ISL6363_14 Datasheet, PDF (25/32 Pages) Intersil Corporation – Multiphase PWM Regulator for VR12™ Desktop CPUs

ISL6363
Overcurrent Protection
Refer to Equation 1 on page 16 and Figures 16, 20 and 22;
resistor R_i sets the droop current I_droop. Tables 2 (page 19)
and 3 (page 19) 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. We
will design I_droop 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 28 gives the DC relationship
of V_cn(s) and I_o(s).
⎛
⎞
V_Cn
=
⎜
⎜
⎜
⎝
-----------^R----ⁿ----^t--^c--ⁿ----^e---^t-----------
R_ntcn
e
t
+
-^R----^s--^u----^m---
N
×
D_-----NC----R_{--⎟⎟⎟}
⎠
× I_o
(EQ. 28)
Substitution of Equation 28 into Equation 1 gives Equation 29:
I_droop
=
-²---
R_i
×
-----------^R----ⁿ---^t---^c--ⁿ----^e---^t-----------
R_ntcnet
+
-^R----^s--^u----^m---
N
×
D_-----C_----R_--
N
×
I_o
(EQ. 29)
Therefore:
R_i
=
--------------²----^R----ⁿ---^t--^c---ⁿ---^e---^t----^×----^D-----^C----^R-----^×-----^I--^o--------------
N × _⎝⎛R_ntcnet + -^R----^s-_N-^u----^m---_⎠⎞ × I_droop
(EQ. 30)
Substitution of Equation 20 and application of the OCP condition
in Equation 30 gives Equation 31:
R_i
=
-----------²-----^×------⁽----^RR--------ⁿⁿ------^tt----^c-^c-----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. 31)
Where I_omax is the full load current, 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 31 gives R_i = 606Ω.
For resistor sensing, Equation 32 gives the DC relationship of
V_cn(s) and I_o(s).
V_Cn
=
-^R----^s--^e----ⁿ-
N
×
I_o
(EQ. 32)
Substitution of Equation 32 into Equation 1 gives Equation 33:
I_droop
=
-²---
R_i
×
-^R----^s--^e----ⁿ-
N
×
I_o
(EQ. 33)
Therefore
R_i
=
-²---^R-----^s--^e----ⁿ----^×-----^I--^o-
N × I_droop
(EQ. 34)
Substitution of Equation 34 and application of the OCP condition
in Equation 30 gives Equation 35:
R_i
=
-²---^R-----^s--^e----ⁿ----^×-----^I--^o---^m-----^a---^x-
N × I_droopmax
(EQ. 35)
Where I_omax is the full load current, I_droopmax is the corresponding
droop current. For example, given N = 3, R_sen = 1mΩ, I_omax = 53A
and I_droopmax = 40.9µA, Equation 35 gives R_i = 863Ω.
LOAD LINE SLOPE
Refer to Figure 9.
For inductor DCR sensing, substitution of Equation 29 into
Equation 2 gives the load line slope expression:
LL = -^V---^d---^r--^o----^o---^p- = -²---^R-----^d---^r--^o---^o---^p-- × -----------^R----ⁿ----^t--^c--ⁿ----^e---^t----------- × ^D-----^C----^R--
I_o
R_i
R_ntc
n
e
t
+
-^R----^s--^u----^m---
N
N
(EQ. 36)
For resistor sensing, substitution of Equation 33 into Equation 2
gives the load line slope expression:
LL
=
-^V---^d---^r--^o----^o---^p-
I_o
=
2_----R_{-----s--e----n----}×_-----R_{----d---r--o---o----p-}
N × R_i
(EQ. 37)
Substitution of Equation 30 and rewriting Equation 36, or
substitution of Equation 34 and rewriting Equation 37 give the
same result in Equation 38:
R_droop
=
------^I--^o------- × LL
I_droop
(EQ. 38)
One can use the full load condition to calculate R_droop. For
example, given I_omax = 51A, I_droopmax = 40.9µA and
LL = 1.9mΩ, Equation 38 gives R_droop = 2.37kΩ.
It is recommended to start with the R_droop value calculated by
Equation 38, and fine tune it on the actual board to get accurate
load line slope. One should record the output voltage readings at
no load and at full load for load line slope calculation. Reading
the output voltage at lighter load instead of full load will increase
the measurement error.
Compensator
Figure 17 shows the desired load transient response waveforms.
Figure 23 shows the equivalent circuit of a voltage regulator (VR)
with the droop function. A VR is equivalent to a voltage source
(= VID) and output impedance Z_out(s). If Z_out(s) is equal to the
load line slope LL, i.e., constant output impedance, in the entire
frequency range, V_O will have square response when I_o has a
square change.
Zout(s) = LL
IO
VID
VR
LOAD ^VO
FIGURE 23. VOLTAGE REGULATOR EQUIVALENT CIRCUIT
Intersil provides a Microsoft Excel-based spreadsheet to help
design the compensator and the current sensing network, so the
VR achieves constant output impedance as a stable system.
Figure 26 shows a screenshot of the spreadsheet.
A VR with an active droop function is a dual-loop system consisting
of a voltage loop and a droop loop which is a current loop.
However, neither loop alone is sufficient to describe the entire
system. The spreadsheet shows two loop gain transfer functions,
T1(s) and T2(s), that describe the entire system. Figure 24
conceptually shows T1(s) measurement set-up and Figure 25
25
FN6898.1
September 5, 2013