ISL62882_14 Datasheet, PDF (23/42 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62882, ISL62882B
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 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_Rsen(s)
(EQ. 20)
A_Rsen(s)
=
----------¹------------
1 + -_ω----_s-^s--_n---_s-
ω_Rsen
=
-------------¹--------------
-^R----^s--^u----^m---
N
×
C_n
(EQ. 21)
(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_ntcnet
+
R_{-----s--u----m---}
N
×
D_-----NC----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
c
net
+
-^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
net
+
-^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------^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. 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)
Substitution of Equation 27 into Equation 1 gives Equation 28:
I_droop
=
-²---
R_i
×
-^R----^s--^e----ⁿ-
N
×
I_o
(EQ. 28)
Therefore
R_i = -^2N---^R--_×---^s--_I^e--_d-ⁿ--_r---_o^×---_o--^I-_po--
(EQ. 29)
Substitution of Equation 29 and application of the OCP condition
in Equation 25 gives Equation 30:
R_i
=
-²---^R-----^s--^e---ⁿ-----^×-----^I-^o----^m-----^a---^x-
N × I_droopmax
(EQ. 30)
23
FN6890.4
June 21, 2011