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ISL6277HRZ Datasheet(PDF) 20 Page - Intersil Corporation |
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ISL6277HRZ Datasheet(HTML) 20 Page - Intersil Corporation |
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20 / 37 page  ISL6277 20 FN8270.1 March 8, 2012 In a 1-phase configuration, the ISEN2_NB pin is tied to +5V. The Northbridge VR operates in 1-phase CCM and enters 1-phase DE when both PSI0_L and PSI1_L are low. The Northbridge VR can be disabled completely by tieing ISEN1_NB to 5V. Dynamic Operation Core VR and Northbridge VR behave the same during dynamic operation. The controller responds to VID-on-the-fly changes by slewing to the new voltage at the slew rate programmed, see Table 4. During negative VID transitions, the output voltage decays to the lower VID value at the slew rate determined by the load. The R3™ modulator intrinsically has voltage feed-forward. The output voltage is insensitive to a fast slew rate input voltage change. FB2 Function The Core VR features an FB2 pin. The FB2 function is only available when the Core VR is in a 2-phase configuration. Figure 18 shows the FB2 function. A switch (called FB2 switch) turns on to short the FB and the FB2 pins when the controller is in 2-phase mode. Capacitors C3.1 and C3.2 are in parallel, serving as part of the compensator. When the controller enters 1-phase mode, the FB2 switch turns off, removing C3.2 and leaving only C3.1 in the compensator. The compensator gain increases with the removal of C3.2. By properly sizing C3.1 and C3.2, the compensator can be optimal for both 2-phase mode and 1-phase mode. When the FB2 switch is off, C3.2 is disconnected from the FB pin. However, the controller still actively drives the FB2 pin voltage to follow the FB pin voltage such that C3.2 voltage always follows C3.1 voltage. When the controller turns on the FB2 switch, C3.2 is reconnected to the compensator smoothly. The FB2 function ensures excellent transient response in both 2-phase and 1-phase mode. If the FB2 function is not used, populate C3.1 only. Adaptive Body Diode Conduction Time Reduction In DCM, the controller turns off the low-side MOSFET when the inductor current approaches zero. During on-time of the low-side MOSFET, phase voltage is negative, and the amount is the MOSFET rDS(ON) voltage drop, which is proportional to the inductor current. A phase comparator inside the controller monitors the phase voltage during on-time of the low-side MOSFET and compares it with a threshold to determine the zero crossing point of the inductor current. If the inductor current has not reached zero when the low-side MOSFET turns off, it will flow through the low-side MOSFET body diode, causing the phase node to have a larger voltage drop until it decays to zero. If the inductor current has crossed zero and reversed the direction when the low-side MOSFET turns off, it will flow through the high-side MOSFET body diode, causing the phase node to have a spike until it decays to zero. The controller continues monitoring the phase voltage after turning off the low-side MOSFET. To minimize the body diode-related loss, the controller also adjusts the phase comparator threshold voltage accordingly in iterative steps such that the low-side MOSFET body diode conducts for approximately 40ns. Resistor Configuration Options The ISL6277 uses the COMP, COMP_NB and FCCM_NB pins to configure some functionality within the IC. Resistors from these pins to GND are read during the first portion of the soft-start sequence. The following sections outline how to select the resistor values for each of these pins to correctly program the output voltage offset of each output, the configuration of the floating DriverX and PWM_Y output, VID-on-the-Fly slew rate, and switching frequency used for both VRs. VR Offset Programming A positive or negative offset is programmed for the Core VR using a resistor to ground from the COMP pin and the Northbridge in a similar manner from the COMP_NB pin. Table 3 provides the resistor value to select the desired output voltage offset R1 E/A R3 C2 C1 VREF R2 C3.2 C3.1 FB FB2 COMP VSEN R1 E/A R3 C2 C1 VREF R2 C3.2 C3.1 FB FB2 COMP VSEN CONTROLLER IN 2-PHASE MODE CONTROLLER IN 1-PHASE MODE FIGURE 18. FB2 FUNCTION TABLE 3. COMP & COMP_NB OUTPUT VOLTAGE OFFSET SELECTION RESISTOR VALUE [kΩ] COMP VCORE OFFSET [mV] COMP_NB OFFSET [mV] 5.62 -43.75 18.75 9.53 -37.5 31.25 13.3 -31.25 43.76 16.9 -25 50 21.0 -18.75 37.5 26.7 -12.5 25 34.0 -6.25 12.5 41.2 6.25 0 57.6 18.75 18.75 73.2 31.25 31.25 95.3 43.76 43.76 121 50 50 154 37.5 37.5 182 25 25 221 12.5 12.5 OPEN 0 0 |
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