ISL6334DIRZ datasheet(11/28 Pages) RENESAS

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ISL6334D

FN6802 Rev 4.00

Page 11 of 28

Apr 29, 2016

can be set to be lower than the fixed internal overcurrent threshold.

During the dynamic VID, the OCP function of this pin is disabled to

avoid false triggering. Tie it to GND if not used.

FS - Use this pin to set up the desired switching frequency. A

resistor placed from FS to ground/VCC will set the switching

frequency. The relationship between the value of the resistor and

the switching frequency will be approximated by Equation 3. This

pin is also used with the SS and PSI# pins for phase dropping

decoding (see Table 2 on page 12).

SS - Use this pin to set up the desired start-up oscillator

frequency. A resistor placed from SS to ground/VCC will set up

the soft-start ramp rate. The relationship between the value of

the resistor and the soft-start ramp-up time will be approximated

by Equations 12 and 13. This pin is also used with the FS and

PSI# pins for phase dropping decoding (see Table 2).

VID7, VID6, VID5, VID4, VID3, VID2, VID1 and VID0 - These are

the inputs to the internal DAC that generate the reference voltage

for output regulation. All VID pins have no internal pull-up current

sources until after TD3. Connect these pins either to open-drain

outputs with external pull-up resistors or to active pull-up outputs,

as high as VCC plus 0.3V.

PSI# - A low input signal indicates the low power mode operation

of the processor. The controller drops the number of active

phases to single or 2-phase operation, according to the logic on

Table 2. The PSI# pin, SS and FS pins are used to program the

controller in operation of non-coupled, 2-phase coupled, or

(n-x)-phase coupled inductors when PSI# is asserted (active low).

Different cases yield different PWM output behavior on both

dropped phase(s) and remained phase(s) as PSI# is asserted and

deasserted. A high input signal pulls the controller back to

normal operation.

Operation

Multiphase Power Conversion

Microprocessor load current profiles have changed to the point

that the advantages of multiphase power conversion are

impossible to ignore. The technical challenges associated with

producing a single-phase converter (which are both cost-effective

and thermally viable), have forced a change to the cost-saving

approach of multiphase. The ISL6334D controller helps reduce

the complexity of implementation by integrating vital functions

and requiring minimal output components. Figures 1, 2 and 3

provide top level views of multiphase power conversion using the

ISL6334D controller.

Interleaving

The switching of each channel in a multiphase converter is timed

to be symmetrically out-of-phase with each of the other channels.

In a 3-phase converter, each channel switches 1/3 cycle after the

previous channel and 1/3 cycle before the following channel. As

a result, the 3-phase converter has a combined ripple frequency

3x greater than the ripple frequency of any one phase. In

addition, the peak-to-peak amplitude of the combined inductor

currents is reduced in proportion to the number of phases (see

Equations 1 and 2). Increased ripple frequency and lower ripple

amplitude mean that the designer can use less per-channel

inductance and lower total output capacitance for any

performance specification.

Figure 4 illustrates the multiplicative effect on output ripple

frequency. The three channel currents (IL1, IL2 and IL3) combine

to form the AC ripple current and the DC load current. The ripple

component has 3x the ripple frequency of each individual

channel current. Each PWM pulse is terminated 1/3 of a cycle

after the PWM pulse of the previous phase. The DC components of

the inductor currents combine to feed the load.

To understand the reduction of ripple current amplitude in the

multiphase circuit, examine Equation 1, which represents an

individual channel’s peak-to-peak inductor current.

In Equation 1, VIN and VOUT are the input and output voltages

respectively, L is the single-channel inductor value, and fSW is the

switching frequency.

The output capacitors conduct the ripple component of the

inductor current. In the case of multiphase converters, the

capacitor current is the sum of the ripple currents from each of

the individual channels. Compare Equation 1 to the expression

FIGURE 4. PWM AND INDUCTOR-CURRENT WAVEFORMS FOR

3-PHASE CONVERTER

1µs/DIV

PWM2, 5V/DIV

PWM3, 5V/DIV

IL2, 7A/DIV

IL3, 7A/DIV

IL1 + IL2 + IL3, 7A/DIV

IL1, 7A/DIV

PWM1, 5V/DIV

IP-P

VIN VOUT

–

 V

OUT

LfSW VIN

------------------------------------------------------

(EQ. 1)

FIGURE 5. CHANNEL INPUT CURRENTS AND INPUT-CAPACITOR

RMS CURRENT FOR 3-PHASE CONVERTER

CHANNEL 3

INPUT CURRENT

10A/DIV

CHANNEL 2

INPUT CURRENT

10A/DIV

CHANNEL 1

INPUT CURRENT

10A/DIV

INPUT-CAPACITOR CURRENT, 10A/DIV

1µs/DIV

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