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LM2775 датащи(PDF) 10 Page - Texas Instruments |
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LM2775 датащи(HTML) 10 Page - Texas Instruments |
10 / 27 page LM2775 SNVSA57 – MAY 2015 www.ti.com Feature Description (continued) 7.3.3 PFM Mode To minimize quiescent current during light load operation, the LM2775 provides a PFM operation option (selectable via the PFM pin. '1' = PFM allowed, '0' = Fixed frequency). By allowing the charge pump to only switch when the VOUT voltage decays to a typical 5.05 V, the quiescent current drawn from the power source is minimized. The frequency of pulsed operation is not limited and can drop into the sub-1-kHz range when unloaded. As the load increases, the frequency of pulsing increases. When PFM mode is disabled, the device operates in a constant frequency mode. In this mode, the quiescent current remains at normal levels even when the load current is decreased. The main advantages of fixed frequency operation include a lower output voltage ripple level due to the constant switching and a predictable switching frequency that stays at 2 MHz which can be important in noise sensitive applications. 7.3.4 Output Discharge The LM2775 provides two different output discharge modes upon entering a shutdown state (EN pin = '0') after running in the on state (EN = '1'). The first mode is high impendance mode (OUTDIS = '0'). In this mode, the output remains high even when the EN pin is driven low. This enables use in applications where the LM2775 output might be tied to a system rail that has another power source connected (USBOTG). When OUTDIS = 0, the output of the LM2775 draws a minimal current from the output supply (1.6 µA typical). In Discharge Mode (OUTDIS pin = '1'), the LM2775 actively pulls down on the output of the device until the output voltage reaches GND. In this mode, the current drawn from the output is approximately 450 µA. 7.3.5 Thermal Shutdown The LM2775 implements a thermal shutdown mechanism to protect the device from damage due to overheating. When the junction temperature rises to 150°C (typical), the part switches into shutdown mode. The LM2775 releases thermal shutdown when the junction temperature of the part is reduced to 130°C (typical). Thermal shutdown is most often triggered by self-heating, which occurs when there is excessive power dissipation in the device and/or insufficient thermal dissipation. LM2775 power dissipation increases with increased output current and input voltage. When self-heating brings on thermal shutdown, thermal cycling is the typical result. Thermal cycling is the repeating process where the part self-heats, enters thermal shutdown (where internal power dissipation is practically zero), cools, turns on, and then heats up again to the thermal shutdown threshold. Thermal cycling is recognized by a pulsing output voltage and can be stopped be reducing the internal power dissipation (reduce input voltage and/or output current) or the ambient temperature. If thermal cycling occurs under desired operating conditions, thermal dissipation performance must be improved to accommodate the power dissipation of the LM2775. The WSON package is designed to have excellent thermal properties that, when soldered to a PCB designed to aid thermal dissipation, allows the LM2775 to operate under very demanding power dissipation conditions. 7.3.6 Undervoltage Lockout The LM2775 has an internal comparator that monitors the voltage at VIN and forces the LM2775 into shutdown if the input voltage drops to 2.4 V. If the input voltage rises above 2.6 V, the LM2775 resumes normal operation 10 Submit Documentation Feedback Copyright © 2015, Texas Instruments Incorporated Product Folder Links: LM2775 |
Аналогичный номер детали - LM2775 |
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Аналогичное описание - LM2775 |
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