ISL6324A Datasheet, PDF (33/39 Pages) Intersil Corporation – Monolithic Dual PWM Hybrid Controller Powering AMD SVI Split-Plane and PVI Uniplane Processors

ISL6324A
R_FB in this situation, please refer to “Compensation Without
Loadline Regulation” on page 33.
Compensation With Loadline Regulation
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.
C₂ (OPTIONAL)
R_C ^CC
COMP
R_FB
FB
ISL6324A
VSEN
FIGURE 23. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6324A CIRCUIT
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.
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 following three, there is a separate set of
equations for the compensation components.
In Equation 49, 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_P-P is the
peak-to-peak sawtooth signal amplitude as described in the
“Electrical Specifications” table on page 6.
Once selected, the compensation values in Equation 49
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 49 unless some performance issue is noted.
The optional capacitor C₂, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 23). 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.
.
Case 1:
---------------1----------------
2⋅π⋅ L⋅C
>
f₀
R_C = R_FB ⋅ ^2-----â‹---^π-----^â‹---f-0-⁰--.--6^â‹---6V-----^P_â‹---V^---^P--_I--_N-^â‹-------L-----^â‹---C---
C_C
=
---------------0---.--6---6-----^â‹---V----^I--^N----------------
2 ⋅ π ⋅ V_P-P ⋅ R_FB ⋅ f₀
Case 2:
---------------1----------------
2⋅π⋅ L⋅C
≤
f₀
<
------------------1-------------------
2 ⋅ π ⋅ C ⋅ ESR
R_C = R_FB ⋅ -V----^P----^P-----^â‹---⁽--20----.-^â‹_6---^π-6---⁾--_⋲----V^â‹----_I-f-_N0--²-----^â‹---L-----^â‹---C---
C_C
=
-------------------------------0---.--6---6-----^â‹---V-----^I-^N---------------------------------
(2 ⋅ π)² ⋅ f₀₂ ⋅ V_P-P ⋅ R_FB ⋅ L ⋅ C
(EQ. 49)
Case 3:
f₀
>
------------------1-------------------
2 ⋅ π ⋅ C ⋅ ESR
R_C
=
R_FB
⋅
2-----⋅_---π_------⋅_---f--0----⋅_{---V----P-------P-----}⋅_---L-
0.66 ⋅ V_IN ⋅ ESR
C_C
=
-----0----.-6----6----^â‹---V-----^I-^N------^â‹---E----S-----R-----^â‹--------C-------
2 ⋅ π ⋅ V_P-P ⋅ R_FB ⋅ f₀ ⋅ L
Compensation Without Loadline 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 24, provides the
necessary compensation.
The first step is to choose the desired bandwidth, f₀, of the
compensated system. Choose a frequency high enough to
assure adequate transient performance but not higher than 1/3
of the switching frequency. The type-III compensator has an
extra high-frequency pole, f_HF. This pole can be used for added
noise rejection or to assure adequate attenuation at the error
amplifier high-order pole and zero frequencies. A good general
rule is to choose f_HF = 10f₀, but it can be higher if desired.
Choosing f_HF to be lower than 10f₀ can cause problems with
too much phase shift below the system bandwidth as shown in
Equation 50.
33
FN6880.1
April 29, 2010