ISL6363 Datasheet, PDF (24/32 Pages) Intersil Corporation – Multiphase PWM Regulator for VR12™ Desktop CPUs

ISL6363
IO
IL
VO
RING
BACK
FIGURE 20. OUTPUT VOLTAGE RING BACK PROBLEM
ISUM+
Rntcs
Rntc
Cn.1
Rp
Rn
OPTIONAL
Cn.2 ^Vcn
Ri
ISUM-
Rip ^Cip
OPTIONAL
FIGURE 21. OPTIONAL CIRCUITS FOR RING BACK REDUCTION
Figure 20 shows the output voltage ring back problem during
load transient response. The load current i_o has a fast step
change, but the inductor current I_L cannot accurately follow.
Instead, I_L responds in first order system fashion due to the
nature of current loop. The ESR and ESL effect of the output
capacitors makes the output voltage V_O dip quickly upon load
current change. However, the controller regulates V_O according to
the droop current I_droop, which is a real-time representation of I_L;
therefore it pulls V_O back to the level dictated by I_L, causing the
ring back problem. This phenomenon is not observed when the
output capacitor have very low ESR and ESL, such as all ceramic
capacitors.
Figure 21 shows two optional circuits for reduction of the ring
back.
C_n is the capacitor used to match the inductor time constant. It
usually takes the parallel of two (or more) capacitors to get the
desired value. Figure 21 shows that two capacitors C_n.1 and C_n.2
are in parallel. Resistor R_n is an optional component to reduce
the V_O ring back. At steady state, C_n.1 + C_n.2 provides the desired
C_n capacitance. At the beginning of i_o change, the effective
capacitance is less because R_n increases the impedance of the
C_n.1 branch. As Figure 18 explains, V_O tends to dip when C_n is too
small, and this effect will reduce the V_O ring back. This effect is
more pronounced when C_n.1 is much larger than C_n.2. It is also
more pronounced when R_n is bigger. However, the presence of
R_n increases the ripple of the V_n signal if C_n.2 is too small. It is
recommended to keep C_n.2 greater than 2200pF. R_n value
usually is a few ohms. C_n.1, C_n.2 and R_n values should be
determined through tuning the load transient response
waveforms on an actual board.
R_ip and C_ip form an R-C branch in parallel with R_i, providing a
lower impedance path than R_i at the beginning of i_o change. R_ip
and C_ip do not have any effect at steady state. Through proper
selection of R_ip and C_ip values, i_droop can resemble i_o rather than
i_L, and V_O will not ring back. The recommended value for R_ip is
100Ω. C_ip should be determined through tuning the load
transient response waveforms on an actual board. The
recommended range for C_ip is 100pF~2000pF. However, it
should be noted that the R_ip -C_ip branch may distort the i_droop
waveform. Instead of being triangular as the real inductor
current, i_droop may have sharp spikes, which may adversely
affect i_droop average value detection and therefore may affect
OCP accuracy. User discretion is advised.
Resistor Current-Sensing Network
Phase1 Phase2 Phase3
L
L
L
DCR
DCR
DCR
Rsen Rsen Rsen
Rsum
Rsum
Rsum
Ro
Ro
Ro
ISUM+
Vcn
Cn
Ri ^ISUM-
Io
FIGURE 22. RESISTOR CURRENT-SENSING NETWORK
Figure 22 shows the resistor current-sensing network for a
2-phase solution. Each inductor has a series current-sensing
resistor R_sen. R_sum and R_o are connected to the R_sen pads to
accurately capture the inductor current information. The R_sum
and R_o resistors are connected to capacitor C_n. R_sum and C_n
form a filter for noise attenuation. Equations 25 thru 27 give
V_Cn(s) expression
V_Cn(s)
=
-^R----^s--^e---ⁿ--
N
×
I_o
(
s
)
×
A_R
se
n
(
s
)
(EQ. 25)
A_Rsen(s)
=
----------¹------------
1
+
------^s------
ω_sns
(EQ. 26)
ω_Rsen
=
-------------¹--------------
R_{-----s--u----m---}
N
×
C_n
(EQ. 27)
Transfer function A_Rsen(s) always has unity gain at DC.
Current-sensing resistor R_sen value will not have significant
variation over-temperature, so there is no need for the NTC
network.
The recommended values are R_sum = 1kΩ and C_n = 5600pF.
24
FN6898.0
September 29, 2011