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LM3552 Datasheet(PDF) 9 Page - Texas Instruments |
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LM3552 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 23 page CFTO SD T/F ITO INTERNAL EN T/F INTERNAL EN VCFTO VTRIP EXTERNAL/INTERNAL FTO SD / EN SD / EN EN LM3551, LM3552 www.ti.com SNVS371C – AUGUST 2005 – REVISED MARCH 2011 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 by RT to flow. In torch mode, FET-F is not enabled. Flash mode enables both FET-T and FET-F allowing the sum total of the current set by the two external resistors, RT and RF, to flow. 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, CFTO. This causes the voltage on the capacitor to rise. If the voltage reaches Vtrip (1.16V(typ)), the timeout circuit 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 voltage on CFTO never reaches Vtrip. For information on component selection, please see the FLASH TIMEOUT EQUATIONS below. TFTO = CFTO × (ΔVFTO ÷ IFTO) (1) ΔVFTO = 1.16V and IFTO= 1.4µA TFTO = Desired Timeout Duration (2) CFTO(µF) = TFTO(sec.) × 1.21(µA/V) (3) To disable the timeout function, ground the FTO pin. Figure 15. Flash Timeout Protection Diagram Copyright © 2005–2011, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3551 LM3552 |
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