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ISL6277 Datasheet(PDF) 18 Page - Renesas Technology Corp

Part No. ISL6277
Description  Multiphase PWM Regulator for AMD Fusion™ Mobile CPUs Using SVI 2.0
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Manufacturer  RENESAS [Renesas Technology Corp]
Direct Link  http://www.renesas.com
Logo RENESAS - Renesas Technology Corp

ISL6277 Datasheet(HTML) 18 Page - Renesas Technology Corp

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ISL6277
FN8270 Rev 1.00
Page 18 of 37
Mar 8, 2012
The ISL6277 will adjust the phase pulse-width relative to the
other phases to make VISEN1 =VISEN2 =VISEN3, thus to achieve
IL1 =IL2 =IL3, when Rdcr1 =Rdcr2 =Rdcr3 and
Rpcb1 =Rpcb2 =Rpcb3.
Using the same components for L1, L2 and L3 provides a good
match of Rdcr1, Rdcr2 and Rdcr3. Board layout determines Rpcb1,
Rpcb2 and Rpcb3. It is recommended to have a symmetrical
layout for the power delivery path between each inductor and the
output voltage rail, such that Rpcb1 =Rpcb2 =Rpcb3.
Sometimes, it is difficult to implement symmetrical layout. For
the circuit shown in Figure 15, asymmetric layout causes
different Rpcb1, Rpcb2 and Rpcb3 values, thus creating a current
imbalance. Figure 16 shows a differential sensing current
balancing circuit recommended for ISL6277. The current sensing
traces should be routed to the inductor pads so they only pick up
the inductor DCR voltage. Each ISEN pin sees the average voltage
of three sources: its own, phase inductor phase-node pad, and
the other two phase inductor output side pads. Equations 8
through 10 give the ISEN pin voltages:
The ISL6277 will make VISEN1 = VISEN2 = VISEN3 as shown in
Equations 11 and 12:
Rewriting Equation 11 gives Equation 13:
Rewriting Equation 12 gives Equation 14:
Combining Equations 13 and 14 gives:
Therefore:
Current balancing (IL1 =IL2 =IL3) is achieved when
Rdcr1 =Rdcr2 =Rdcr3. Rpcb1, Rpcb2 and Rpcb3 do not have any
effect.
Since the slave ripple capacitor voltages mimic the inductor
currents, the R3™ modulator can naturally achieve excellent
current balancing during steady state and dynamic operations.
Figure 17 shows the current balancing performance of the
evaluation board with load transient of 12A/51A at different rep
rates. The inductor currents follow the load current dynamic
change with the output capacitors supplying the difference. The
inductor currents can track the load current well at a low
repetition rate, but cannot keep up when the repetition rate gets
into the hundred-kHz range, where it is out of the control loop
bandwidth. The controller achieves excellent current balancing in
all cases installed.
FIGURE 16. DIFFERENTIAL-SENSING CURRENT BALANCING CIRCUIT
INTERNAL
TO IC
Vo
ISEN3
L3
Risen
Cisen
ISEN2
Risen
Cisen
ISEN1
Risen
Cisen
L2
L1
Rdcr3
Rdcr2
Rdcr1
PHASE3
PHASE2
PHASE1
IL3
IL2
IL1
Rpcb3
Rpcb2
Rpcb1
Risen
Risen
Risen
Risen
Risen
Risen
V3p
V3n
V2p
V2n
V1p
V1n
VISEN1
V1p V2n V3n
++
=
(EQ. 8)
VISEN2
V1n V2p V3n
++
=
(EQ. 9)
VISEN3
V1n V2n V3p
++
=
(EQ. 10)
V1p V2n V3n
++
V1n V2p V3n
++
=
(EQ. 11)
V1n V2p V3n
++
V1n V2n V3p
++
=
(EQ. 12)
V1p V1n
V2p V2n
=
(EQ. 13)
V2p V2n
V3p V3n
=
(EQ. 14)
V1p V1n
V2p V2n
V3p V3n
==
(EQ. 15)
Rdcr1 IL1
Rdcr2 IL2
Rdcr3 IL3
==
(EQ. 16)


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