ISL62882 Datasheet, PDF (23/42 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62882, ISL62882B
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 19 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
L
L
DCR
DCR
RSEN
RSEN
RSUM
RSUM
RO
RO
ISUM+
VCN
CN
RI
ISUM-
IO
FIGURE 23. RESISTOR CURRENT-SENSING NETWORK
Figure 23 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 a filter for noise
attenuation. Equations 20 thru 22 give V_Cn(s) expression
V_Cn(s)
=
-^R----^s---^e---ⁿ--
N
×
I_o
(
s)
×
A_Rs
en⁽
s
)
(EQ.20)
A_Rsen(s)
=
----------¹------------
1
+
------^s------
ω_sns
(EQ.21)
ω_Rsen
=
--------------¹--------------
R_{-----s---u---m---}
N
×
C_n
(EQ.22)
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.
Overcurrent Protection
Refer to Equation 1 on page 16 and Figures 12, 17 and
23; resistor R_i sets the droop current I_droop. Table 4
shows the internal OCP threshold. It is recommended to
design I_droop without using the R_comp resistor.
For example, the OCP threshold is 40µA for 2-phase
solution. We will design I_droop to be 34.3µA at full load,
so the OCP trip level is 1.16x of the full load current.
For inductor DCR sensing, Equation 23 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.23)
Substitution of Equation 23 into Equation 1 gives
Equation 24:
I_droop
=
--²---
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.24)
Therefore:
R_i
=
--------------²---^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.25)
Substitution of Equation 15 and application of the OCP
condition in Equation 25 gives Equation 26:
R_i
=
-----------²-----^×----^{(------RR--------nn--------tt----cc------ss--------++----------RR--------nn-------t}-^{t--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.26)
where I_omax is the full load current, I_droopmax is the
corresponding droop current. For example, given N = 2,
R_sum = 3.65kΩ, R_p = 11kΩ, R_ntcs = 2.61kΩ, R_ntc = 10kΩ,
DCR = 0.88mΩ, I_omax = 51A and I_droopmax = 34.3µA,
Equation 26 gives R_i = 998Ω.
For resistor sensing, Equation 27 gives the DC
relationship of V_cn(s) and I_o(s).
V_Cn
=
-^R----^s---^e---ⁿ--
N
×
I_o
(EQ.27)
23
FN6890.2
April 29, 2010