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RT8800AGQV Datasheet(PDF) 11 Page - Richtek Technology Corporation

No. de pieza RT8800AGQV
Descripción Electrónicos  General Purpose 3-Phase PWM Controller for High-Density Power Supply
PDF  21 Pages
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Fabricante Electrónico  RICHTEK [Richtek Technology Corporation]
Página de inicio  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT8800AGQV Datasheet(HTML) 11 Page - Richtek Technology Corporation

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RT8800A
11
DS8800A-04 August 2007
www.richtek.com
All brand name or trademark belong to their owner respectively
Figure 2
COMP
VCORE
SSE_Internal
SS_Internal
Cross-over
VRAMP_Valley
Time-Sharing DCR Current Sensing
RT8800A adopts an innovative time-sharing DCR current
sensing technique to sense the phase currents for phase
current balance (phase thermal balance), over current
protection and load line regulation as shown in Figure 3.
The current sensing amplifier GM samples and holds
voltages VX across the current sensing capacitor CX by
turns in a switching cycle. According to the Basic Circuit
Theory, if
LX
LX
X
X
X
LX
X
R
I
V
then
C
R
R
L
×
=
×
=
Consequently, the sensing current IX is proportional to
inductor current ILX and is expressed as
The sensed current IX is used for current balance, over
current protection, and droop tuning as described as
followed. Since all phases share one common GM, GM
offset and linearity variation effect are eliminated in
practical applications. As sub-milli-ohmgrade inductors are
widely used in modern motherboards, slight mismatch of
GM amplifiers offset and linearity results in considerable
current shift between phases. The time sharingDCR current
sensing technical is extremely important to guarantee
phase current balance at mass production.
COMM
LX
LX
X
R
R
I
I
×
=
Figure 3
Figure 4 and 5 show the linearity of GM amplifier and its
test circuit respectively. A voltage source is applied to
ISPx while other ISPx pins are short to VOUT. The voltage
VADJ across resistor RADJ is measured. It is observed from
Figure 5 shows that all ISPx share the same
transconductance linearity and voltage offset.
Figure 4. The Linearity of GMx
VOUT
T1
T3
T2
IX
S/H CKT
+
-
LX
RLX
RCOMM
ILX
RX
CX
+ VX -
GM
0
10
20
30
40
50
60
70
0
20406080
100
VC (mV)
GM1
GM2
GM3
VX



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