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LM3686 Fiches technique(PDF) 13 Page - Texas Instruments |
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LM3686 Fiches technique(HTML) 13 Page - Texas Instruments |
13 / 27 page LM3686 www.ti.com SNVS520E – AUGUST 2008 – REVISED MAY 2013 Operation Description DEVICE INFORMATION The LM3686 incorporates a high efficiency synchronous switching step-down DC-DC converter, a very low dropout linear regulator (LILO), and ultra low noise linear regulator. The DC-DC converter delivers a constant voltage from a single Li- Ion battery and input voltage rails from 2.7V to 5.5V to portable devices such as cell phones and PDAs. Using a voltage mode architecture with synchronous rectification, it has the ability to deliver up to 600 mA load current (when not powering the LILO) depending on the input voltage, output voltage, ambient temperature and the inductor chosen. The linear regulator delivers a constant voltage biased from VIN_LILO power input typically the output voltage of the DC-DC converter is used (post regulation) with a maximum load current of 350 mA. The other linear regulator delivers a constant voltage biased from VIN_LDO power input - with a maximum load current of 300 mA. Three enable pins allow the independent control of the three outputs. Shutdown mode turns off the device, offering the lowest current consumption (ISHUTDOWN = 2.5 µA typ). Besides the shutdown feature, for the DC-DC converter there are two more modes of operation depending on the current required: - PWM (Pulse Width Modulation), and - PFM (Pulse Frequency Modulation). The device operates in PWM mode at load current of approximately 80 mA or higher. Lighter load current cause the device to automatically switch into PFM for reduced current consumption (IQ_VBATT = 28 µA typ) and a longer battery life. Additional features include soft-start, startup mode of the linear regulator, under-voltage protection, current overload protection, and over-temperature protection. An internal reference generates a 1.8V biasing an internal resistive divider to create a reference voltage range from 0.7V to 1.8V (in 50 mV steps) for the LILO and the 0.5V reference used for the DC-DC converter. The ultra low noise linear regulator also has internal reference that generates a 1.8V biasing for a internal resistor divider. Thus, creating a reference voltage ranging from 1.5V to 3.3V The Under-voltage lockout feature enables the device to startup once VBATT has reached 2.65V typically and turns the device off if VBATT drops below 2.41V typically. Post Regulation Note: In the case that the DC-DC converter is switched off while the Linear Regulator is still enabled, the LILO can still support up to 50 mA. The linear regulator LILO is turned on via a small NMOS device supplied by VIN_LDO . The maximum current is 50 mA when this small NMOS is ON. If higher current > 50 mA is desired the following condition must be done: 1) EN_DC = HIGH When the condition is met, the LILO transitions to the large NMOS and can support up to 350 mA. DC-DC CONVERTER OPERATION During the first part of each switching cycle, the control block in the LM3686 turns on the internal PFET switch. This allows current to flow from the input VBATT through the switch pin SW and the inductor to the output filter capacitor and load. The inductor limits the current to a ramp with a slope of (VBATT - VOUT_DCDC) / L, by storing energy in the magnetic field. During the second part of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor current down with a slope of (- VOUT_DCDC / L). The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage across the load. Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 13 Product Folder Links: LM3686 |
Numéro de pièce similaire - LM3686_15 |
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Description similaire - LM3686_15 |
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