ISL6322_07 Datasheet, PDF (34/41 Pages) Intersil Corporation – Four-Phase Buck PWM Controller with Integrated MOSFET Drivers and I2C Interface for Intel VR10, VR11, and AMD Applications

ISL6322
2. Plug the inductor L and DCR component values, and the
value for C₁ chosen in step 1, into Equation 33 to
calculate the value for R₁.
R₁
=
------------^L-------------
DCR ⋅ C₁
I_OCP = I_{OCP, min} ^{(EQ. 33)}
3. Resistor R₂ should be left unpopulated.
If the desired overcurrent trip level, I_OCP, is greater than the
minimum overcurrent trip level, I_OCP,min, then a resistor
divider R-C circuit should be used to set the desired trip
level. Do the following steps to choose the component
values for the resistor divider R-C current sensing
network:
1. Choose an arbitrary value for C₁. The recommended
value is 0.1μF.
2. Plug the inductor L and DCR component values, the
value for C₁ chosen in step 1, the number of active
channels N, and the desired overcurrent protection level
I_OCP into Equations 34 and 35 to calculate the values for
R₁ and R₂.
R₁
=
---------^L-----^â‹---^I--^O----^C-----^P----------
C₁ ⋅ 0.0375 ⋅ N
I_OCP > I_{OCP, min} ^{(EQ. 34)}
R₂ = -_C----₁-----_â‹---₍--_I--_O----_C-----_P-----_â‹^L---_D--^â‹--_C-^I--^O--_R--^C----–^P-----₀---_.--₀---₃---₇---₅-----_â‹---_N-----₎
(EQ. 35)
Due to errors in the inductance or DCR, it may be necessary
to adjust the value of R₁ and R₂ to match the time constants
correctly. The effects of time constant mismatch can be seen
in the form of droop overshoot or undershoot during the
initial load transient spike, as shown in Figure 22. Do the
following steps to ensure the R-C and inductor L/DCR
time constants are matched accurately.
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 Equation 36 and Equation 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.
Load-line Regulation Resistor
If load-line 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 load-line regulation
resistor, labeled R_FB in Figure 6. This resistor’s value sets
the desired load-line required for the application. The
desired load-line, 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 load-line, the load-line 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)
ΔV₂
ΔV₁
V_OUT
I_TRAN
ΔI
FIGURE 22. TIME CONSTANT MISMATCH BEHAVIOR
1. Capture a transient event with the oscilloscope set to
about L/DCR/2 (sec/div). For example, with L = 1µH and
DCR = 1mΩ, set the oscilloscope to 500µs/div.
2. Record ΔV1 and ΔV2 as shown in Figure 22.
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 load-line 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 load-line 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, see “Compensation without
Load-line Regulation” on page 36.
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 (Droop) Regulation” on page 20,
there are two distinct methods for achieving these goals:
34
FN6328.1
February 15, 2007