Datasheet

DLY

2V/Div

ILED

100 mA/Div

Vcathode

2V/Div

1 ms/DIV

LM3431

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SNVS547G –NOVEMBER 2007–REVISED MAY 2013

Where D

DIM

is the dimming duty cycle. Figure 24 below shows the DLY pin charging during dimming due to a

Type 1 fault:

Figure 24. DLY Charging, 1 LED Open Fault

When the LED string turns on, there is a 1.6 µs typical blanking time for fault detection. This ensures that the

LED cathode voltage will reach its regulation point and faults will not be falsely triggered. However, faults can not

be detected during short dimming cycles of less than 1.6 µs.

When a Type 3 fault occurs, DLY does not charge. The LM3431 will enter standby mode and restart from

softstart when the fault condition is removed.

Fault Shutdown and Automatic Restart

In normal operation, the LM3431 must be powered off or put into standby via the EN pin to restart after a fault

shutdown. However, the LEDOFF pin can be connected to GND to enable the automatic restart feature. During

startup, the LEDOFF voltage is monitored and if grounded, auto-restart mode is enabled.

In auto-restart mode, the DLY pin will be discharged by a 1.8 µA sink current after a Type 2 shutdown. In this

mode, DLY will not reach 3.6V, but will start discharging from the shut down threshold of 2.8V. When the DLY

pin voltage falls to 350 mV (typical) the LM3431 will restart from softstart mode. In this way, the DLY capacitor

controls the restart delay time. If the LEDOFF pin is used to control additional LED strings (see LEDOFF: Adding

Additional channels), then the automatic restart feature cannot be enabled.

In the case of an output over-voltage fault (all LEDs open), DLY will not discharge until the AFB voltage falls

below the OVP threshold. Figure 25 below shows an OVP fault with auto-restart activated. The output voltage

increases when all LEDs are opened, causing DLY to charge. DLY remains at 2.8V until Vout falls below the

OVP threshold. When DLY discharges to 350 mV, softstart begins. In auto-restart mode, the LM3431 will re-start

continually until the fault is removed. In this example, the fault is removed and normal operation continues after

one attempted re-start.

Product Folder Links: LM3431