ISL6569 Datasheet, PDF (18/22 Pages) Intersil Corporation – Multi-Phase PWM Controller

ISL6569
less than 50%. Nevertheless, both inequalities should be
evaluated, and L should be selected based on the lower of
the two results. In each equation, L is the per-channel
inductance, and C is the total output capacitance.
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 effected 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
IOUT
VDIFF
FIGURE 13. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6569 CIRCUIT
The feedback resistor, R_FB, has already been chosen as
outlined in Load-Line Regulation Resistor. 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 which follow, there is 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 = _2----π----0V---.-_P-7---5_P----VR----^I-_F-^N--_B----f--₀--
Case 2:
---------1----------
2π LC
≤
f₀
<
2----π^----C-----(^{-1-E-----S----R-----})
R_C
=
R_FB
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_PPR_FB LC
(EQ. 27)
Case 3:
f₀ > _2----π----C-----₍-1-E-----S----R-----₎
R_C = R_{FB 0----.--7--2-5---}^π--V--f--^0I--_N-V----₍-^p-E--^p---SL----R-----₎
C_C
=
-0---.--7---5----V----^I--^N----⁽--E-----S----R-----⁾-------C---
2πV_PPR_FBf₀ L
In Equations 27, L is the per-channel filter inductance
divided by 2 (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
Figure 5 and Electrical Specifications.
Once selected, the compensation values in Equations 27
assure a stable converter with reasonable transient perfor-
mance. 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
Equations 27 unless some performance issue is noted.
The optional capacitor C₂, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 13). Keep
a position available for C₂, and be prepared to install a high-
frequency capacitor of between 22pF and 150pF in case any
trailing edge jitter problem 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 14, provides the necessary
compensation.
18
FN9085.7
December 29, 2004