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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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LM3551 Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM3551 Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
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10 / 15 page  Circuit Description (Continued) voltage drops to 10.6V(typ), switching will resume. The LM3551 and LM3552 will go back into OVP if the failure is still present resulting in a pulsed output condition. Note: To disable OVP, ground to OVP pin. CAUTION: The LM3551 and LM3552 may be damaged if an OVP condition occurs and OVP is disabled. Under-Voltage Protection (UVP) Both the LM3551 and LM3552 have under-voltage protec- tion 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 R T 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, R T 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, C FTO. 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 C FTO never reaches Vtrip. For in- formation on component selection, please see the FLASH TIMEOUT EQUATIONS below. 20151204 Flash Timeout Protection Diagram T FTO =CFTO x( ∆V FTO ÷IFTO) ∆V FTO = 1.16V and IFTO= 1.4µA T FTO = Desired Timeout Duration C FTO(µF) = TFTO(sec.) x 1.21(µA/V) To disable the timeout function, ground the FTO pin. Soft-Start The LM3551 and LM3552 have a soft-start pin that can be used to limit the inductor inrush current on start-up. The external SS pin is used to tailor the soft-start for a specific application but is not required for all applications and can be left open when not needed. When used, a current source charges the external soft-start capacitor, C SS, forcing the internal reference to ramp-up at a user determined rate. Typical Start-Up Times V IN = 3.6V, TA = +25˚C C SS (µF) Load (mA) Start-Up Time (msec.) 0.1 Sharp LED @ 75mA Torch 3 Sharp LED @ 250mA Flash 8 Lumiled LED @ 200mA Torch 1.6 Lumiled LED @ 700mA Flash 6 0.47 Sharp LED @ 75mA Torch 12 Sharp LED @ 250mA Flash 35 Lumiled LED @ 200mA Torch 6 Lumiled LED @ 700mA Flash 35 1.0 Sharp LED @ 75mA Torch 25 Sharp LED @ 250mA Flash 75 Lumiled LED @ 200mA Torch 30 Lumiled LED @ 700mA Flash 70 Application Information LM3551 AND LM3552 FUNCTIONALITY TRUTH TABLE SD (LM3551) or EN (LM3552) T/F LM3551 Result LM3552 Result 1 0 Shut-down Torch Mode 1 1 Shut-down Flash Mode 0 0 Torch Mode Shut-down 0 1 Flash Mode Shut-down The LM3551 has a 1.2M Ω pull-up to V IN on SD and the LM3552 has a 1.2M Ω pull-down to GND on EN. CURRENT SET EQUATIONS The LM3551/2 utilize inline resistors to set the Torch and Flash LED currents. The Torch-Mode current (continuous) and the Flash-Mode current (pulsed) are programmed by placing the appropriately selected resistors between the feedback pin (FB) and FET-T (torch FET) and FET-F (flash FET) pins. Torch-mode is set by utilizes the current through www.national.com 10 |
Podobny numer części - LM3551_06 |
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Podobny opis - LM3551_06 |
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