Datasheet

R17 = Rth @ T x

VCC - 1.23V

1.23V

ILED

200 mA/Div

DLY

1V/Div

Vcathode

5V/Div

400 Ps/DIV

LM3431

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SNVS547G –NOVEMBER 2007–REVISED MAY 2013

Figure 26. 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 approximately 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 continue 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. Therefore, the DLY and soft-start time should be set short enough for the LED string to withstand the

burst of unregulated current.

Thermal Considerations

To optimize performance under all conditions, the LM3431 controls the temperature coefficients of critical

parameters 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 27 as R17 and Rth. As the thermistor temperature increases, 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 specified 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.

(32)

where Rth@T is the thermistor resistance at the desired shutdown 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 sufficient hysteresis. Larger values of R17 (and Rth) will generate larger hysteresis.

If more hysteresis is required, a resistor can be added in series with THM as shown below:

Product Folder Links: LM3431