Texas Instruments LM3431 Evaluation Board LM3431EVAL/NOPB LM3431EVAL/NOPB 데이터 시트
제품 코드
LM3431EVAL/NOPB
Vout
10V/Div
10 ms/DIV
SS/SH
1V/Div
DLY
2V/Div
ILED
500 mA/Div
SNVS547G – NOVEMBER 2007 – REVISED MAY 2013
Figure 25. OVP and Auto-Restart
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
condition is shown in
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
condition is shown in
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 output 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 threshold of 2.0V (typical), a Type 2 fault will be triggered, the DLY pin will begin sourcing current,
and the LM3431 will shut down.
will be held low and cannot sense the failure. Instead, this failure mode is sensed by AFB. When AFB reaches its
over-voltage threshold 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.
be affected by the dimming duty cycle and any over-voltage condition will cause DLY to charge.
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
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.
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.
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 dimming duty cycle.
UVP threshold is monitored during both LED on and LED off cycles. A UVP fault will cause DLY to charge,
unaffected by the dimming duty cycle.
below shows the sudden cathode voltage increase due to an
LED string short. DLY begins charging and charges continuously when an AFB under-voltage is detected,
eventually causing a shutdown.
eventually causing a shutdown.
24
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