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LM3243 Datasheet(PDF) 13 Page - Texas Instruments |
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LM3243 Datasheet(HTML) 13 Page - Texas Instruments |
13 / 32 page ![]() LM3243 www.ti.com SNVS782C – OCTOBER 2010 – REVISED AUGUST 2015 Feature Description (continued) 7.3.3 Mode Pin The MODE pin changes the state of the converter to one of the two allowed modes of operation. Setting the MODE pin HIGH (> 1.2 V) sets the device for automatic transition between pulse frequency modulation (PFM) and PWM mode operation. In this mode, the converter operates in PFM mode to maintain the output voltage regulation at very light loads and transitions into PWM mode at loads exceeding 95 mA (typical). The PWM switching frequency is 2.7 MHz (typical). Setting the MODE pin LOW (< 0.5 V) sets the device for PWM mode operation. The switching operation is in PWM mode only, and the switching frequency is also 2.7 MHz (typical). 7.3.4 Dynamic Adjustment Of Output Voltage The output voltage of the LM3243 can be dynamically adjusted by changing the voltage on the VCON pin. In RF PA applications, peak power is required when the handset is far away from the base station. To maximize the power savings, the LM3243 output should be set just high enough to achieve the desired PA linearity. Hence, during low-power requirements, reduction of supply voltage to the PA can reduce power consumption from the PA, making the operation more efficient and promote longer battery life. Please refer to Setting The Output Voltage for further details. 7.3.5 Internal Synchronous Rectification The LM3243 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop, thus increasing efficiency. The reduced forward voltage drop in the internal NFET synchronous rectifier significantly improves efficiency for low output voltage operation. The NFET is designed to conduct through its intrinsic body diode during the transient intervals, eliminating the need of an external diode. 7.3.6 Current Limit The LM3243 current limit feature protects the converter during current overload conditions. Both SW and ACB pins have positive and negative current limits. The positive and negative current limits bound the SW and ACB currents in both directions. The SW pin has two positive current limits. The ILIM,PFET,SteadyState current limit triggers the ACB circuit. Once the peak inductor current exceeds ILIM,PFET,SteadyState, the ACB circuit starts assisting the switcher and provides just enough current to keep the inductor current from exceeding ILIM,PFET,SteadyState allowing the switcher to operate at maximum efficiency. Transiently a second current limit ILIM,PFET,Transient of 1.9 A (typical) or 2.1 A (maximum) limits the maximum peak inductor current possible. The output voltage will fall out of regulation only after both SW and ACB output pin currents reach their respective current limits of ILIM,PFET,Transient and ILIM,P-ACB. 7.3.7 Timed Current Limit If the load or output short circuit pulls the output voltage to 0.3 V or lower and the peak inductor current sustains ILIM, PFET Transient more than 10 µs, the LM3243 switches to a timed current limit mode. In this mode, the internal PFET switch is turned off. After approximately 30 µs, the device will return to the normal operation. 7.3.8 Thermal Overload Protection The LM3243 device has a thermal overload protection that protects itself from short-term misuse and overload conditions. If the junction temperature exceeds 150°C, the LM3243 shuts down. Normal operation resumes after the temperature drops below 130°C. Prolonged operation in thermal overload condition may damage the device and is therefore not recommended. 7.4 Device Functional Modes 7.4.1 PWM Operation When the LM3243 operates in PWM mode, the switching frequency is constant, and the switcher regulates the output voltage by changing the energy-per-cycle to support the load required. During the first portion of each switching cycle, the control block in the LM3243 turns on the internal PFET switch. This allows current to flow from the input through the inductor and to the output filter capacitor and load. The inductor limits the current to a ramp with a slope of (VIN – VOUT)/L, by storing energy in its magnetic field. Copyright © 2010–2015, Texas Instruments Incorporated Submit Documentation Feedback 13 Product Folder Links: LM3243 |
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