LX1677

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 11

AMD64 Processor VRM Controller

Copyright

© 2000

Rev 1.1, 2003-03-03

TM

®

TH EORY OF O PERATION (CONTINUED)

phase buck converter topology is that if multiple, identical,

inductors have the same identical voltage impressed across

their leads, they must then have the same identical current

passing through them. The current that we would like to

balance between inductors is mainly the DC component

along with as much as possible the transient current. All

inductors in a multiphase buck converter topology have

their output side tied together at the output filter capacitors.

Therefore this side of all the inductors has the same

identical voltage.

If the input side of the inductors can be forced to have

the same equivalent DC potential on this lead, then they

will have the same DC current flowing. To achieve this

requirement, phase 1 will be the control phase that sets the

output operating voltage, under normal PWM operation.

To force the current of phase 2 to be equal to the current of

phase 1; a second feedback loop is used. Phase 2 has a low

pass filter connected from the input side of each inductor.

This side of the inductors has a square wave signal that is

proportional to its duty cycle. The output of each LPF is a

DC

(+ some AC) signal that is proportional to the

magnitude and duty cycle of its respective inductor signal.

The second feedback loop will use the output of the

phase 1 LPF as a reference signal for an error amplifier

that will compare this reference to the output of the phase 2

LPF. This error signal will be amplified and used to

control the PWM circuit of phase 2. Therefore, the duty

cycle of phase 2 will be set so that the equivalent voltage

potential will be forced across the phase 2 inductor as

compared to the phase 1 inductor. This will force the

current in the phase 2 inductor to follow and equal the

phase 1 inductor current.

imbalance the phases so that one phase will supply more

current than the other under unique situations. The LX1677

will normally be used with the same supply voltages on

phase 1 and 2 PWM inputs and will have equal currents in

both phases.

LP2

LP1

Phase 1

Comparator

+

_

+

_

gm

Phase 2

Comparator

0°

180°

Phase 2

Integrating

Amp

Differential

Amp

+

-

DC Bias

Phase 1 Error

Amp

+

-

HO1

EA -

EO2

LO1

LO2

HO2

Phase 1

Low Pass Filter

Phase 2

Low Pass Filter

Vout

DC Bias

DC Bias

DACOUT

Differential Feedbach From Vout

Phase 1 Diff

Amp

Ramp .75 V to 3 V

Both PWMs

Figure 5 – LoadSHARE Control Loop