Using the ISL43120 SPST Switch in a Multi-Phase PWM Power Application ® Technical Brief January 2003 TB401 Author: Don LaFontaine Introduction This Techbrief will illustrate the use of a dual SPST Switch in a multi-Phase PWM High current, high slew-rate power application. The ISL43120 enables the user to change the supply’s output voltage via logic control. Typical application would be for testing of product at 10% high supplies as part of an outgoing inspection. The 3 phase power converter (Figure 2) shows the connection of the ISL43120 dual SPST switch. Single logic control is achieved by shorting the VIN1 and VIN2 pins of the ISL43120 together. RFB VOUT FB ROS DROOP RFB VSEN ROS ISL6558 Operation Figure 2 shows a simplified diagram of the voltage regulation and current control loops for a three-phase converter. The ISL6558 Precision Multi-Phase PWM Controller provides both voltage and current feedback to precisely regulate output voltage and tightly control phase currents of the three power channels. Voltage Loop Output voltage feedback is applied via the resistor combination of RFB and ROS to the inverting input of the error amplifier. The output voltage can now be changed to a predetermined value via logic control of the ISL43120. The output voltage with the parallel resistor RP is calculated using Equation 1. The process of determining the values of RFB and ROS first starts with Equation 2, or Equation 3 if output droop compensation is required. Once the values of RFB and ROS are determined, Equation 1 can be used to determine the output voltage as a function of the parallel combination of RP and ROS. Note: The value of RP includes the switch resistance. 0.8V ( R FB + ( R OS || R P ) ) V OUT = -----------------------------------------------------------------|| R R OS Current Loop The current control loop keeps the channel currents (IL1, IL2 IL3) in balance. During the PWM off-time of each channel, the voltage developed across the rDS(ON) of the lower MOSFET is sampled (Q1, Q2, Q3) and fed back to a current sensing network. This signal keeps each channel’s output current contribution balanced relative to the other active channels. Reference the ISL6558 data sheet for further details of the internal operation of the ISL6558. SELECTING RFB AND ROS If output droop compensation is not required the DROOP pin must be left open. Simply select a value for RFB and calculate ROS based on the following equation: 0.8V R OS = R FB x ---------------------------------V OUT – 0.8V (EQ. 2) Equation 1 can then be used to determine the output voltage as a function of RP. (EQ. 1) P The output voltage, VOUT, must be fed back to the VSEN pin separately from the feedback components to the FB pin. If VSEN and FB are tied together, the error amplifier will hold the VSEN voltage at the reference level while the actual output voltage level could be much different. This would mask the output voltage and prevent the protection features from reacting to undervoltage or overvoltage conditions at the proper time. It is for this reason, that the second SPST switch and the duplication of resistors ROS and RP are required as shown in Figure 2. If the output voltage is required to be 0.8V, the user could then tied the output directly back to the VSEN pin without a resistor divider. 1 FIGURE 1. VSEN RESISTOR DIVIDER CONFIGURATION In applications where droop compensation is desired, tie the DROOP and FB pins together. Select RFB first given the following equation, where VDROOP is the desired amount of output voltage droop at full load. This equation is contingent upon the correct selection of the ISEN resistors discussed in the Fault Protection section of the data sheet. V DROOP 3 - = 20 ×10 xV DROOP R FB = -----------------------50µA (EQ. 3) Calculate ROS based on RFB using the following equation. Where VOUT,NL is the desired output voltage under no-load conditions. 0.8V R OS = R FB x -------------------------------------------– 0.8V V OUT, NL 1-888-INTERSIL or 321-724-7143 | (EQ. 4) Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright © Intersil Americas Inc. 2003. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Technical Brief 401 Typical Application - 3 Phase Converter VOUT = 0.8V(RFB + ROS || RP)/ ROS || RP +12V +5V RC +5V BOOT IL1 PVCC UGATE CC VCC RFB FB DROOP + 0.8V ROS RP PHASE COMP DRIVER HIP6601B PWM VCC Q1 LGATE ERROR AMPLIFIER RISEN3 GND RFB VOUT PWM4 PWM3 +12V VSEN PWM2 LOGIC CONTROL +5V ROS BOOT PWM1 PVCC PWM CONTROL ISL6558 RP PHASE ISEN4 PGOOD ISEN3 FS/EN NC VCC PWM DRIVER HIP6601B ISEN2 RT GND IL2 UGATE LGATE Q2 RISEN2 GND ISEN1 +12V +5V BOOT PVCC PHASE VCC PWM IL3 UGATE DRIVER HIP6601B Q3 LGATE RISEN1 GND FIGURE 2. SCHEMATIC OF THE ISL6558 3 PHASE CONVERTER WITH VARIABLE OUTPUT VOLTAGE VIA LOGIC CONTROL All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. 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