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LM3481QMM 데이터시트(PDF) 8 Page - Texas Instruments |
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LM3481QMM 데이터시트(HTML) 8 Page - Texas Instruments |
8 / 30 page FB FA/SYNC/SD COMP ISEN AGND PGND DR VIN Shutdown Detect SYNC/Fixed Frequency detect Soft-start + EA + - + Slope Compensation Thermal Shutdown Set/Blankout Switch Logic Switch Driver 1.275V Reference Oscillator + 220 mV Level Shifter 6 + One Shot Bias Voltages 6V Overvoltage Comparator Short-circuit Comparator Ramp Adjust V-I Converter I-V Converter PWM Comparator R S Q VCC UVLO UVLO 40 é A Vfb + Vovp - - - - LM3481 SNVS346E – NOVEMBER 2007 – REVISED APRIL 2012 www.ti.com FUNCTIONAL BLOCK DIAGRAM FUNCTIONAL DESCRIPTION The LM3481 uses a fixed frequency, Pulse Width Modulated (PWM), current mode control architecture. In a typical application circuit, the peak current through the external MOSFET is sensed through an external sense resistor. The voltage across this resistor is fed into the ISEN pin. This voltage is then level shifted and fed into the positive input of the PWM comparator. The output voltage is also sensed through an external feedback resistor divider network and fed into the error amplifier (EA) negative input (feedback pin, FB). The output of the error amplifier (COMP pin) is added to the slope compensation ramp and fed into the negative input of the PWM comparator. At the start of any switching cycle, the oscillator sets the RS latch using the SET/Blank-out and switch logic blocks. This forces a high signal on the DR pin (gate of the external MOSFET) and the external MOSFET turns on. When the voltage on the positive input of the PWM comparator exceeds the negative input, the RS latch is reset and the external MOSFET turns off. The voltage sensed across the sense resistor generally contains spurious noise spikes, as shown in Figure 18. These spikes can force the PWM comparator to reset the RS latch prematurely. To prevent these spikes from resetting the latch, a blank-out circuit inside the IC prevents the PWM comparator from resetting the latch for a short duration after the latch is set. This duration, called the blank-out time, is typically 250 ns and is specified as tmin (on) in the electrical characteristics section. Under extremely light load or no-load conditions, the energy delivered to the output capacitor when the external MOSFET is on during the blank-out time is more than what is delivered to the load. An over-voltage comparator inside the LM3481 prevents the output voltage from rising under these conditions by sensing the feedback (FB pin) voltage and resetting the RS latch. The latch remains in a reset state until the output decays to the nominal value. Thus the operating frequency decreases at light loads, resulting in excellent efficiency. 8 Submit Documentation Feedback Copyright © 2007–2012, Texas Instruments Incorporated Product Folder Links: LM3481 |
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