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ISL6277A Datasheet(PDF) 20 Page - Renesas Technology Corp |
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ISL6277A Datasheet(HTML) 20 Page - Renesas Technology Corp |
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20 / 38 page  ISL6277A FN8322 Rev 2.00 Page 20 of 38 December 4, 2015 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 tying ISEN1_NB to 5V. The Core and Northbridge VRs have an overcurrent threshold of 1.5V on IMON and IMON_NB respectively, and this level does not vary based on channel configuration. See “Overcurrent” on page 26 for more details. 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 on page 21. 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 20 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 ISL6277A 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 on page 21 provides the resistor value to select the desired output voltage offset. The 1% tolerance resistor value shown in the table must be used to program the corresponding Core or NB output voltage offset. The MIN and MAX tolerance values provide margin to insure the 1% tolerance resistor will be read correctly. TABLE 2. NORTHBRIDGE VR MODES OF OPERATION ISEN2_NB CONFIG. PSL0_L AND PSI1_L MODE IMON OCP THRESHOLD To Power Stage 2-phase NB VR Config. 11 2-phase CCM 1.5V 01 1-phase DE 00 1-phase DE Tied to 5V 1-phase NB VR Config. 11 1-phase CCM 1.5V 01 1-phase DE 00 1-phase DE 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 20. FB2 FUNCTION |
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