ISL6312_07 Datasheet, PDF (29/35 Pages) Intersil Corporation – Four-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR10, VR11, and AMD Applications

ISL6312
3. Select new values, R_1,NEW and R_2,NEW, for the time
constant resistors based on the original values, R_1,OLD
and R_2,OLD, using Equations 36 and 37.
R_{1, NEW}
=
R_1,
O
L
D
⋅
Δ_----V_----1--
ΔV₂
(EQ. 36)
R_{2, NEW}
=
R_2,
O
L
D
⋅
Δ_----V_----1--
ΔV₂
(EQ. 37)
4. Replace R₁ and R₂ with the new values and check to see
that the error is corrected. Repeat the procedure if
necessary.
Loadline Regulation Resistor
If loadline regulation is desired, the IDROOP pin should be
shorted to the FB pin in order for the internal average
sense current to flow out across the loadline regulation
resistor, labeled R_FB in Figure 6. This resistor’s value sets
the desired loadline required for the application. The
desired loadline, R_LL, can be calculated by Equation 38
where V_DROOP is the desired droop voltage at the full load
current I_FL.
R_LL
=
V-----_{D----R----O-----O----P--}
I_FL
(EQ. 38)
Based on the desired loadline, the loadline regulation
resistor, R_FB, can be calculated from Equation 39 or
Equation 40, depending on the R-C current sense circuitry
being employed. If a basic R-C sense circuit consisting of C1
and R1 is being used, use Equation 39. If a resistor divider
R-C sense circuit consisting of R1, R2, and C1 is being
used, use Equation 40.
R_FB
=
-^R----^L---^L-----^â‹---^N------^â‹---³---⁰---⁰--
DCR
(EQ. 39)
R_FB = -^R----^L---^L-----^â‹---^N----_D--^â‹---^3C---⁰--_R-⁰----_â‹-^â‹--_R-⁽---^R-₂---¹-----+----^R-----²---⁾-
(EQ. 40)
In Equations 39 and 40, R_LL is the loadline resistance; N is
the number of active channels; DCR is the DCR of the
individual output inductors; and R1 and R2 are the current
sense R-C resistors.
If no loadline regulation is required, the IDROOP pin should
be left open and not connected to anything. To choose the
value for R_FB in this situation, please refer to the
Compensation Without Loadline Regulation section.
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.
COMPENSATION WITH LOAD-LINE 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
FB
ISL6312
IDROOP
R_FB
VDIFF
FIGURE 20. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6312 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 41, 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 VPP is the peak-to-
peak sawtooth signal amplitude as described in the
Electrical Specifications.
Once selected, the compensation values in Equation 41
assure a stable converter with reasonable transient
performance. In most cases, transient performance can be
improved by making adjustments to RC. Slowly increase the
value of RC while observing the transient performance on an
oscilloscope until no further improvement is noted. Normally,
CC will not need adjustment. Keep the value of CC from
Equation 41 unless some performance issue is noted.
29
FN9289.3
February 14, 2007