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LM25007MMX Datasheet(PDF) 10 Page - Texas Instruments |
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LM25007MMX Datasheet(HTML) 10 Page - Texas Instruments |
10 / 19 page VIN VIN RON RON/SD 7V SERIES REGULATOR ON TIMER VIN START COMPLETE RON STOP RUN LM25007 LM25007 SNVS401C – JANUARY 2006 – REVISED FEBRUARY 2013 www.ti.com Figure 8. Shutdown Implementation Current Limit The LM25007 contains an intelligent current limit off-timer intended to reduce the foldback characteristic inherent with fixed off-time over-current protection. If the current in the Buck switch exceeds 725mA the present cycle on- time is immediately terminated (cycle by cycle current limit). Following the termination of the cycle a non- resetable current limit off timer is initiated. The duration of the off time is a function of the external resistor (RCL) and the FB pin voltage. When the FB pin voltage equals zero, the current limit off-time is internally preset to 17µs. This condition occurs in short circuit operation when a maximum amount of off-time is required. In cases of overload (not complete short circuit) the current limit off-time is reduced as a function of the output voltage (measured at the FB pin). Reducing the off-time with smaller overloads reduces the amount of foldback and also reduces the initial start-up time. The current limit off time for a given FB pin voltage and RCL resistor can be calculated by the following equation: Toff = 10 -5 / (0.59 + (V FB / 7.22 x 10 -6 x R CL)) (5) Applications utilizing low resistance inductors and/or a low voltage drop rectifier may require special evaluation at high line, short circuit conditions. In this special case the preset 17µs (FB = 0V) off-time may be insufficient to balance the inductor volt*time product. Additional inductor resistance, output resistance or a larger voltage drop rectifier may be necessary to balance the inductor cycle volt*time product and limit the short circuit current. N - Channel Buck Switch and Driver The LM25007 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. The bootstrap capacitor is charged by VCC through the internal high voltage diode. A 0.01uF ceramic capacitor connected between the BST pin and SW pin is recommended. During each cycle when the Buck switch turns off, the SW pin is approximately 0V. When the SW pin voltage is low, the bootstrap capacitor is charged from Vcc through the internal diode. The minimum off timer, set to 300ns, ensures that there will be a minimum interval every cycle to recharge the bootstrap capacitor. An external re-circulating diode from the SW pin to ground is necessary to carry the inductor current after the internal Buck switch turns off. This external diode must be of the Ultra-fast or Schottky type to reduce turn-on losses and current over-shoot. The reverse voltage rating of the re-circulating diode must be greater than the maximum line input voltage. 10 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated Product Folder Links: LM25007 |
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