Setting configuration bit D0 to 1 enables multiplex

operation. MUX0 and MUX1 alternate the LED anode

drive voltage between two sets of LEDs. The Y and Z

ports provide individual PWM control during alternate

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 MAX6972/MAX6973 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 MAX6972/MAX6973 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 MAX6972/MAX6973 detect die temperatures

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

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).

16-Output PWM LED Drivers

for Message Boards

______________________________________________________________________________________

13

PART

MUX

BIT

OPERATION

PWM

RES.

TOTAL CLOCKS PER

PWM SUBFRAME

USEABLE CLOCKS PER

PWM SUBFRAME

MAXIMUM PWM

DUTY CYCLE

0

Nonmultiplex

MAX6972

1

Multiplex

12 bits

4096

4064

4064 / 4096 = 99.22%

0

Nonmultiplex

MAX6973

1

Multiplex

14 bits

16,384

16,320

16,320 / 16,384 = 99.61%

Table 3. MAX6972/MAX6973 Timing Comparison

PART

GLB4

BIT

MUX

BIT

OPERATION

GLOBAL

PDM

RES.

SUBFRAMES

PER FRAME

CLOCKS

PER

FRAME

CLOCK

FREQUENCY (MHz)

FOR 50fps

CLOCK

FREQUENCY (MHz)

FOR 60fps

X0

Nonmultiplex

7 bits

128

MAX6972

X1

Multiplex

6 bits

64

524,288

26.2144

31.45728

0

Nonmultiplex

5 bits

32

0

1

Multiplex

4 bits

16

524,288

26.2144

31.45728

0

Nonmultiplex

3 bits

8

MAX6973

1

1

Multiplex

2 bits

4

131,072

6.5536

7.8643