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LM3151 数据表(PDF) 11 Page - Texas Instruments |
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LM3151 数据表(HTML) 11 Page - Texas Instruments |
11 / 26 page tSS = Vref x CSS ISS LM3151, LM3152, LM3153 www.ti.com SNVS562G – SEPTEMBER 2008 – REVISED MARCH 2011 Short-Circuit Protection The LM3151/2/3 will sense a short-circuit on the output by monitoring the output voltage. When the attenuated feedback voltage has fallen below 60% of the reference voltage, Vref x 0.6 (≈ 0.36V), short-circuit mode of operation will start. During short-circuit operation, the SS pin is discharged and the output voltage will fall to 0V. The SS pin voltage, VSS, is then ramped back up at the rate determined by the SS capacitor and ISS until VSS reaches 0.7V. During this re-ramp phase, if the short-circuit fault is still present the output current will be equal to the set current limit. Once the soft-start voltage reaches 0.7V the output voltage is sensed again and if the attenuated VFB is still below Vref x 0.6 then the SS pin is discharged again and the cycle repeats until the short- circuit fault is removed. Soft-Start The soft-start (SS) feature allows the regulator to gradually reach a steady-state operating point, which reduces start-up stresses and current surges. At turn-on, while VCC is below the under-voltage threshold, the SS pin is internally grounded and VOUT is held at 0V. The SS capacitor is used to slowly ramp VFB from 0V to it's final output voltage as programmed by the internal resistor divider. By changing the soft-start capacitor value, the duration of start-up can be changed accordingly. The start-up time can be calculated using the following equation: where • tSS is measured in seconds • Vref = 0.6V • ISS is the soft-start pin source current, which is typically 7.7 µA (refer to electrical characteristics table) An internal switch grounds the SS pin if VCC is below the under-voltage lockout threshold, if a thermal shutdown occurs, or if the EN pin is grounded. By using an externally controlled switch, the output voltage can be shut off by grounding the SS pin. During startup the LM3151/2/3 will operate in diode emulation mode, where the low-side gate LG will turn off and remain off when the inductor current falls to zero. Diode emulation mode allows for start up into a pre-biased output voltage. When soft-start is greater than 0.7V, the LM3151/2/3 will remain in continuous conduction mode. During diode emulation mode at current limit the low-gate will remain off when the inductor current is off. The soft start time should be greater than the rise time specified by, tSS ≥ (VOUT x COUT) / (IOCL - IOUT) Enable/Shutdown The EN pin can be activated by either leaving the pin floating due to an internal pull up resistor to VIN or by applying a logic high signal to the EN pin of 1.26V or greater. The LM3151/2/3 can be remotely shut down by taking the EN pin below 1.02V. Low quiescent shutdown is achieved when VEN is less than 0.4V. During low quiescent shutdown the internal bias circuitry is turned off. The LM3151/2/3 has certain fault conditions that can trigger shutdown, such as over-voltage protection, current limit, under-voltage lockout, or thermal shutdown. During shutdown, the soft-start capacitor is discharged. Once the fault condition is removed, the soft-start capacitor begins charging, allowing the part to start up in a controlled fashion. In conditions where there may be an open drain connection to the EN pin, it may be necessary to add a 1000 pF bypass capacitor to this pin. This will help decouple noise from the EN pin and prevent false disabling. Thermal Protection The LM3151/2/3 should be operated such that the junction temperature does not exceed the maximum operating junction temperature. An internal thermal shutdown circuit, which activates at 165°C (typical), takes the controller to a low-power reset state by disabling the buck switch and the on-timer, and grounding the SS pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction temperature falls back below 150°C the SS pin is released and normal operation resumes. Copyright © 2008–2011, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3151 LM3152 LM3153 |
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