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The MAX6978 serial-interfaced LED driver provides
eight open-drain, constant-current sinking LED driver
outputs rated at 5.5V. The MAX6978 operates from a 3V
to 5.5V supply. The MAX6978 supply and the LEDs'
supply can power up in any order. The constant-current
outputs are programmed together to up to 55mA using
a single external resistor. The MAX6978 operates with a
25Mb, industry-standard, 4-wire serial interface.
The MAX6978 includes circuitry that automatically
detects open-circuit LEDs. Fault status is loaded into the
serial-interface shift register when LE goes high and is
automatically shifted out on DOUT when the next data
transmission is shifted in.
The MAX6978 includes a fail-safe feature for safety related
applications, which blanks the display if the serial
interface fails. If the serial interface becomes
inactive for more than 1s, all driver output latches are
automatically cleared. This turns off all LEDs connected
to the outputs. The shift register data is not disturbed.
The outputs remain off until the driver output latches are
updated with data turning them on, so recovery is automatic
if the transmission failure is temporary. The
watchdog function requires no software change to the
application driving the MAX6978.
The MAX6978 uses the industry-standard shift-register-plus-
latch-type serial interface. The driver accepts data
shifted into an 8-bit shift register using data input DIN
and clock input CLK. Input data appears at the output
DOUT eight clock cycles later to allow cascading of
multiple MAX6978s. The latch-enable input LE loads
the 8 bits of shift-register data into an 8-bit output latch
to set which LEDs are on and which are off. The outputenable
input OE-bar gates all eight outputs on and off, and
is fast enough to be used as a PWM input for LED
intensity control.
The MAX6978 is one of a family of 12 shift-register-plus-latch-
type LED drivers. The family includes 8-port and
16-port types, with 5.5V- or 36V-rated LED outputs, with
and without open-circuit LED detection and watchdog.
All versions operate from a 3V to 5.5V supply, and are
specified over the -40°C to +125°C temperature range.
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