ISL6307A Datasheet, PDF(15/33 Page) Intersil Corporation – Ultra-high bandwidth 6-Phase PWM Controller with 8 Bit VID Code Capable of Precision RDS(ON) or DCR Differential Current Sensing

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ISL6307A Datasheet, PDF (15/33 Pages) Intersil Corporation – Ultra-high bandwidth 6-Phase PWM Controller with 8 Bit VID Code Capable of Precision RDS(ON) or DCR Differential Current Sensing

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ISL6307A

Resistive Sensing

For accurate current sense, a dedicated current-sense

resistor RSENSE in series with each output inductor can

serve as the current sense element (see Figure 5). This

technique is more accurate, but reduces overall converter

efficiency due to the additional power loss on the current

sense element RSENSE.

Equation 6 shows the ratio of the channel current to the

sensed current ISEN.

ISEN

R-----S----E----N----S----E--

RISEN

(EQ. 6)

RSENSE VOUT

COUT

ISL6307A INTERNAL CIRCUIT

RISEN(n)

SAMPLE

HOLD

ISEN-(n)

ISEN+(n)

ISEN

-R----S-----E----N-----S----E---

RISEN

FIGURE 5. SENSE RESISTOR IN SERIES WITH INDUCTORS

MOSFET RDS(ON) Sensing

The controller can also sense the channel load current by

sampling the voltage across the lower MOSFET RDS(ON)

(see Figure 6). The amplifier is ground-reference by

connecting the ISEN- pin to the source of the lower

MOSFET. ISEN+ pin is connected to the PHASE node

through the current sense resistor RISEN. The voltage

across RISEN is equivalent to the voltage drop across the

RDS(ON) of the lower MOSFET while it is conducting. The

resulting current out of the ISEN+ pin is proportional to the

channel current IL.

ISEN

-R----D-----S----(---O-----N-----)

RISEN

SAMPLE

HOLD

ISEN+(n)

RISEN

(PTC)

ISEN-(n)

VIN

ILxRDS(ON)

N-CHANNEL

MOSFETs

ISL6307A INTERNAL CIRCUIT EXTERNAL CIRCUIT

FIGURE 6. MOSFET RDS(ON) CURRENT-SENSING CIRCUIT

Equation 7 shows the ratio of the channel current to the

sensed current ISEN.

ISEN

R-----D----S----(--O-----N----)

RISEN

(EQ. 7)

Both inductor DCR and MOSFET RDS(ON) value will

increase as the temperature increases. Therefore the

sensed current will increase as the temperature of the

current sense element increases. In order to compensate

the temperature effect on the sensed current signal, a

Positive Temperature Coefficient (PTC) resistor can be

selected for the sense resistor RISEN, or the integrated

temperature compensation function of ISL6307A should be

utilized. The integrated temperature compensation function

is described in the Temperature Compensation section.

Channel-Current Balance

The sensed current In from each active channel are summed

together and divided by the number of active channels. The

resulting average current IAVG provides a measure of the

total load current. Channel current balance is achieved by

comparing the sampled current of each channel to the

average current to make an appropriate adjustment to the

PWM duty cycle of each channel. Intersilâs patented current-

balance method is illustrated in Figure 7. The average

current combines with the channel 1 current I1 to create an

error signal IER. 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.

FN9236.0

February 6, 2006

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