ISL6312_07 Datasheet, PDF(12/35 Page) Intersil Corporation – Four-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR10, VR11, and AMD Applications

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ISL6312_07 Datasheet, PDF (12/35 Pages) Intersil Corporation – Four-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR10, VR11, and AMD Applications

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ISL6312

In order to realize the thermal advantage, it is important that

each channel in a multiphase converter be controlled to

carry equal amounts of current at any load level. To achieve

this, the currents through each channel must be sampled

every switching cycle. The sampled currents, In, from each

active channel are summed together and divided by the

number of active channels. The resulting cycle average

current, IAVG, provides a measure of the total load-current

demand on the converter during each switching cycle.

Channel-current balance is achieved by comparing the

sampled current of each channel to the cycle average

current, and making the proper adjustment to each channel

pulse width based on the error. Intersilâs patented current-

balance method is illustrated in Figure 3, with error

correction for channel 1 represented. In the figure, the cycle

average current, IAVG, is compared with the channel 1

sample, I1, to create an error signal IER.

VCOMP

FILTER f(s)

MODULATOR

RAMP

WAVEFORM

PWM1

IER

IAVG

Ã·N

Î£

TO GATE

CONTROL

LOGIC

NOTE: Channel 3 and 4 are optional.

FIGURE 3. CHANNEL-1 PWM FUNCTION AND CURRENT-

BALANCE ADJUSTMENT

The filtered error signal modifies the pulse width

commanded by VCOMP to correct any unbalance and force

IER toward zero. The same method for error signal

correction is applied to each active channel.

PWM

SWITCHING PERIOD

ISEN

TIME

FIGURE 4. CONTINUOUS CURRENT SAMPLING

Continuous Current Sampling

In order to realize proper current-balance, the currents in

each channel are sensed continuously every switching

cycle. During this time the current-sense amplifier uses the

ISEN inputs to reproduce a signal proportional to the

inductor current, IL. This sensed current, ISEN, is simply a

scaled version of the inductor current.

The ISL6312 supports inductor DCR current sensing to

continuously sense each channelâs current for channel-

current balance. The internal circuitry, shown in Figure 5

represents channel n of an N-channel converter. This

circuitry is repeated for each channel in the converter, but

may not be active depending on how many channels are

operating.

MOSFET

DRIVER

VIN

UGATE(n)

LGATE(n)

DCR

INDUCTOR

VL(s)

VC(s)

VOUT

COUT

ISL6312 INTERNAL CIRCUIT

R2*

SAMPLE

ISEN

VC(s)

RISEN

ISEN-(n)

ISEN+(n)

*R2 is OPTIONAL

FIGURE 5. INDUCTOR DCR CURRENT SENSING

CONFIGURATION

Inductor windings have a characteristic distributed

resistance or DCR (Direct Current Resistance). For

simplicity, the inductor DCR is considered as a separate

lumped quantity, as shown in Figure 5. The channel current

IL, flowing through the inductor, passes through the DCR.

Equation 3 shows the s-domain equivalent voltage, VL,

across the inductor.

VL(s) = IL â (s â L + DCR)

(EQ. 3)

A simple R-C network across the inductor (R1 and C)

extracts the DCR voltage, as shown in Figure 5. The voltage

across the sense capacitor, VC, can be shown to be

proportional to the channel current IL, shown in Equation 4.

FN9289.3

February 14, 2007

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