ISL6277A Datasheet, PDF (32/38 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion Mobile CPUs Using SVI 2.0

ISL6277A
DCR = 0.88mΩ, I_omax = 65A and I_droopmax = 45μA. Equation 30
gives R_i = 439Ω.
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 2 gives Equation 32:
I_droop
=
5_--
4
×
--¹---
R_i
×
R_{-----s---e---n--}
N
×
I_o
(EQ. 32)
Therefore:
R_i
=
5_--
4
×
N^--R----×^s---e--I^--nd---_r^-×--_o^---_o^I--o-_p^-
(EQ. 33)
Substitution of Equation 33 and application of the OCP condition
in Equation 29 gives Equation 34:
R_i
=
5_--
4
×
N^--R----s×^---e--I^--nd---_r^-×--_o^---_o^I--o-_p^--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 = 65A and I_droopmax = 45µA, Equation 34
gives R_i = 602Ω.
Load Line Slope
See Figure 14 for load-line implementation.
For inductor DCR sensing, substitution of Equation 28 into
Equation 3 gives the load-line slope expression:
LL
=
V_{-----d---r--o----o---p-}
I_o
=
5_--
4
×
-^R----^d---^r---^o---^o---^p-
R_i
×
-----------^R-----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_ntcnet
+
-^R----^s---^u---^m---
N
×
D_-----C_-----R_---
N
(EQ. 35)
For resistor sensing, substitution of Equation 32 into Equation 3
gives the load line slope expression:
LL
=
-^V----^d---^r--^o----^o---^p-
I_o
=
5_--
4
×
-^R----^s---^e---ⁿ-_N----^×--_×---^R--_R--^d--_i--^r--^o---^o---^p--
(EQ. 36)
Substitution of Equation 29 and rewriting Equation 35, or
substitution of Equation 33 and rewriting Equation 36, gives the
same result as in Equation 37:
R_droop
=
-------^I-^o--------
I_droop
×
LL
(EQ. 37)
One can use the full-load condition to calculate R_droop. For
example, given I_omax = 65A, I_droopmax = 45µA and LL = 2.1mΩ,
Equation 37 gives R_droop = 3.03kΩ.
It is recommended to start with the R_droop value calculated by
Equation 37 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 23 shows the desired load transient response waveforms.
Figure 29 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., a constant output impedance, then in the
entire frequency range, V_o will have a square response when I_o
has a square change.
Z_out(s) = LL
io
VID
VR
LOAD ^V_o
FIGURE 29. VOLTAGE REGULATOR EQUIVALENT CIRCUIT
Intersil provides a Microsoft Excel-based spreadsheet to help
design the compensator and the current sensing network so that
VR achieves constant output impedance as a stable system.
A VR with 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 30 conceptually
shows T1(s) measurement set-up, and Figure 31 conceptually
shows T2(s) measurement set-up. The VR senses the inductor
current, multiplies it by a gain of the load-line slope, adds it on top
of the sensed output voltage, and then feeds it to the
compensator. T1 is measured after the summing node, and T2 is
measured in the voltage loop before the summing node. The
spreadsheet gives both T1(s) and T2(s) plots. However, only T2(s)
can actually be measured on an ISL6277A regulator.
L
VO
Q1
VIN
GATE Q2
DRIVER
COUT
iO
LOAD LINE SLOPE
20Ω
MOD.
EA ^-
COMP
+
VID
CHANNEL B
LOOP GAIN = _{CHANNEL A}
+
+
ISOLATION
TRANSFORMER
CHANNEL A
NETWORK
ANALYZER
EXCITATION OUTPUT
CHANNEL B
FIGURE 30. LOOP GAIN T1(s) MEASUREMENT SET-UP
T1(s) is the total loop gain of the voltage loop and the droop loop.
It always has a higher crossover frequency than T2(s), therefore
has a higher impact on system stability.
T2(s) is the voltage loop gain with closed droop loop, thus having
a higher impact on output voltage response.
32
FN8322.0
December 19, 2012