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16-Port, 5.5V Constant-Current LED Driver with
LED Fault Detection and Watchdog
OE
LE
t LRF
OUT_
LE
CLK
DOUT
D15
D14 D13
D12
LE
OUT_
t LRR
Figure 4. Fault Timing Diagram
FAULT STATUS BITS
CLK
OUT_
CLK
t CRF
t CRR
OE . The fault data is latched on the rising edge of LE
and is shifted out when new LED data is loaded into the
output latches from the shift register. If one or more out-
put ports are detected with an open-circuit fault, the
D14 and D13 bits of DOUT go high. If no open-circuit
faults are detected, the D14 and D13 are set to low.
The data in the other 14 bit positions in DOUT are not
OUT_
Figure 3. LE and CLK to OUT_ Timing
Watchdog
The MAX6979 includes a watchdog circuit that monitors
the CLK, DIN, and LE inputs. If there is no transition on
all of these inputs for nominally 1s, then the output latch-
es 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, which 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
on. Recovery is therefore automatic if the transmission
failure is temporary, because the MAX6979 does not
lock up in the watchdog timeout state. The MAX6979
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
driving the MAX6979.
LED Fault Detection
The MAX6979 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
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.
Applications Information
Selecting External Component R SET to
Set LED Output Current
The MAX6979 uses an external resistor R SET to set the
LED current for outputs OUT0–OUT15. The minimum
allowed value of R SET is 327.3 ? , which sets the output
currents to 55mA. The maximum allowed value of R SET
is 1.5k ? . The reference value, 360 ? , sets the output
currents to 50mA. To set a different output current, use
the formula:
R SET = 18,000/I OUT
where I OUT is the desired output current in mA.
8
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