LM3431MHX National Semiconductor, LM3431MHX Datasheet - Page 18

• Manufacturer Part Number: LM3431MHX
• Description: 3-Channel Constant Current LED Driver with Integrated Boost Controller
• Manufacturer: National Semiconductor

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regulate, although the DLY pin remains high. In this condition,
the DLY pin will charge to a maximum of 3.6V (typical).
In case of a Type 2 fault, when the DLY voltage reaches 2.8V
(typical), the LM3431 will shut down and the DLY pin will re-
main at 3.6V.
For evaluation and debugging purposes, Type 2 shutdown
can be disabled by grounding the DLY pin. It is not recom-
mended to leave the DLY pin open.
For any fault other than a cathode short, the DLY pin will dis-
charge (sinking 1.8 µA) when the fault is removed before
shutdown occurs. Since most fault conditions can only be
sensed during the LED-on dimming period, the DLY pin will
not charge during LED-off times. When the LEDs are off, DLY
is in a high impedance state and its voltage will remain con-
stant. If a fault is removed during the LED-off period, DLY will
begin discharging at the next LED-on cycle. If the fault is not
removed, DLY will continue charging at the next LED-on cy-
cle. Therefore, the DLY charging time is controlled by both the
DLY capacitor and the dimming duty cycle. The time for the
DLY pin to charge to the shutdown threshold can be calcu-
lated as shown:
Where D
the DLY pin charging during dimming due to a Type 1 fault:
When the LED string turns on, there is a 1.6 µs typical blank-
ing 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 en-
able the automatic restart feature. During startup, the LED-
OFF 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
FIGURE 11. DLY Charging, 1 LED Open Fault
DIM
is the dimming duty cycle. Figure 11 below shows
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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 section), 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 12 below shows an OVP fault with au-
to-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 dis-
charges 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 con-
tinues after one attempted re-start.
Open LED
If any LED string fails open, the boost regulator will sense a
low voltage at the CFB pin. This will cause the output voltage
to increase, causing the other LED string cathode voltages to
also increase. When the SC pin voltage rises to 3.1V, a Type
1 fault will be triggered and the DLY pin will begin sourcing
current. In this mode, the SC voltage will be clamped at 3.5V
(typical) and the regulators will continue to operate. At this
higher cathode voltage, power dissipation will increase in the
external NPN regulators. Power dissipation will also increase
in the LM3431 since any open string will cause the NDRV pin
to source its maximum current. A one LED open fault condi-
tion is shown in Figure 11 above. The open LED causes the
cathode voltage to increase, and DLY charges during each
LED on cycle while current continues to be regulated in the
other LED strings.
If all LED strings fail open, the same action will cause the out-
put voltage to increase. However, in this case, SC will be held
low and cannot sense the failure. Instead, this failure mode is
sensed by AFB. When AFB reaches its over-voltage thresh-
old of 2.0V (typical), a Type 2 fault will be triggered, the DLY
pin will begin sourcing current, and the LM3431 will shut
down.
Unlike the SC and CFB fault detection, the AFB pin is always
monitored. Therefore, DLY charging time will not be affected
by the dimming duty cycle and any over-voltage condition will
cause DLY to charge.
FIGURE 12. OVP and Auto-Restart
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