ISL6267 Datasheet, PDF (26/33 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion Mobile CPUs

ISL6267
should be determined through tuning the load transient response
waveforms on an actual board.
R_ip and C_ip form an R-C branch in parallel with R_i, providing a
lower impedance path than R_i at the beginning of i_o change. R_ip
and C_ip do not have any effect at steady state. Through proper
selection of R_ip and C_ip values, i_droop can resemble i_o rather than
i_L, and V_o will not ring back. The recommended value for R_ip is
100Ω. C_ip should be determined through tuning the load
transient response waveforms on an actual board. The
recommended range for C_ip is 100pF~2000pF. However, it
should be noted that the R_ip -C_ip branch may distort the i_droop
waveform. Instead of being triangular as the real inductor
current, i_droop may have sharp spikes, which may adversely
affect i_droop average value detection and therefore may affect
OCP accuracy. User discretion is advised.
Resistor Current-Sensing Network
PHASE1 PHASE2 PHASE3
L
L
L
DCR
DCR
DCR
RSEN RSEN RSEN
RSUM
RSUM
RSUM
RO
RO
RO
VCN⁺ CN
-
RI
ISUM+
ISUM-
IO
FIGURE 26. RESISTOR CURRENT-SENSING NETWORK
Figure 26 shows the resistor current-sensing network for a
2-phase solution. Each inductor has a series current sensing
resistor, R_sen. R_sum and R_o are connected to the R_sen pads to
accurately capture the inductor current information. The R_sum
and R_o resistors are connected to capacitor C_n. R_sum and C_n
form a filter for noise attenuation. Equations 24 thru 26 give the
V_Cn(s) expression.
V_Cn(s)
=
-^R----^s--^e----ⁿ-
N
×
I_o(s)
×
A_Rsen(s)
(EQ. 24)
A_Rsen(s)
=
----------¹------------
1
+
------^s------
ω_sns
(EQ. 25)
ω_Rsen
=
-------------¹--------------
-^R----^s--^u----^m---
N
×
C_n
(EQ. 26)
Transfer function A_Rsen(s) always has unity gain at DC. Current-
sensing resistor R_sen value does not have significant variation
over-temperature, so there is no need for the NTC network.
The recommended values are R_sum = 1kΩ and C_n = 5600pF.
Overcurrent Protection
Refer to Equation 1 on page 20 and Figures 20, 24 and 26;
resistor R_i sets the droop current, I_droop. Tables 6 and 7 show the
internal OCP threshold. It is recommended to design I_droop
without using the R_comp resistor.
For example, the OCP threshold is 60µA for 3-phase solution.
I_droop is designed to be 40.9µA at full load, so the OCP trip level
is 1.5x of the full load current.
For inductor DCR sensing, Equation 27 gives the DC relationship
of V_cn(s) and I_o(s):
⎛
⎞
V_Cn
=
⎜
⎜
⎜
⎝
-----------^R----ⁿ----^t--^c--ⁿ----^e---^t-----------
R_ntcnet
+
R_{-----s--u----m---}
N
×
D_-----NC----R_{--⎟⎟⎟}
⎠
× I_o
(EQ. 27)
Substitution of Equation 27 into Equation 1 gives Equation 28:
I_droop
=
-²---
R_i
×
-----------^R----ⁿ---^t---^c--ⁿ----^e---^t-----------
R_n
t
c
net
+
-^R----^s--^u----^m---
N
×
D_-----C_----R_--
N
×
I_o
(EQ. 28)
Therefore:
R_i
=
--------------²----^R----ⁿ---^t--^c---ⁿ---^e---^t---^×-----^D-----^C----^R-----^×-----^I--^o--------------
N
×
⎛
⎝
R_n
t
c
net
+
-^R----^s-_N-^u----^m---_⎠⎞
×
I_droop
(EQ. 29)
Substitution of Equation 19 and application of the OCP condition
in Equation 29 gives Equation 30:
R_i
=
-----------²-----^×-----^(-----RR--------ⁿⁿ------^tt-----^cc-----ss--------++----------^RR--------ⁿⁿ------^tt-----^cc-----)---+-----^×----^R------^R--^p------^p-----^×-----^D-----^C---^R------^×-----^I-^o----^m-----^a---^x-----------
N
×
⎛
⎜
⎝
(_---R_{----n---t--c---s----+-----}R_{----n---t--c---})_----×_-----R_----p-
R_ntcs + R_ntc + R_p
+
-^R----^s-_N-^u----^m---_{⎠⎟⎞}
×
I_droopmax
(EQ. 30)
where I_omax is the full load current and I_droopmax is the
corresponding droop current. For example, given N = 3,
R_sum = 3.65kΩ, R_p = 11kΩ, R_ntcs = 2.61kΩ, R_ntc = 10kΩ,
DCR = 0.88mΩ, I_omax = 51A and I_droopmax = 40.9µA.
Equation 30 gives R_i = 606Ω.
For resistor sensing, Equation 31 gives the DC relationship of
V_cn(s) and I_o(s).
V_Cn
=
-^R----^s--^e----ⁿ-
N
×
I_o
(EQ. 31)
Substitution of Equation 31 into Equation 1 gives Equation 32:
I_droop
=
-²---
R_i
×
-^R----^s--^e----ⁿ-
N
×
I_o
(EQ. 32)
Therefore:
R_i
=
-²---^R-----^s--^e---ⁿ-----^×-----^I-^o--
N × I_droop
(EQ. 33)
Substitution of Equation 33 and application of the OCP condition
in Equation 29 gives Equation 34:
R_i = -^2N---^R--_×---^s--_I^e--_d-ⁿ--_r--_o-^×---_o--^I-_po---_m-^m----_a-^a--_x-^x-
(EQ. 34)
where I_omax is the full load current and I_droopmax is the
corresponding droop current. For example, given N = 3,
R_sen = 1mΩ, I_omax = 51A and I_droopmax = 40.9µA, Equation 34
gives R_i = 831Ω.
26
January 31, 2011
FN7801.0