ISL6277HRZ datasheet(20/37 Pages) RENESAS

20 / 37 page

ISL6277

FN8270 Rev 1.00

Page 20 of 37

Mar 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

E/A

VREF

C3.2

C3.1

FB2

COMP

VSEN

E/A

VREF

C3.2

C3.1

FB2

COMP

VSEN

CONTROLLER

2-PHASE MODE

CONTROLLER

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

21.0

-18.75

37.5

26.7

-12.5

34.0

-6.25

12.5

41.2

6.25

57.6

18.75

73.2

31.25

95.3

43.76

121

154

37.5

182

221

12.5

OPEN

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