ISL62882_14 Datasheet, PDF (21/42 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62882, ISL62882B
Care should be taken in layout that the resistor is placed very
close to the RBIAS pin and that a good quality signal ground is
connected to the opposite side of the R_BIAS resistor.
R_is and C_is
As Figures 1 thru 4 show, the ISL62882 needs the R_is - C_is
network across the ISUM+ and the ISUM- pins to stabilize the
droop amplifier. The preferred values are R_is = 82.5Ω and
C_is = 0.01µF. Slight deviations from the recommended values
are acceptable. Large deviations may result in instability.
Inductor DCR Current-Sensing Network
Phase1 Phase2
Rsum
Rsum
ISUM+
L
L
DCR
DCR
Rntcs
Rp
Rntc
Ro
Ro
Cn ^Vcn
Ri ^ISUM-
Io
FIGURE 17. DCR CURRENT-SENSING NETWORK
Figure 17 shows the inductor DCR current-sensing network for a
2-phase solution. An inductor current flows through the DCR and
creates a voltage drop. Each inductor has two resistors in R_sum and
R_o connected to the pads to accurately sense the inductor current by
sensing the DCR voltage drop. The R_sum and R_o resistors are
connected in a summing network as shown, and feed the total
current information to the NTC network (consisting of R_ntcs, R_ntc
and R_p) and capacitor C_n. R_ntc is a negative temperature coefficient
(NTC) thermistor, used to temperature-compensate the inductor
DCR change.
The inductor output side pads are electrically shorted in the
schematic, but have some parasitic impedance in actual board
layout, which is why one cannot simply short them together for the
current-sensing summing network. It is recommended to use
1Ω~10Ω R_o to create quality signals. Since R_o value is much
smaller than the rest of the current sensing circuit, the following
analysis will ignore it for simplicity.
The summed inductor current information is presented to the
capacitor C_n. Equations 14 thru 18 describe the
frequency-domain relationship between inductor total current
I_o(s) and C_n voltage V_Cn(s).
⎛
⎞
V_Cn(s)
=
⎜
⎜
⎜
⎝
-----------^R----ⁿ----^t--^c--ⁿ----^e---^t-----------
R_ntc
ne
t
+
-^R----^s--^u----^m---
N
×
D_-----NC----R_{--⎟⎟⎟}
⎠
× I_o(s) × A_cs(s)
(EQ. 14)
R_ntcnet
=
-⁽--^R----ⁿ---^t--^c---^s----+-----^R----ⁿ---^t--^c---⁾----^×-----^R----^p-
R_ntcs + R_ntc + R_p
A_cs(s)
=
---¹-----+-----^{ω------s----L-----}
1 + -_ω----_s-^s--_n---_s-
ω_L
=
D_-----C_----R_--
L
(EQ. 15)
(EQ. 16)
(EQ. 17)
ω_sns
=
--------------------------¹----------------------------
-^R----ⁿ---^t--^c---ⁿ---^e---^t---^×------^R--------^s---^N--u-------m-------
R_n
t
c
net
+
-^R----^s--^u----^m---
N
×
C_n
(EQ. 18)
where N is the number of phases.
Transfer function A_cs(s) always has unity gain at DC. The inductor
DCR value increases as the winding temperature increases,
giving higher reading of the inductor DC current. The NTC R_ntc
values decreases as its temperature decreases. Proper
selections of R_sum, R_ntcs, R_p and R_ntc parameters ensure that
V_Cn represent the inductor total DC current over the temperature
range of interest.
There are many sets of parameters that can properly
temperature-compensate the DCR change. Since the NTC network
and the R_sum resistors form a voltage divider, V_cn is always a
fraction of the inductor DCR voltage. It is recommended to have a
higher ratio of V_cn to the inductor DCR voltage, so the droop circuit
has higher signal level to work with.
A typical set of parameters that provide good temperature
compensation are: R_sum = 3.65kΩ, R_p = 11kΩ, R_ntcs = 2.61kΩ
and R_ntc = 10kΩ (ERT-J1VR103J). The NTC network parameters
may need to be fine tuned on actual boards. One can apply full
load DC current and record the output voltage reading
immediately; then record the output voltage reading again when
the board has reached the thermal steady state. A good NTC
network can limit the output voltage drift to within 2mV. It is
recommended to follow the Intersil evaluation board layout and
current-sensing network parameters to minimize engineering
time.
V_Cn(s) also needs to represent real-time I_o(s) for the controller to
achieve good transient response. Transfer function A_cs(s) has a
pole ω_sns and a zero ω_L. One needs to match ω_L and ω_sns so
A_cs(s) is unity gain at all frequencies. By forcing ω_L equal to ω_sns
and solving for the solution, Equation 19 gives C_n value.
C_n
=
-----------------------------^L------------------------------
-^R----ⁿ---^t--^c---ⁿ---^e---^t---^×-----^R---------s---^N--u-------m------- × DCR
R_ntcn
e
t
+
R_{-----s--u----m---}
N
(EQ. 19)
21
FN6890.4
June 21, 2011