SD

T/F

INTERNAL EN

T/F

INTERNAL EN

EXTERNAL/INTERNAL

FTO

SD / EN

SD / EN

EN

LM3551, LM3552

www.ti.com

SNVS371D – AUGUST 2005 – REVISED MAY 2013

Under-Voltage Protection (UVP)

Both the LM3551 and LM3552 have under-voltage protection circuitry (UVP). This protects the NMOS power

device during startup and shutdown by preventing operation at voltages less than the minimum input voltage.

The UVP protection is enabled at 2.48V(typ.) and will not disable until the input voltage rises above 2.58V(typ.) .

Torch/Flash Pin (T/F)

The TORCH/FLASH pin (T/F) controls whether the LM3551/2 is in continuous torch mode, or in flash mode. A

logic '0' places the part into torch mode and a logic '1' places the part into flash mode. There are no pull-ups or

pull-downs internally connected to T/F. When placed into torch mode, FET-T is enabled allowing the current set

Flash Timeout Protection (FTP)

When SD is low(LM3551) or EN is high(LM3552), and T/F is high, a current is output to an external capacitor,

forces the INTERNAL_EN signal to go low, which in turn shuts-off the low-side torch and flash FETs in addition

to disabling the main power SW FET. At such time, the LED will be turned off. The part will remain disabled until

SD is pulled high (LM3551) or EN is pulled low (LM3552) and/or T/F is pulled low. At that point, the part will

return to normal operating mode. The diagram below shows a first pulse which exceeds the timeout period and

internal_EN being driven low. The second FLASH pulse is shorter than the timeout period and therefore the

EQUATIONS below.

(1)

(2)

(3)

To disable the timeout function, ground the FTO pin.

Figure 15. Flash Timeout Protection Diagram

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Product Folder Links: LM3551 LM3552