MAX6975 Maxim Integrated Products, MAX6975 Datasheet - Page 9

• Manufacturer Part Number: MAX6975
• Description: (MAX6974 / MAX6975) 24-Output PWM LED Drivers
• Manufacturer: Maxim Integrated Products

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
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).
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
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.
Figure 2. Relationship Among the CALDACs, Global-Intensity, and Individual-Intensity PWM Controls
(mA)
30
25
20
15
10
100%
5
50%
0%
0
_______________________________________________________________________________________
6mA MIN
Table
Figure
CURRENT
CALDAC
4). The B CALDAC data is
LED-Intensity Control
2). The individual-intensity
CALDAC
30mA MAX
= 169
Calibration DACs
21.8mA
255
100%
50%
0%
0
GLOBAL-INTENSITY
24-Output PWM LED Drivers
PDM
GLOBAL
= 96
The MAX6974/MAX6975 adjust global and individual
intensities over a time period called a frame. One frame
requires 2
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
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.
127
100%
50%
0%
0
for Message Boards
INDIVIDUAL-INTENSITY
19
Rn, Gn, OR Bn PWM
(524,288) periods of CLKI and corre-
CLKI
PWM
= 2560
= 31.5MHz allows a video-frame
Global-Intensity Control
4095
Rn, Gn, OR Bn I
AVE
= 10.22mA
9