Rainbow Electronics MAX6973 - Manual (Page 9)

LED Intensity Control
The MAX6972/MAX6973 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 MAX6972/MAX6973 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 Y and Z CALDACs set the output current level
for all 8 outputs in the Y and Z ports, respectively (see
the MAX6972/MAX6973 Block Diagrams). The Y
CALDAC and Z CALDAC range from a low of 11mA
(0x00) to a maximum of 55mA (0xFF), providing
172µA/step of current trimming. The CALDACs are
loaded by the serial interface using command 01 (see
Table 4). The Z CALDAC data is loaded first, followed
by the Y CALDAC data (see the Serial Interface section).
The loaded data takes effect immediately.
Global-Intensity Control
The MAX6972/MAX6973 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 MAX6972/
MAX6973 at f
CLKI = 31.5MHz allows a video-frame
update rate of 60fps for full-motion video (see the
MAX6972 Video-Frame Timing and MAX6973 Video-
Frame Timing sections).
The MAX6972/MAX6973 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 MAX6972 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 MAX6973 features 5-, 4-, 3-, and 2-bit
global-intensity control to yield 32, 16, 8, and 4 sub-
frames per frame, respectively.
The MAX6972/MAX6973 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.
MAX6972/MAX6973
16-Output PWM LED Drivers
for Message Boards
_______________________________________________________________________________________ 9
10
20
30
50
(mA)
CALDAC
CURRENT
40
55mA MAX
11mA MIN
127 0 GLOBAL
= 96
GLOBAL-INTENSITY
PDM
INDIVIDUAL-INTENSITY
PWM
4095 0 Yn or Zn PWM
= 2560
Yn or Zn I AVE = 18.750mA
CALDAC
= 169
255 0
40mA
100%
0%
50%
100%
0%
50%
100%
0%
50%
Figure 2. Relationship Among the CALDACs, Global-Intensity, and Individual-Intensity PWM Controls