ISL6559 Datasheet, PDF (16/21 Pages) Intersil Corporation – Multi-Phase PWM Controller

ISL6559
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_FB
2----π_{----f--0---V-----p---p--------L---C---}
0.75 V_{I N}
C_C
=
--------0---.--7---5----V----^I--^N----------
2πV_PPR_FBf₀
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. 23)
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_PPR_FBf₀ L
In Equations 23, 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
Figure 4 and Electrical Specifications.
Once selected, the compensation values in Equations 23
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
Equations 23 unless some performance issue is noted.
The optional capacitor C₂, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 12). 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.
C₂
R_C ^CC
COMP
C₁
R₁
R_FB
FB
IOUT
VDIFF
FIGURE 13. COMPENSATION CIRCUIT FOR ISL6559 BASED
CONVERTER WITHOUT LOAD-LINE
REGULATION.
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 13, 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 chose 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.
In the solutions to the compensation equations, there is a single
degree of freedom. For the solutions presented in Equations
24, R_FB is selected arbitrarily. The remaining compensation
components are then selected according to Equations 24.
R₁
=
R_F
B
----------C-----⁽--E----S-----R-----⁾---------
LC – C(ESR)
C₁
=
-----L----C-----^–-----C-----⁽--E-----S----R-----⁾
R_FB
C₂
=
-----------------------0----.-7----5---V-----^I-^N-------------------------
(2π)²f₀f_HF LCR_FBV_PP
R_C
=
---V-----^P----^P----^--2----^π---^---²----f--⁰---f--^H----^F----L----C----R-----^F----^B----
0.75 V_{IN }2πf_HF LC–1_
C_C
=
--0----.-7----5----V----^I--^N----^-2----^π----f--^H----^F--------L----C-----^–---1---^---
(2π)²f₀f_HF LCR_FBV_PP
(EQ. 24)
16
FN9084.8
December 29, 2004