.22mA
CALDAC
= 169
255 0
21.8mA
100%
0%
50%
100%
0%
50%
100%
0%
50%
Figure 2. Relationship Among the CALDACs, Global-Intensity, and Individual-Intensity PWM Controls