ISL6377 Datasheet, PDF (29/36 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion™ Desktop CPUs Using SVI 2.0

ISL6377
ISUM+
Resistor Current-Sensing Network
PHASE1 PHASE2 PHASE3 PHASE4
Rntcs
Rntc
Cn.1
Rp
Rn
OPTIONAL
Cn.2 ^Vcn
Ri
ISUM-
Rip ^Cip
OPTIONAL
FIGURE 25. OPTIONAL CIRCUITS FOR RING-BACK REDUCTION
Figure 24 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 the
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 has
very low ESR and ESL, as is the case with all ceramic capacitors.
Figure 25 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 25 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 22 shows, V_o tends
to dip when C_n is too small, and this effect reduces 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.
L
L
L
L
DCR
DCR
DCR
DCR
RSEN RSEN RSEN RSEN
RSUM
RSUM
RSUM
RSUM
RO
RO
RO
RO
ISUM+
VCN⁺ CN
-
RI
ISUM-
IO
FIGURE 26. RESISTOR CURRENT-SENSING NETWORK
Figure 26 shows the resistor current-sensing network for a
4-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 28 thru 30 give the
V_Cn(s) expression.
V_Cn(s)
=
R_{-----s---e---n--}
N
×
I_o
(
s
)
×
A_Rs
en⁽
s
)
(EQ. 28)
A_Rsen(s)
=
----------¹------------
1 + _ω-----_s-^s--_n---_s-
(EQ. 29)
ω_Rsen
=
-------------¹---------------
R_{-----s---u---m---}
N
×
C_n
(EQ. 30)
Transfer function A_Rsen(s) always has unity gain at DC.
Current-sensing resistor R_sen value does 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.
Overcurrent Protection
Refer to Equation 2 on page 16 and Figures 20, 24 and 26;
resistor R_i sets the I_sum current which is proportional to droop
current and IMON current. Tables 1 and 2 show the internal OCP
threshold based on the IMON pin voltage. Since the R_i resistor
impacts both the droop current and the IMON current, fine
adjustments to I_droop will require changing the R_comp resistor.
For example, the OCP threshold is 1.5V on the IMON pin which
equates to an IMON current of 11.25µA using a 133kΩ IMON
resistor. The corresponding I_sum is 45µA which results in an
29
FN8336.0
August 6, 2012