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IR3084MTRPBF Datasheet(PDF) 10 Page - International Rectifier |
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IR3084MTRPBF Datasheet(HTML) 10 Page - International Rectifier |
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10 / 45 page  IR3084 Page 10 of 45 07/20/2005 Body Braking TM In a conventional synchronous buck converter, the minimum time required to reduce the current in the inductor in response to a load step decrease is; TSLEW = [L x (IMAX − IMIN)] / Vout The slew rate of the inductor current can be significantly increased by turning off the synchronous rectifier in response to a load step decrease. The switch node voltage is then forced to decrease until conduction of the synchronous rectifier’s body diode occurs. This increases the voltage across the inductor from Vout to Vout + VBODY DIODE. The minimum time required to reduce the current in the inductor in response to a load transient decrease is now; TSLEW = [L x (IMAX − IMIN)] / (Vout + VBODY DIODE) Since the voltage drop in the body diode is often higher than output voltage, the inductor current slew rate can be increased by 2X or more. This patent pending technique is referred to as “body braking” and is accomplished through the “0% Duty Cycle Comparator” located in the Phase IC. If the Error Amp’s output voltage drops below 91% of the VDAC voltage this comparator turns off the low side gate driver. Figure 5 depicts PWM operating waveforms under various conditions PHASE IC CLOCK PULSE EAIN VDAC PWMRMP GATEH GATEL STEADY-STATE OPERATION DUTY CYCLE DECREASE DUE TO VIN INCREASE (FEED-FORWARD) DUTY CYCLE INCREASE DUE TO LOAD INCREASE DUTY CYCLE DECREASE DUE TO LOAD DECREASE (BODY BRAKING) OR FAULT (VCC UV, VCCVID UV, OCP, VID=11111X) STEADY-STATE OPERATION Body-Braking Threshold Figure 5 – PWM Operating Waveforms Lossless Average Inductor Current Sensing Inductor current can be sensed by connecting a series resistor and a capacitor network in parallel with the inductor and measuring the voltage across the capacitor. The equation of the sensing network is, S S L L S S L C C sR sL R s i C sR s v s v + + = + = 1 ) ( 1 1 ) ( ) ( Usually the resistor Rcs and capacitor Ccs are chosen so that the time constant of Rcs and Ccs equals the time constant of the inductor which is the inductance L over the inductor DCR. If the two time constants match, the voltage across Ccs is proportional to the current through L, and the sense circuit can be treated as if only a sense resistor with the value of RL was used. The mismatch of the time constants does not affect the measurement of inductor DC current, but affects the AC component of the inductor current. |
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