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IR3522 Datasheet(PDF) 11 Page - International Rectifier |
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IR3522 Datasheet(HTML) 11 Page - International Rectifier |
11 / 32 page IR3522 Page 11 V3.01 Figure 5 PWM Operating Waveforms Lossless Average Inductor Current Sensing Inductor current can be sensed by connecting a series RC network in parallel with the inductor and measuring the voltage across the capacitor, as shown in Figure 6. The equation of this sensing network is, ( ) . 1 1 ) ( 1 1 ) ( ) ( CS CS L L L CS CS L C C sR R L s R s i C sR s v s v + + = + = Usually, the resistor Rcs and capacitor Ccs are chosen so that the RC time constant equals the time constant of the inductor which is the inductance L divided by the inductor’s DCR (RL). 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. Figure 6 Inductor Current Sensing and Current Sense Amplifier The advantage of sensing the inductor current versus high side or low side sensing is that actual output current being delivered to the load is obtained rather than peak or sampled information about the switch currents. The output voltage can be positioned to meet a load line based on real time information. Except for a sense resistor in series with the inductor, this is the only sense method that can support a single cycle transient response. Other methods provide no information during either load increase (low side sensing) or load decrease (high side sensing). CO L RL RCS CCS VO Current Sense Amp CSOUT iL vL vCS c PHASE IC CLOCK PULSE VDAC EAIN PWMRMP GATEL GATEH DUTY CYCLE DECREASE DUE TO VIN INCREASE (FEED-FORWARD) DUTY CYCLE INCREASE DUE TO LOAD INCREASE STEADY-STATE OPERATION STEADY-STATE OPERATION DUTY CYCLE DECREASE DUE TO LOAD DECREASE (BODY BRAKING) OR FAULT (VCC UV, OCP, VID FAULT) |
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