LM3431_08 Datasheet, PDF (19/24 Pages) National Semiconductor (TI) – 3-Channel Constant Current LED Driver with Integrated Boost Controller

Shorted LED
If an LED fails short circuit, the SC voltage will increase. When
SC reaches 3.1V, the same Type 1 fault as an open LED will
be triggered. Current in the affected string will continue to be
regulated, with the cathode clamped at one diode V_f above
3.5V. As in the case of 1 LED open, the power dissipation will
increase in the external NPN regulator of the shorted string.
However, if enough LEDs or an entire string are shorted, the
SC pin will rise to the short circuit threshold of 6.0V. This will
cause a Type 2 fault, and the LM3431 will shut down when
the DLY threshold is reached.
When an LED string is shorted, the LM3431 will attempt to
reduce the SC voltage to 3.5V. As a result, switching will stop,
and the cathode voltage will be brought to the minimum level,
which is Vin. If Vin is less than approximately 6V and the DLY
time is long enough, SC will fall below the 6V short circuit fault
threshold. In this case, the shorted string fault will be detected
as an AFB under-voltage (UVP) fault.
When AFB falls below 0.85V (typical) a Type 2 fault will be
triggered. As is the case with OVP detection, the AFB UVP
threshold is monitored during both LED on and LED off cycles.
A UVP fault will cause DLY to charge, unaffected by the dim-
ming duty cycle. Figure 13 below shows the sudden cathode
voltage increase due to an LED string short. DLY begins
charging and charges continuously when an AFB under-volt-
age is detected, eventually causing a shutdown.
and provides over-temperature protection for both the IC and
LEDs.
THM
The THM pin is designed to monitor for over-temperature
conditions at the LED array. This is done with a negative TC
thermistor mounted at the LED panel. The THM circuit is a
resistor divider from a reference voltage to ground, shown in
Figure 14 as R17 and Rth. As the thermistor temperature in-
creases, the THM pin voltage will decrease. When THM drops
to 1.23V (typical), a Type 3 fault is triggered and the LM3431
will enter standby until the thermistor temperature decreases
and THM voltage increases. Thermistors are typically speci-
fied by their resistance at 25°C, and by their beta constant
which describes the temperature coefficient. The resistance
value at the desired shutdown temperature can be calculated
from the beta constant or found in the thermistor datasheet
table. Once the shutdown temperature resistance is known,
the R17 value can be calculated as shown below.
where Rth@T is the thermistor resistance at the desired shut-
down temperature. Although VCC is shown in the typical
application schematic, any regulated voltage source can be
used in its place, including V_REF.
In shutdown, THM sinks 10 µA to create some hysteresis. An
R17 value of at least 20 kΩ is recommended to create suffi-
cient hysteresis. Larger values of R17 (and Rth) will generate
larger hysteresis.
If more hysteresis is required, a resistor can be added in se-
ries with THM as shown below:
30041155
FIGURE 13. LED String Short Fault and UVP Detection
Shorted Cathode
At the end of softstart, the CFB voltage is monitored. In normal
startup, the LED strings are off and CFB voltage increases
with the output voltage. If the CFB voltage stays below ap-
proximately 1.9V, a cathode short to ground condition is
detected and a Type 2 fault is triggered. At the end of soft-
start, the DLY pin will begin sourcing current and it will con-
tinue sourcing until the shutdown threshold is reached, even
if the short condition is removed.
When a cathode short occurs, the LEDs in the affected string
will be driven on during the soft-start and DLY periods. There-
fore, the DLY and soft-start time should be set short enough
for the LED string to withstand the burst of unregulated cur-
rent.
THERMAL CONSIDERATIONS
To optimize performance under all conditions, the LM3431
controls the temperature coefficients of critical parameters
30041157
FIGURE 14. THM Circuit with Hysteresis
The THM hysteresis can be determined by calculating the
restart threshold as shown below. If R_HYS is not installed, cal-
culate Rth@restart using an R_HYS value of 0Ω.
Where 10 µA is the THM sink current, and Rth@restart is the
thermistor resistance at the restart temperature. Refer to the
manufacturer datasheet to find the restart temperature at the
calculated resistance or use the beta constant to calculate the
restart temperature.
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