RT8802A
Consequently, the sensing current IX is proportional to
inductor current ILX and is expressed as :
PWM4
VCORE
ILX ×DCRx
RCSNX
IX =
The sensed current IX is used for current balance and droop
tuning as described as followed. Since all phases share
one common GM, GM offset and linearity variation effect
is eliminated in practical applications. As sub-milli-ohm-
grade inductors are widely used in modern mother boards,
slight mismatch ofGM amplifiers offset and linearity results
in considerable current shift between phases. The time-
sharing DCR current sensing technical is extremely
important to guarantee phase current balance in mass
production.
VID7
Figure 2. Vcore Response at VID on the Fly
DCR Current Sensing
Phase Current Balance
RT8802Aadopts an innovative time-sharingDCR current
sensing technique to sense the phase currents for phase
current balance (phase thermal balance) and load line
regulation as shown in Figure 3. Current sensing amplifier
GM samples and holds voltages VCx across the current
sensing capacitor Cx by turns in a switching cycle.
According to the Basic Circuit Theory, if
The sampled and held phase current IX are summed and
averaged to get the averaged current
. Each phase
I
X
current IX then is compared with the averaged current.
The difference between IX and is injected to
I
X
corresponding PWM comparator. If phase current IX is
smaller than the averaged current , RT8802A increases
the duty cycle of corresponding phase to increase the
phase current accordingly and vice versa.
Lx
= Rx×Cx then VCx = I ×DCRx
LX
DCRx
T1
T2
T3
T4
L1
DCR1
R1
C1
+ VC1 -
L3
DCR3
I
= I x DCR /R
X
LX
X
CSNX
ISP1
ISN1
T1
T3
+
CSA
-
I
S/H CKT
X
R3
C3
+ VC3 -
T1
T3
R
CSN1
ISP3
CSA: Current Sense Amplifier
ISN35
R
CSN3
L2
DCR2
R2
C2
+ VC2 -
L4
DCR4
ISP2
ISN24
ISP4
T2
T4
T2 or T4
R
CSN24
R4
C4
+ VC4 -
Figure 3
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22
DS8802A-04 August 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
