ISL6277A Datasheet, PDF (31/38 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion Mobile CPUs Using SVI 2.0

ISL6277A
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 24 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.
Resistor Current-Sensing Network
PHASE1 PHASE2 PHASE3
L
L
L
DCR
DCR
DCR
R_SEN
R_SEN
R_SEN
RSUM
RSUM
RSUM
RO
RO
RO
+
VCN CN
-
RI
ISUM+
ISUM-
IO
FIGURE 28. RESISTOR CURRENT-SENSING NETWORK
Figure 28 shows the resistor current-sensing network for a
3-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 24 thru 26 give the
V_Cn(s) expression.
V_Cn(s)
=
-^R----^s---^e---ⁿ--
N
×
I_o(s)
×
A_Rsen(s)
(EQ. 24)
A_Rsen(s)
=
----------¹------------
1 + _ω-----_s-^s--_n---_s-
(EQ. 25)
ω_Rsen
=
-------------¹---------------
R_{-----s---u---m---}
N
×
C_n
(EQ. 26)
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 18 and Figures 22, 26 and 28;
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
I_droop of 56.25µA. At full load current, I_omax, the I_sum current is
36µA and the resulting I_droop is 45µA. The ratio of I_sum at OCP
relative to full load current is 1.25. Therefore, the OCP current
trip level is 25% higher than the full load current.
For inductor DCR sensing, Equation 27 gives the DC relationship
of V_cn(s) and I_o(s):
⎛
⎞
V_Cn
=
⎜
⎜
⎜
⎝
------------^R----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_n
t
cne
t
+
-^R----^s---^u---^m---
N
×
D_-----CN_-----R_{---⎟⎟⎟}
⎠
× I_o
(EQ. 27)
Substitution of Equation 27 into Equation 2 gives Equation 28:
I_droop
=
5_--
4
×
--¹---
R_i
×
------------^R----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_n
t
cne
t
+
-^R----^s---^u---^m---
N
×
D_-----C_-----R_---
N
×
I_o
(EQ. 28)
Therefore:
R_i
=
5_--
4
×
---------------^R-----ⁿ---^t--^c---ⁿ----^e---^t---^×-----^D----^C-----^R------^×-----^I--^o----------------
N
×
⎛
⎝
R_n
t
c
ne
t
+
-^R----^s-_N--^u---^m---_⎠⎞
×
I_droop
(EQ. 29)
Substitution of Equation 19 and application of the OCP condition
in Equation 29 gives Equation 30:
R_i
=
5_--
4
×
----------------^(-----RR---------^nn-------tt----^cc------^ss--------++----------^RR--------ⁿⁿ--------^tt---^cc-------)--+----^×-----^R-----R----^p-----^p------×-----^D-----^C----^R------^×-----^I--^o---^m-----^a---^x----------------
N
×
⎛
⎜
⎝
(_---R_{-----n---t--c---s----+-----}R_{----n----t-c----})_---×_-----R_----p--
R_ntcs + R_ntc + R_p
+
-^R----^s-_N--^u---^m---_{⎠⎟⎞}
×
I_droopmax
(EQ. 30)
where I_omax is the full load current and I_droopmax is the
corresponding droop current. For example, given N = 3,
R_sum = 3.65kΩ, R_p = 11kΩ, R_ntcs = 2.61kΩ, R_ntc = 10kΩ,
31
FN8322.0
December 19, 2012