ISL6557A Datasheet, PDF (15/19 Pages) Intersil Corporation – Multi-Phase PWM Controller for Core-Voltage Regulation

ISL6557A
The feedback resistor, R_FB, has already been chosen as out-
lined in Load-Line Regulation Resistor. Select a target band-
width for the compensated system, f₀. The target bandwidth
must be large enough to assure adequate transient perfor-
mance, but smaller than 1/3 of the per-channel switching fre-
quency. 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
defined below, there is a separate set of equations for the
compensation components.
Case 1:
Case 2:
Case 3:
---------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
---------¹----------
2π LC
≤
f₀
<
-₂---_π----_C-----₍-¹-_E-----_S----_R-----₎
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_PPR_FB LC
f₀ > ₂----_π----_C-----₍-¹-_E-----_S----_R-----₎
R_C
=
R_F
B
--------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
(EQ. 20)
In Equations 20, 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 5 and Electrical Specifications.
Once selected, the compensation values in Equations 20
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 20 unless some performance issue is noted.
The optional capacitor C₂, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 5). Keep
a position available for C₂, and be prepared to install a high-
frequency capacitor of between 22pF and 150pF in case any
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 13, provides the necessary
compensation.
C₂
R_C ^CC
COMP
C₁
+
R₁
R_FB
V_DROOP
-
FB
IOUT
VDIFF
FIGURE 13. COMPENSATION CIRCUIT FOR ISL6557A BASED
CONVERTER WITHOUT LOAD-LINE
REGULATION
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 = 10 f₀, but it can be higher
if desired. Choosing f_HF to be lower than 10 f₀ 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 21, R_FB is selected arbitrarily. The remaining
compensation components are then selected according to
Equations 21.
R₁
=
R_FB
----------^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
=
-^R-----^F----^B---^V-----^P----^P----^--²----^π---^---²----^f--⁰---^f--^H----^F----^L----^C---
0.75V_{IN}2πf_HF LC –1_
(EQ. 21)
C_C
=
-⁰---^.--⁷---⁵----^V----^I--^N----^---²----^π----^f--^H----^F--------^L----^C-----–-----¹---^--
^{2π 2V}PP^RFB^{f0fHF LC}
15