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L6919ETR Datenblatt(PDF) 11 Page - STMicroelectronics |
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L6919ETR Datenblatt(HTML) 11 Page - STMicroelectronics |
11 / 33 page 11/33 L6919E Figure 4. Current Reading Timing (Left) and Circuit (Right) Where RSENSE is an external sense resistor or the rds,on of the low side mosfet and Rg is the transconductance resistor used between ISENx and PGNDSx pins toward the reading points; IPHASE is the current carried by each phase and, in particular, the current measured in the middle of the oscillator period The current information reproduced internally is represented by the second term of the previous equation as follow: Since the current is read in differential mode, also negative current information is kept; this allow the device to check for dangerous returning current between the two phases assuring the complete equalization between the phase's currents. From the current information of each phase, information about the total current delivered (IFB =IINFO1 +IINFO2) and the average current for each phase (IAVG =(IINFO1 +IINFO2)/2 ) is taken. IINFOX is then com- pared to IAVG to give the correction to the PWM output in order to equalize the current carried by the two phases. The transconductance resistor Rg can be designed in order to have current information of 25 µA per phase at full nominal load; the over current intervention threshold is set at 140% of the nominal (IINFOx = 35µA). According to the above relationship, the over current threshold (IOCPx) for each phase, which has to be placed at one half of the total delivered maximum current, results: Since the device senses the output current across the low-side mosfets (or across a sense resistors in series with them) the device limits the bottom of the inductor current triangular waveform: an over current is detected when the current flowing into the sense element is greater than IOCPx (IINFOx > 35µA). Introducing now the maximum ON time dependence with the delivered current (where T is the switching period T=1/fSW): This linear dependence has a value at zero load of 0.80·T and at maximum current of 0.40·T typical and results in two different behaviors of the device: Rg 50 µµµµA IISENx Rg LGATEX ISENX PGNDSX ILS1 ILS2 Track & Hold Total current information I IN FO x R SENSE I PHASE ⋅ R g ---------------------------------------------- = I OCPx 35 µARg ⋅ R SEN SE --------------------------- = Rg I OC P x R SE NSE ⋅ 35 µA ------------------------------------------- = T ON,MAX 0.80 I FB 5.73 k ⋅ () T ⋅ – 0.80 R SENSE Rg ---------------------- I OUT 5.73k ⋅⋅ T 0.80 T I FB ⋅ 0 µA = 0.40 T I FB ⋅ 70 µA = ⋅ – == |
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