ISL6219 Datasheet, PDF (14/17 Pages) Intersil Corporation – Microprocessor CORE Voltage Regulator Precision Multi-Phase BUCK PWM Controller for Mobile Applications

ISL6219
C₂ (OPTIONAL)
R_C ^CC
COMP
R_FB
+
V_DROOP
-
FB
VSEN
FIGURE 11. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6219 CIRCUIT
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
<
--------------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_PPR_FB LC
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
(EQ. 17)
In Equations 17, 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 17
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 17 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 12, provides the necessary
compensation.
C₂
R_C ^CC
COMP
C₁
+
R₁
R_FB
V_DROOP
-
FB
VSEN
FIGURE 12. COMPENSATION CIRCUIT FOR ISL6219 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 18, R_FB is selected arbitrarily. The remaining
compensation components are then selected according to
Equations 18.
C₁
=
-----^L---^C------–-----^C-----⁽--^E-----^S----^R-----⁾
R_FB
R_C = ----------^V----^P----^P----^---²----^π---^---²---^f--⁰----^f--^H----^F----^L---^C------------
0.75 V_IN
2
π
f_H
F


L C –1_
14