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LX1677CLQ Datasheet(PDF) 11 Page - Microsemi Corporation |
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LX1677CLQ Datasheet(HTML) 11 Page - Microsemi Corporation |
11 / 16 page LX1677 PRODUCTION DATA SHEET Microsemi Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 11 AMD64 Processor VRM Controller Copyright © 2000 Rev 1.1, 2003-03-03 TM ® TH EORY OF O PERATION (CONTINUED) THEORY OF OPERATION FOR A BI-PHASE, LOADSHARE TM CONFIGURATION The basic principle used in LoadSHARETM in a multiple phase buck converter topology is that if multiple, identical, inductors have the same identical voltage impressed across their leads, they must then have the same identical current passing through them. The current that we would like to balance between inductors is mainly the DC component along with as much as possible the transient current. All inductors in a multiphase buck converter topology have their output side tied together at the output filter capacitors. Therefore this side of all the inductors has the same identical voltage. If the input side of the inductors can be forced to have the same equivalent DC potential on this lead, then they will have the same DC current flowing. To achieve this requirement, phase 1 will be the control phase that sets the output operating voltage, under normal PWM operation. To force the current of phase 2 to be equal to the current of phase 1; a second feedback loop is used. Phase 2 has a low pass filter connected from the input side of each inductor. This side of the inductors has a square wave signal that is proportional to its duty cycle. The output of each LPF is a DC (+ some AC) signal that is proportional to the magnitude and duty cycle of its respective inductor signal. The second feedback loop will use the output of the phase 1 LPF as a reference signal for an error amplifier that will compare this reference to the output of the phase 2 LPF. This error signal will be amplified and used to control the PWM circuit of phase 2. Therefore, the duty cycle of phase 2 will be set so that the equivalent voltage potential will be forced across the phase 2 inductor as compared to the phase 1 inductor. This will force the current in the phase 2 inductor to follow and equal the phase 1 inductor current. With the LoadSHARETM topology it is possible to imbalance the phases so that one phase will supply more current than the other under unique situations. The LX1677 will normally be used with the same supply voltages on phase 1 and 2 PWM inputs and will have equal currents in both phases. LP2 LP1 Phase 1 Comparator + _ + _ gm Phase 2 Comparator 0° 180° Phase 2 Integrating Amp Differential Amp + - DC Bias Phase 1 Error Amp + - HO1 EA - EO2 LO1 LO2 HO2 Phase 1 Low Pass Filter Phase 2 Low Pass Filter Vout DC Bias DC Bias DACOUT Differential Feedbach From Vout Phase 1 Diff Amp Ramp .75 V to 3 V Both PWMs Figure 5 – LoadSHARE Control Loop |
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