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LM25018 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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LM25018 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 31 page + VIN UVLO VIN RUV1 CIN RUV2 2 3 LM25018 LM25018 www.ti.com SNVS953D – DECEMBER 2012 – REVISED DECEMBER 2014 Feature Description (continued) 7.3.9 Undervoltage Detector The LM25018 device contains a dual-level undervoltage lockout (UVLO) circuit. A summary of threshold voltages and operational states is provided in Device Functional Modes. When the UVLO pin voltage is below 0.66 V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.66 V but less than 1.225 V, the controller is in standby mode. In standby mode the VCC bias regulator is active while the regulator output is disabled. When the VCC pin exceeds the VCC undervoltage threshold and the UVLO pin voltage is greater than 1.225 V, normal operation begins. An external set-point voltage divider from VIN to GND can be used to set the minimum operating voltage of the regulator. UVLO hysteresis is accomplished with an internal 20- μA current source that is switched on or off into the impedance of the set-point divider. When the UVLO threshold is exceeded, the current source is activated to quickly raise the voltage at the UVLO pin. The hysteresis is equal to the value of this current times the resistance RUV2. If the UVLO pin is wired directly to the VIN pin, the regulator will begin operation once the VCC undervoltage is satisfied. Figure 11. UVLO Resistor Setting 7.3.10 Thermal Protection The LM25018 device should be operated so the junction temperature does not exceed 150°C during normal operation. An internal Thermal Shutdown circuit is provided to protect the LM25018 in the event of a higher than normal junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state, disabling the buck switch and the VCC regulator. This feature prevents catastrophic failures from accidental device overheating. When the junction temperature reduces below 145°C (typical hysteresis = 20°C), the VCC regulator is enabled, and normal operation is resumed. 7.3.11 Ripple Configuration LM25018 uses constant on-time (COT) control scheme, in which the on-time is terminated by an on-timer, and the off-time is terminated by the feedback voltage (VFB) falling below the reference voltage (VREF). Therefore, for stable operation, the feedback voltage must decrease monotonically, in phase with the inductor current during the off-time. Furthermore, this change in feedback voltage (VFB) during off-time must be large enough to suppress any noise component present at the feedback node. Table 1 shows three different methods for generating appropriate voltage ripple at the feedback node. Type 1 and Type 2 ripple circuits couple the ripple at the output of the converter to the feedback node (FB). The output voltage ripple has two components: 1. Capacitive ripple caused by the inductor current ripple charging/discharging the output capacitor. 2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor. The capacitive ripple is not in phase with the inductor current. As a result, the capacitive ripple does not decrease monotonically during the off-time. The resistive ripple is in phase with the inductor current and decreases monotonically during the off-time. The resistive ripple must exceed the capacitive ripple at the output node (VOUT) for stable operation. If this condition is not satisfied unstable switching behavior is observed in COT converters, with multiple on-time bursts in close succession followed by a long off-time. Copyright © 2012–2014, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM25018 |
Số phần tương tự - LM25018_15 |
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Mô tả tương tự - LM25018_15 |
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