Rainbow Electronics MAX6975 - Manual (Page 20)

MAX6974/MAX6975
When the MAX6974/MAX6975 send individual-intensity
PWM data, the data segment bit length is large due to
the 12-bit or 14-bit PWM data for each of the 24 outputs
(see
Figure 11). The various data segment bit lengths
for each of the four commands and different operating
modes is shown in
Table 4. Data capturing is the same
as described above with the header segment outputs
and data being delayed by the full length of the data bit
stream being captured plus one clock cycle.
LED Open-Circuit and
Overtemperature Detection Counter
The MAX6974/MAX6975 feature LED open-circuit
detection and overtemperature detection that use the
counter section of the header segment to record
detected faults. Using commands 01 or 11 force the
counter to record LED open-circuit detection faults.
Using commands 00 or 10 force the counter to record
overtemperature faults.
The MAX6974/MAX6975 detect an open circuit on a driver
output by monitoring for output voltages below 200mV.
When an open circuit is detected, the MAX6974/
MAX6975 increment the counter segment data,
CNTR[9:0], received on DIN by 1 before transmitting a
header and new counter value out DOUT. Regardless
of the number of open-circuit outputs on a device, the
counter increment is 1.
The MAX6974/MAX6975 detect die temperatures above
T
DIE
= +165°C and disable all output drivers by setting
all PWM data to zero. During an overtemperature event,
the MAX6974/MAX6975 increment the counter segment
data, CNTR[9:0], received on DIN by 1 before transmitting
a header and new counter value out DOUT. The output
drivers are allowed to be on when the die temperature
falls below T
DIE = +150°C.
When there is no fault detected, the counter data is
passed directly to DOUT unaltered.
Applications Information
Terminations and PC Board Layout
The MAX6974/MAX6975’s layout simplifies cascading
multiple devices, as the interface signals flow through
from each device. The synchronous and buffered
nature of the interface simplifies the board design, but
pay attention to signal routing and termination, as with
other high-speed logic circuits.
Terminate the differential input pairs, CLKI+ and CLKI-,
as well as DIN+ and DIN-, with a termination resistor as
close as possible to the package. When using the
MAX6974/MAX6975 as the signal source, use a 200
Ω
termination resistor. When using a level translator or clock
retimer as the signal source, use a 110
Ω termination
resistor. Route each differential input pair as close
parallel tracks with spacing or a GND trace between
the track pair and the next signal track to minimize
cross-coupling. Track lengths up to a few inches do not
require termination-matched tracks (transmission lines).
24-Output PWM LED Drivers
for Message Boards
20 ______________________________________________________________________________________
DATA 1 PWM 288 BITS H1
289 CLOCKS
289 CLOCKS
289 CLOCKS
D0
D1
D2
D3
DATA 2 PWM 288 BITS DATA 3 PWM 288 BITS T
DATA 2 PWM 288 BITS DATA 3 PWM 288 BITS T H2
DATA 3 PWM 288 BITS T H3
T H4
Figure 11. Long (288 Bits) PWM Data Cascading Shown for
MAX6974 in Nonmultiplexed Mode
CLKI- CLKO-
CLKO+ CLKI+
200 Ω
DIN- DOUT-
DOUT+ DIN+
LOADO LOADI
n-2
200 Ω
MAX6974
CLKI- CLKO-
CLKO+ CLKI+
110 Ω
DIN- DOUT-
DOUT+ DIN+
LOADO LOADI
n-1
110 Ω
MAX6974
DO2-
DO2+
DIN2 DO1-
DO1+ DIN1
MAX9112
CLK
DIN
LOAD
HOST
n MORE DEVICES WITH
200 Ω TERMINATION
Figure 12. Typical Cascaded Serial-Interface Termination Circuit