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24-Output PWM LED Drivers
for Message Boards
Table 3. MAX6974/MAX6975 Timing Comparison
PART
MAX6974
MAX6975
MUX
BIT
0
1
0
1
OPERATION
Nonmultiplex
Multiplex
Nonmultiplex
Multiplex
PWM
RES.
12 bits
14 bits
TOTAL CLOCKS PER
PWM SUBFRAME
4096
16,384
USEABLE CLOCKS PER
PWM SUBFRAME
4064
16,320
MAXIMUM PWM
DUTY CYCLE
4064 / 4096 = 99.22%
16,320 / 16,384 = 99.61%
PART
MAX6974
MAX6975
GLB4
BIT
X
X
0
1
MUX
BIT
0
1
0
1
0
1
OPERATION
Nonmultiplex
Multiplex
Nonmultiplex
Multiplex
Nonmultiplex
Multiplex
GLOBAL
PDM
RES.
7 bits
6 bits
5 bits
4 bits
3 bits
2 bits
SUBFRAMES
PER FRAME
128
64
32
16
8
4
CLOCKS
PER
FRAME
524,288
524,288
131,072
CLOCK
FREQUENCY (MHz)
FOR 50fps
26.2144
26.2144
6.5536
CLOCK
FREQUENCY (MHz)
FOR 60fps
31.45728
31.45728
7.8643
MUX cycles as shown in Figure 3. The alternating MUX
cycles reduce the global-intensity resolution (the num-
ber of subframes) by half, which reduces the average
LED current by half.
Watchdog
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.
LED Open-Circuit and
Overtemperature Detection
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 DIE = +165°C and disable all output drivers by
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 DIE = +150°C. The
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).
Commands
The MAX6974/MAX6975 have four commands used to
load all operating mode and LED output current data.
______________________________________________________________________________________
13
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MAX6975ATL 功能描述:LED显示驱动器 RoHS:否 制造商:Micrel 数位数量:5 片段数量: 安装风格:SMD/SMT 封装 / 箱体:PLCC-44 工作电源电压:4.75 V to 11 V 最大电源电流:10 mA 最大工作温度:+ 85 C 最小工作温度:- 40 C 封装:Tube
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MAX6975ATL+T 功能描述:LED照明驱动器 24-Output PWM LED Driver RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
MAX6975ATL-T 功能描述:LED显示驱动器 RoHS:否 制造商:Micrel 数位数量:5 片段数量: 安装风格:SMD/SMT 封装 / 箱体:PLCC-44 工作电源电压:4.75 V to 11 V 最大电源电流:10 mA 最大工作温度:+ 85 C 最小工作温度:- 40 C 封装:Tube