ISL6327A Datasheet, PDF (25/29 Pages) Intersil Corporation – Enhanced 6-Phase PWM Controller with 8-Bit VID Code and Differential Inductor DCR or Resistor Current Sensing

ISL6327A
Compensation
The two opposing goals of compensating the voltage
regulator are stability and speed. Depending on whether the
regulator employs the optional load-line regulation as
described in Load-Line Regulation, there are two distinct
methods for achieving these goals.
COMPENSATING LOAD-LINE REGULATED
CONVERTER
The load-line regulated converter behaves in a similar
manner to a peak-current mode controller because the two
poles at the output-filter L-C resonant frequency split with
the introduction of current information into the control loop.
The final location of these poles is determined by the system
function, the gain of the current signal, and the value of the
compensation components, R_C and C_C.
Since the system poles and zero are affected by the values
of the components that are meant to compensate them, the
solution to the system equation becomes fairly complicated.
Fortunately there is a simple approximation that comes very
close to an optimal solution. Treating the system as though it
were a voltage-mode regulator by compensating the L-C
poles and the ESR zero of the voltage-mode approximation
yields a solution that is always stable with very close to ideal
transient performance.
.
C₂ (OPTIONAL)
R_C ^CC
COMP
R_FB
+
V_DROOP
-
FB
IDROOP
VDIFF
FIGURE 17. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6327A CIRCUIT
The feedback resistor, R_FB, has already been chosen as
outlined in “Load-Line Regulation Resistor” on page 24.
Select a target bandwidth for the compensated system, f₀.
The target bandwidth must be large enough to assure
adequate transient performance, but smaller than 1/3 of the
per-channel switching frequency. The values of the
compensation components depend on the relationships of f₀
to the L-C pole frequency and the ESR zero frequency. For
each of the three cases in Equation 33, there are a separate
set of equations for the compensation components.
Case 1:
---------1----------
2π LC
>
f₀
R_C
=
R_F
B
2----π_{----f--0---V-----P------P--------L----C---}
0.75 V _{I N}
C_C
=
--------0----.-7----5----V----^I--^N----------
2 π V_P-P R_{F B} f₀
Case 2:
---------1----------
2π LC
≤
f₀
<
--------------1---------------
2πC(ESR)
R_C
=
R_F
B
V-----_P------P----(_--2----π_----)_--²_-----f--0--²_----L---C---
0.75 V_IN
C_C
=
---------------------0---.--7---5----V----^I--^N-----------------------
(2π)² f₀₂ V_P-PR_FB LC
(EQ. 33)
Case 3:
f₀ > _2----π----C-----₍-1-E-----S----R-----₎
R_C
=
R_FB
-------2---^π-----f--⁰---V----^P----^---^P----L-------
0.75 V_IN (ESR)
C_C
=
0----.--7---5----V----_I--N----(_{--E-----S----R-----})_-------C---
2πV_P-PR_FBf₀ L
In Equation 33, L is the per-channel filter inductance divided
by the number of active channels; C is the sum total of all
output capacitors; ESR is the equivalent-series resistance of
the bulk output-filter capacitance; and V_PP is the peak-to-
peak sawtooth signal amplitude as described in “Electrical
Specifications” on page 6.
The optional capacitor C₂, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 18). Keep
a position available for C₂, and be prepared to install a high
frequency capacitor of between 22pF and 150pF in case any
leading-edge jitter problem is noted.
Once selected, the compensation values in Equation 33
assure a stable converter with reasonable transient
performance. In most cases, transient performance can be
improved by making adjustments to R_C. Slowly increase the
value of R_C while observing the transient performance on an
oscilloscope until no further improvement is noted. Normally,
C_C will not need adjustment. Keep the value of C_C from
Equation 33 unless some performance issue is noted.
COMPENSATION WITHOUT LOAD-LINE REGULATION
The non load-line regulated converter is accurately modeled
as a voltage-mode regulator with two poles at the L-C
resonant frequency and a zero at the ESR frequency. A
type III controller, as shown in Figure 18, provides the
necessary compensation.
25
FN6833.0
February 17, 2009