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LM3481-Q1 Datasheet(PDF) 17 Page - Texas Instruments |
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LM3481-Q1 Datasheet(HTML) 17 Page - Texas Instruments |
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17 / 38 page  UVLO VIN R7 R8 IUVLO VUVLOSEN - + + - 2 R7 = § ¨ © -1 VEN 1.43V x R8 R8 = § ¨ © 1 + 1.43V ± VSH VEN ± 1.43V IUVLO x 1.43V FA/SYNC/SD 40 k: DR 30 Ps LM3481 LM3481, LM3481-Q1 www.ti.com SNVS346F – NOVEMBER 2007 – REVISED NOVEMBER 2014 Feature Description (continued) Figure 28. Shutdown Operation in Synchronization Mode 7.3.5 Undervoltage Lockout (UVLO) Pin The UVLO pin provides user programmable enable and shutdown thresholds. The UVLO pin is compared to an internal reference of 1.43 V (typical), and a resistor divider programs the enable threshold, VEN. When the IC is enabled, a 5- μA current is sourced out of the UVLO pin, which effectively causes a hysteresis, and the UVLO shutdown threshold, VSH, is now lower than the enable threshold. Setting these thresholds requires two resistors connected from the VIN pin to the UVLO pin and from the UVLO pin to GND (see Figure 29). Select the desired enable, VEN, and UVLO shutdown, VSH, threshold voltages and use the Equation 17 and Equation 18 to determine the resistance values: (17) (18) Figure 29. UVLO Pin Resistor Divider If the system is designed to work over wide input voltages, the voltage at the UVLO pin could exceed the voltage limit for the UVLO pin. In this case a zener diode can be connected between the UVLO pin and ground to prevent the UVLO voltage from rising above the maximum value. If the UVLO pin function is not desired, select R8 and R7 of equal magnitude greater than 100 k Ω. This will allow VIN to be in control of the UVLO thresholds. The UVLO pin may also be used to implement the enable/disable function. If a signal pulls the UVLO pin below the 1.43 V (typical) threshold, the converter will be disabled. 7.3.6 Short-Circuit Protection When the voltage across the sense resistor (measured on the ISEN Pin) exceeds 220 mV, short-circuit current limit gets activated. A comparator inside the LM3481 reduces the switching frequency by a factor of 8 and maintains this condition until the short is removed. Copyright © 2007–2014, Texas Instruments Incorporated Submit Documentation Feedback 17 Product Folder Links: LM3481 LM3481-Q1 |
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