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RT8802A Datasheet(PDF) 22 Page - Richtek Technology Corporation

Part # RT8802A
Description  2/3/4/5-Phase PWM Controller for High-Density Power Supply
PDF  28 Pages
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Manufacturer  RICHTEK [Richtek Technology Corporation]
Direct Link  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT8802A Datasheet(HTML) 22 Page - Richtek Technology Corporation

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RT8802A
22
DS8802A-04 August 2007
www.richtek.com
All brandname or trademark belong to their owner respectively
Consequently, the sensing current IX is proportional to
inductor current ILX and is expressed as :
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 of GM 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.
Phase Current Balance
The sampled and held phase current IX are summed and
averaged to get the averaged current
. Each phase
current IX then is compared with the averaged current.
The difference between IX and
is injected to
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.
X
I
X
I
Figure 2. Vcore Response at VID on the Fly
VCORE
VID7
PWM4
DCR Current Sensing
RT8802A adopts an innovative time-sharing DCR 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
Figure 3
DCRx
I
VCx
then
Cx
Rx
DCRx
Lx
LX ×
=
×
=
CSNX
LX
X
R
DCRx
I
I
×
=
CSA
R
CSN1
R
CSN3
R
CSN24
T1
T3
T2 or T4
T1
T3
T2
T4
CSA: Current Sense Amplifier
I
X
I
X
= I
LX
x DCR
X
/R
CSNX
T1
T2
T3
T4
ISP1
ISN1
ISP3
ISN35
ISP2
ISP4
ISN24
S/H CKT
+
-
L1
DCR1
L2
DCR2
L3
DCR3
L4
DCR4
R1
C1
+ VC1 -
R3
C3
+ VC3 -
R2
C2
+ VC2 -
R4
C4
+ VC4 -



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