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NCP81243 Datasheet(PDF) 19 Page - ON Semiconductor |
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NCP81243 Datasheet(HTML) 19 Page - ON Semiconductor |
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19 / 26 page  NCP81243 www.onsemi.com 19 Remote Sense Amplifier A high performance high input impedance true differential amplifier is provided to accurately sense the output voltage of the regulator. The VSP and VSN inputs should be connected to the regulator’s output voltage sense points. The remote sense amplifier takes the difference of the output voltage with the DAC voltage and adds the droop voltage to VDIFOUT = V VSP * VVSN ) 1.3 V * VDAC ) VDROOP * VCSREF This signal then goes through a standard error compensation network and into the inverting input of the error amplifier. The non−inverting input of the error amplifier is connected to the same 1.3 V reference used for the differential sense amplifier output bias. High Performance Voltage Error Amplifier A high performance error amplifier is provided for high bandwidth transient performance. A standard type III compensation circuit is normally used to compensate the system. Differential Current Feedback Amplifiers Each phase has a low offset differential amplifier to sense that phase current for current balance. The inputs to the CSNx and CSPx pins are high impedance inputs. It is recommended that any external filter resistor RCSN does not exceed 10 k W to avoid offset issues with leakage current. It is also recommended that the voltage sense element be no less than 0.5 k W for accurate current balance. Fine tuning of this time constant is generally not required. The individual phase current is summed into the PWM comparator feedback this way current is balanced via a current mode control approach. CCSN RCSN DCR LPHASE 1 2 SWNx VOUT R CSN + L PHASE C CSN @ DCR Total Current Sense Amplifier The NCP81243 uses a patented approach to sum the phase currents into a single temperature compensated total current signal. This signal is then used to generate the output voltage droop, total current limit, and the output current monitoring functions. The total current signal is floating with respect to CSREF. The current signal is the difference between CSCOMP and CSREF. The Ref(n) resistors sum the signals from the output side of the inductors to create a low impedance virtual ground. The amplifier actively filters and gains up the voltage applied across the inductors to recover the voltage drop across the inductor series resistance (DCR). Rth is placed near an inductor to sense the temperature of the inductor. This allows the filter time constant and gain to be a function of the Rth NTC resistor and compensate for the change in the DCR with temperature. The DC gain equation for the current sensing: V CSCOMP−CSREF + * Rcs2 ) Rcs1@Rth Rcs1 )Rth Rph @ Iout Total @ DCR Set the gain by adjusting the value of the Rph resistors. The DC gain should be set to the output voltage droop. If the voltage from CSCOMP to CSREF is less than 100 mV at ICCMAX then it is recommend increasing the gain of the CSCOMP amp. This is required to provide a good current signal to offset voltage ratio for the ILIMIT pin. When no droop is needed, the gain of the amplifier should be set to provide ~100 mV across the current limit programming resistor at full load. The values of Rcs1 and Rcs2 are set based on the 220k NTC and the temperature effect of the inductor and should not need to be changed. The NTC should be placed near the closest inductor. The output voltage droop should be set with the droop filter divider. The pole frequency in the CSCOMP filter should be set equal to the zero from the output inductor. This allows the circuit to recover the inductor DCR voltage drop current signal. Ccs1 and Ccs2 are in parallel to allow for fine tuning of the time constant using commonly available values. It is best to fine tune this filter during transient testing. |
Similar Part No. - NCP81243_17 |
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Similar Description - NCP81243_17 |
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