FAN5240 Просмотр технического описания (PDF) - Fairchild Semiconductor

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FAN5240

Multi-Phase PWM Controller for AMD Mobile Athlon™ and Duron™

Fairchild Semiconductor 

Current Processing Section

The following discussion refers to Figure 6.

Setting RSENSE

Each phase current is sampled about 200nS after the

SW node crosses 0V. For proper converter operation,

choose an RSENSE value of:

RSENSE

=

R-----D----S---(--O----N----)---•----I--M----A----X-

40µA

With Active Droop, the output voltage varies with the load

as if a resistor were connected in series with the con-

verter’s output, in other words, it's effect is to raise the

output resistance of the converter.

1.2

VDROOP

which is about 1K for the components in Figure 1.

Active Droop

The core converter incorporates a proprietary output

voltage droop method for optimum handling of fast load

transients found in modern processors.

“Active droop” or voltage positioning is now widely used

in the computer power applications. The technique is

based on raising the converter voltage at light load in

anticipation of a step increase in load current, and con-

versely, lowering VCORE in anticipation of a step

decrease in load current.

ILOAD

IMAX

Figure 5. Active Droop

To get the most from the Active Droop, its magnitude

should be scaled to match the output capacitor’s ESR

voltage drop.

VDROOP = IMAX × ESR

(6)

Active Droop allows the size and cost of the output

capacitors required to handle CPU current transients to

be reduced. The reduction may be almost a factor of 2

when compared to a system without Active Droop.

ISNS1-ISNS2

ISNS2

B-A

A ΣB

ISNS2-ISNS1

A-B

ISNS2

5

To A1 (+)

S/H

ISNS1

ISNS1

5

V to I

in +

ISNS1

8 in D

0.9V

ISNS1 RSENSE

LDRV1

PGND1

ILIM RILIM

ILIM det. 1

2.5V

I2 =

ILIM

ILIM mirror

Figure 6. Current Limit and Active Droop Circuits

10

www.fairchildsemi.com

FAN5240 Rev. 1.2.0

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