ISL62882_14 Datasheet, PDF (24/42 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62882, ISL62882B
where I_omax is the full load current, I_droopmax is the
corresponding droop current. For example, given N = 2,
R_sen = 1mΩ, I_omax = 51A and I_droopmax = 34.3µA, Equation 30
gives R_i = 1.487kΩ.
A resistor from COMP to GND can adjust the internal OCP
threshold, providing another dimension of fine-tune flexibility.
Table 4 shows the detail. It is recommended to scale I_droop such
that the default OCP threshold gives approximately the desired
OCP level, then use R_comp to fine tune the OCP level if necessary.
Load Line Slope
Refer to Figure 12.
For inductor DCR sensing, substitution of Equation 24 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_n
t
c
net
+
R_{-----s--u----m---}
N
N
(EQ. 31)
For resistor sensing, substitution of Equation 28 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. 32)
Substitution of Equation 25 and rewriting Equation 31, or
substitution of Equation 29 and rewriting Equation 32 give the
same result in Equation 33:
R_droop
=
------^I--^o------- × LL
I_droop
(EQ. 33)
One can use the full load condition to calculate R_droop. For
example, given I_omax = 51A, I_droopmax = 34.3µA and
LL = 1.9mΩ, Equation 33 gives R_droop = 2.825kΩ.
It is recommended to start with the R_droop value calculated by
Equation 33, 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.
Current Monitor
Refer to Equation 13 for the IMON pin current expression.
Refer to Figures 1 and 2, the IMON pin current flows through
R_imon. The voltage across R_imon is expressed in Equation 34:
V_Rimon = 3 × I_droop × R_imon
(EQ. 34)
Rewriting Equation 33 gives Equation 35:
I_droop
=
--------^I-^o--------- × LL
R_droop
(EQ. 35)
Substitution of Equation 35 into Equation 34 gives Equation 36:
V_Rimon
=
3_----I_--o-----×_----L_----L_-
R_droop
×
R_imon
(EQ. 36)
Rewriting Equation 36 and application of full load condition gives
Equation 37:
R_imon = ^V----^R----ⁱ-^m---₃--^o--_I--^no----^××-----^RL----_L^d---^r--^o---^o----^p-
(EQ. 37)
For example, given LL = 1.9mΩ, R_droop = 2.825kΩ,
V_Rimon = 963mV at I_omax = 51A, Equation 37 gives
R_imon = 9.358kΩ.
A capacitor C_imon can be paralleled with R_imon to filter the IMON
pin voltage. The R_imonC_imon time constant is the user’s choice. It
is recommended to have a time constant long enough such that
switching frequency ripples are removed.
Compensator
Figure 18 shows the desired load transient response waveforms.
Figure 24 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 24. 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 27 shows a screenshot of the spreadsheet.
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 25 conceptually
shows T1(s) measurement set-up and Figure 26 conceptually
shows T2(s) measurement set-up. The VR senses the inductor
current, multiplies it by a gain of the load line slope, then adds it
on top of the sensed output voltage and feeds it to the
compensator. T(1) is measured after the summing node, and T2(s)
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 be actually measured on an ISL62882 regulator.
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) and has
more meaning of system stability.
T2(s) is the voltage loop gain with closed droop loop. It has more
meaning of output voltage response.
Design the compensator to get stable T1(s) and T2(s) with
sufficient phase margin, and output impedance equal or smaller
than the load line slope.
24
FN6890.4
June 21, 2011