MAX6975 Maxim Integrated Products, MAX6975 Datasheet - Page 13

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

Setting configuration bit D0 to 1 enables multiplex
operation. MUX0 and MUX1 alternate the LED anode
drive voltage between two sets of LEDs. The R, G, and
B ports provide individual PWM control during alternate
MUX cycles as shown in
cycles reduce the global-intensity resolution (the num-
ber of subframes) by half, which reduces the average
LED current by half.
A selectable watchdog timer monitors serial-interface
inputs CLKI, DIN, and LOADI. Enabling the watchdog
timer requires that CLKI, DIN, and LOADI toggle at
least once every 40ms. If any of these transitions fails to
occur, then the individual-intensity PWM data latches
clear. This condition effectively blanks the LEDs.
Update the individual-intensity PWM data registers to
turn the LEDs back on. The watchdog timeout does not
affect the calibration or global-intensity data, the clock
synchronization, or multiplexed/nonmultiplexed setting.
Use the watchdog functionality in safety-critical appli-
cations where a blanked display is safer than an incor-
rect display.
Table 3. MAX6974/MAX6975 Timing Comparison
MAX6974
MAX6975
MAX6974
MAX6975
PART
PART
GLB4
BIT
MUX
X
X
0
1
BIT
0
1
0
1
______________________________________________________________________________________
MUX
BIT
OPERATION
Nonmultiplex
Nonmultiplex
0
1
0
1
0
1
Multiplex
Multiplex
Figure
OPERATION
Nonmultiplex
Nonmultiplex
Nonmultiplex
Multiplex
Multiplex
Multiplex
3. The alternating MUX
12 bits
14 bits
PWM
RES.
GLOBAL
7 bits
6 bits
5 bits
4 bits
3 bits
2 bits
RES.
PDM
Watchdog
TOTAL CLOCKS PER
PWM SUBFRAME
24-Output PWM LED Drivers
SUBFRAMES
PER FRAME
16,384
4096
128
64
32
16
8
4
The MAX6974/MAX6975 feature two fault detection func-
tions: open-circuit LED outputs and overtemperature. An
LED open circuit is detected on driver outputs by moni-
toring for output voltages below 200mV. When an open
circuit is detected, the MAX6974/MAX6975 increments
a fault counter included in the serial-interface protocol
that can be routed back to the host transmitter for diag-
nostics. Any number of open-circuit LEDS, multiplexed
or nonmultiplexed, can be detected, however, only one
counter increment occurs per device.
The MAX6974/MAX6975 detect die temperatures
above T
setting all PWM data to zero. The fault counter in the
serial-interface protocol is incremented by one count
for each cascaded device with an overtemperature
condition. The output drivers are turned back on when
the die temperature falls below T
fault counter value is distinguished between LED open-
circuit and overtemperature conditions by the serial-
interface command used at the time of detection (see
the Serial Interface section for more details).
CLOCKS
524,288
524,288
131,072
FRAME
USEABLE CLOCKS PER
for Message Boards
PER
DIE
PWM SUBFRAME
= +165°C and disable all output drivers by
16,320
4064
FREQUENCY (MHz)
Overtemperature Detection
FOR 50fps
CLOCK
26.2144
26.2144
6.5536
LED Open-Circuit and
16,320 / 16,384 = 99.61%
4064 / 4096 = 99.22%
DIE
MAXIMUM PWM
FREQUENCY (MHz)
DUTY CYCLE
= +150°C. The
FOR 60fps
31.45728
31.45728
CLOCK
7.8643
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