MAX6975ATL+ Datasheet MAX6974ATL+, MAX6975ATL+, MAX6974ATL+T | P13-P15

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.

MAX6974/MAX6975

24-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

MAX6974

1 Multiplex

12 bits 4096 4064 4064 / 4096 = 99.22%

0 Nonmultiplex

MAX6975

1 Multiplex

14 bits 16,384 16,320 16,320 / 16,384 = 99.61%

Table 3. MAX6974/MAX6975 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

X 0 Nonmultiplex 7 bits 128

MAX6974

X 1 Multiplex 6 bits 64

524,288 26.2144 31.45728

0 Nonmultiplex 5 bits 32

1 Multiplex 4 bits 16

524,288 26.2144 31.45728

0 Nonmultiplex 3 bits 8

MAX6975

1 Multiplex 2 bits 4

131,072 6.5536 7.8643

MAX6974/MAX6975

Each command is uniquely identified by two bits, C1

and C0, embedded in the serial-interface protocol

structure. The commands Load CALDAC, Load Global-

Intensity PDM, and Load Configuration each require 24

bits of data (3 bytes) for every cascaded device. The

number of bits required for the command load individual

PWM varies by device and multiplex mode of operation.

Each cascaded device can receive unique data for

CALDACs, global intensity, configuration, and individual

PWM output drivers. Generally, all cascaded devices

are operated in the same configuration mode. The data

bytes are transmitted MSB first for all commands. The

commands are communicated to all cascaded devices

by the host using the synchronous serial-interface and

protocol structure (see the

Serial Interface

section for

details). The four commands and the data lengths for

each command are shown in Table 4.

The MAX6974, operating in nonmultiplexed mode,

requires twenty-four 12-bit individual PWM data (288

bits total) and requires forty-eight 12-bit data (576 bits

total) in multiplexed operation mode. Similarly, the

MAX6975, operating in nonmultiplexed mode, requires

twenty-four 14-bit individual-intensity PWM data (336

bits total) and requires forty-eight 14-bit (672 bits total)

data in multiplexed mode. The individual PWM data are

loaded into an intermediate latch and transferred to the

actual PWM latches at subframe 0 and PWM clock 0.

The R, G, and B calibration DACs are loaded with 8-bit

data each in nonmultiplexed and multiplexed modes.

Data is updated immediately into the CALDAC latches

(see Table 8)

The MAX6974/MAX6975 require one data byte to set the

global-intensity PDM for all output drivers. The global-

intensity PDM data has a variable number of active bits

depending on the multiplex operating mode and, for

the MAX6975, the global-quarter setting. The number of

bits used for global-intensity control is always justified

to the LSB of the data byte, as shown in Table 5. One

byte of data is sent three times with the global-intensity

PDM data bits justified to the LSB. Data is updated into

the PWM latches at subframe 0 and PWM clock 0 (see

Table 9)

When using the MAX6975 5-bit global-intensity setting,

the settings range from 0 to 63 to set the global intensity

from 1 to 64 subframes ON to 64 out of 64 subframes ON.

When using the MAX6974 7-bit global-intensity setting,

the settings range from 0 to 127 to set the global inten-

sity from 1 out of 128 subframes ON to 128 out of 128

subframes ON.

24-Output PWM LED Drivers

for Message Boards

14 ______________________________________________________________________________________

CMD[1:0]

C1 C0

COMMAND DATA LENGTH PER CASCADED DEVICE

288 bits (MAX6974 nonmultiplexed)

576 bits (MAX6974 multiplexed)

336 bits (MAX6975 nonmultiplexed)

0 0 Load individual PWM

672 bits (MAX6975 multiplexed)

0 1 Load CALDAC 24 bits

1 0 Load global-intensity PDM 24 bits

1 1 Load configuration 24 bits

Table 4. Commands and Data Length

PART GLB4 MUX TOTAL BITS MSB D7 D6 D5 D4 D3 D2 D1 LSB D0

X 0 7 0 Bit[6] Bit[5] Bit[4] Bit[3] Bit[2] Bit[1] Bit[0]

MAX6974

X 1 6 0 0 Bit[5] Bit[4] Bit[3] Bit[2] Bit[1] Bit[0]

0 0 5 0 0 0 Bit[4] Bit[3] Bit[2] Bit[1] Bit[0]

0 1 4 0 0 0 0 Bit[3] Bit[2] Bit[1] Bit[0]

1 0 3 0 0 0 0 0 Bit[2] Bit[1] Bit[0]

MAX6975

1 1 2 0 0 0 0 0 0 Bit[1] Bit[0]

Table 5. Global-Intensity Data Bit Justification

The global-intensity data is received in an intermediate

PWM clock 0.

The MAX6974/MAX6975 have one byte of configuration

data with 5 active bit settings as shown in Table 6. One

byte of data containing configuration bit settings is sent

three times. Data is updated immediately into the CAL-

DAC latches. See Table 10

The loaded configuration

settings take effect immediately.

Serial Interface

The MAX6974/MAX6975 feature a fully synchronous

and fully buffered serial interface that allows cascading

of multiple devices. The serial interface consists of

inputs (CLKI, DIN, and LOADI) and outputs (CLKO,

DOUT, and LOADO). The MAX6974/MAX6975 can

pass different data to each cascaded device without

any additional inputs to identify the position of the

devices in the cascaded chain.

MAX6974/MAX6975

24-Output PWM LED Drivers

for Message Boards

______________________________________________________________________________________ 15

CONFIGURATION BIT ACRONYM FUNCTION DESCRIPTION

MSB D7 — 0 Not used

D6 — 0 Not used

D5 — 0 Not used

D4 GLB4 Global quarter

Enables the reduced global-intensity setting in the MAX6975 when set to

1. When set, the MAX6975 uses eight (or four, if multiplexing) PWM

subframes. GLB4 is set to 0 as power-on default. Setting bit D4 has no

effect in the MAX6974.

D3 PWM-ON

Enable

individual

PWMs

Turns all individual PWM outputs on when set to 1. Power-on default is

PWM-ON set to 0 to disable all current output drivers. PWM-ON can be

used to turn all LEDs on or off without affecting the global-intensity or

individual PWM settings.

D2 CRST

Reset frame

and PWM

counters

Setting CRST to 1 synchronously resets internal counters to 0. This action

sets the MAX6974/MAX6975 to subframe 0 of the global-intensity

subframe counter and clock 0 of all individual PWM counters. The CRST

bit is a nonlatching control function that resets to 0 after the counters are

set to 0.

D1 WDOG

Watchdog

enable

Setting WDOG to 1 enables the watchdog timer operation. Power-on

default is 0.

LSB D0 MUX

Multiplex

enable

Setting MUX to 1 turns multiplex mode on. Power-on default is 0.

Table 6. Load Configuration Bit Definitions

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P23

MAX6975ATL+

Mfr. #:

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LED Lighting Drivers 24-Output PWM LED Driver

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MAX6975ATL+

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