Rainbow Electronics MAX6975 - Manual (Page 8)

MAX6974/MAX6975
Detailed Description
The MAX6974/MAX6975 drive 24 nonmultiplexed LEDs
or 48 multiplexed LEDs for various indoor and outdoor
display applications. The EZCascade serial interface
enables large multidriver display panels to be con-
structed with interconnected MAX6974/MAX6975
devices (see
Figure 1).
The drivers provide 12-bit (MAX6974) or 14-bit
(MAX6975) individual PWM steps for each LED output.
Four to seven global-intensity bits provide additional
pulse-density modulation (PDM) intensity control (see
Table 1). The MAX6974/MAX6975 provide 19 bits of
total current/intensity control range per color per pixel,
or 18 bits if multiplexing. The total PWM dynamic range
encompasses gamma correction and, if desired, indi-
vidual LED calibration.
LED outputs are grouped in ports (R, G, and B) with
eight LED outputs per port. Each port features its own
current calibration control DAC (CALDAC) with 0.31%
resolution to set the current. The MAX6974/
MAX6975 current calibration feature allows unmatched
LEDS from different lots and manufacturers to be
color matched.
Power-Up
On power-up, the MAX6974/MAX6975 set the calibration
current to the minimum current for all LED outputs and
clear the global-intensity PDM data, individual-intensity
PWM data, and the timing counters. The display
remains blank after CLKI starts running. The watchdog
function is inactive after power-up.
24-Output PWM LED Drivers
for Message Boards
8 _______________________________________________________________________________________
HOST
CLKO
DOUT
LOADO
CLKI
DIN
LOADI
MAX6974/
MAX6975
1
MAX6974/
MAX6975
2
MAX6974/
MAX6975
3
MAX6974/
MAX6975
N
CLKI
DIN
LOADI
CLKO
DOUT
LOADO
CLKI
DIN
LOADI
CLKO
DOUT
LOADO
CLKI
DIN
LOADI
CLKO
DOUT
LOADO
CLKI
DIN
LOADI
CLKO
DOUT
LOADO
OPTIONAL FEEDBACK
Figure 1. Generic Cascaded Connection Scheme
GLOBAL PDM
PART
LED DRIVE
OUTPUTS
LED DRIVE
CURRENT
CALIBRATION
DAC RANGE DIRECT MULTIPLEXED
INDIVIDUAL
PWM
MAX6974 7 bits 6 bits 12 bits
5 bits 4 bits
MAX6975
24
(7V rated)
30mA 6mA to 30mA
3 bits 2 bits
14 bits
Table 1. Comparison of MAX6974/MAX6975