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16-Port, 36V Constant-Current LED Driver with
LED Fault Detection and Watchdog
LE
OUT_
t LRF
OE
LE
CLK
DOUT
D15
D14 D13
D12
LE
OUT_
t LRR
Figure 4. Fault Timing
FAULT STATUS BITS
CLK
OUT_
CLK
t CRF
t CRR
on. Recovery is therefore automatic if the transmission
failure is temporary because the MAX6983 does not
lock up in the watchdog timeout state. The MAX6983
operates correctly when the serial interface is next acti-
vated, and the watchdog circuit is reset and starts
monitoring the serial interface again. The watchdog
function requires no software change to the application
OUT_
Figure 3. LE and CLK to OUT_ Timing
The fourth input provides output-enable control of the out-
put drivers. OE is high to force outputs OUT0–OUT15
high impedance, without altering the contents of the out-
put latches, and low to enable outputs OUT0–OUT15 to
follow the state of the output latches.
OE is independent of the operation of the serial inter-
face. Data can be shifted into the serial-interface shift
register and latched regardless of the state of OE .
DOUT is the serial-data output, which shifts data out
from the MAX6983’s 16-bit shift register on the rising
edge of CLK. Data at DIN is propagated through the
shift register and appears at DOUT 16 clock cycles later.
See Figure 2.
Watchdog
The MAX6983 includes a watchdog circuit that monitors
the CLK, DIN, and LE inputs. If there is no transition on
any one of these inputs for nominally 1s, the output
latches are cleared and outputs OUT0–OUT15 go high
impedance like the initial power-up condition. This turns
off all LEDs connected to the outputs. The shift-register
data does not change, just the output-latch data.
The watchdog is intended to be used as a fail-safe fea-
ture for applications that prefer a blank display to an
incorrect display if the serial interface fails. When the
watchdog triggers, the outputs remain off until the dri-
ver output latches are updated with data turning them
driving the MAX6983.
LED Fault Detection
The MAX6983 includes circuitry that detects open-cir-
cuit LEDs automatically. An open-circuit fault occurs
when an output is programmed to sink current but less
than about 50% of the programmed current flows.
Open circuits are checked just after the falling edge of
OE . The fault data is latched on the rising edge of LE and
is shifted out when new LED data is loaded into the out-
put latches from the shift register. If one or more output
ports are detected with an open-circuit fault, the D14 and
D13 bits of DOUT go high. If no open-circuit faults are
detected, D14 and D13 are set to low. The data in the
other 14 bit positions in DOUT are not altered.
Fault status is shifted out on DOUT when the next data
transmission is shifted in after the rising edge of LE. LE
is normally taken high after all 16 bits of new LED data
have been clocked into the shift register(s), and then
DOUT outputs data bit D15. On the next two rising
edges of CLK, the 2 fault status bits, D14 and D13, are
clocked out in that order, followed by the remaining 13
unchanged data bits D12 to D0.
A typical fault-detecting application tests all the shifted
out data. Bits D0–D12 and D15 are checked against
the originally transmitted data to check data-link integri-
ty. Bits D13 and D14 are checked first to see that they
contain the same data (validating the status) and sec-
ond, whether faults are reported or not by the actual
logic level.
Figure 4 is the LE and CLK to OUT_ timing diagram.
8
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