ISL6561 Datasheet, PDF (21/26 Pages) Intersil Corporation – Multi-Phase PWM Controller with Precision Rds(on) or DCR Differential Current Sensing for VR10.X Application

ISL6561
In Equation 22, make sure that ∆T₂ is the desired temperature
rise above the ambient temperature, and ∆T₁ is the measured
temperature rise above the ambient temperature. While a
single adjustment according to Equation 22 is usually
sufficient, it may occasionally be necessary to adjust R_ISEN
two or more times to achieve optimal thermal balance
between all channels.
Load-Line Regulation Resistor
The load-line regulation resistor is labeled R_FB in Figure 7.
Its value depends on the desired full-load droop voltage
(V_DROOP in Figure 7). If Equation 21 is used to select each
ISEN resistor, the load-line regulation resistor is as shown
in Equation 23.
R_FB
=
-V----^D----^R----^O-----^O----^P--
70 ×10^–6
(EQ. 23)
If one or more of the ISEN resistors is adjusted for thermal
balance, as in Equation 23, the load-line regulation resistor
should be selected according to Equation 24 where I_FL is the
full-load operating current and R_ISEN(n) is the ISEN resistor
connected to the n^th ISEN pin.
∑ ^R_FB
=
-----V----^D----^R----^O-----^O----^P------
I_FL r_DS(ON)
R_ISEN(n)
n
(EQ. 24)
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
IDROOP
VDIFF
FIGURE 13. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6561 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
=
--------0---.--7---5----V----^I--^N----------
2πV_PPR_FBf₀
Case 2:
---------1----------
2π LC
≤
f₀
<
--------------1---------------
2πC(ESR)
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. 25)
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 25, 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 6 and Electrical Specifications.
21
FN9098.5
May 12, 2005