ISL6277HRZ Datasheet, PDF (28/37 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion™ Mobile CPUs Using SVI 2.0

ISL6277
Key Component Selection
Inductor DCR Current-Sensing Network
PHASE1 PHASE2 PHASE3
RSUM
RSUM
RSUM
ISUM+
L
L
L
DCR DCR
DCR
RNTCS
RP
RNTC
RO
RO
RO
+
CNVCN
-
RI
ISUM-
IO
FIGURE 22. DCR CURRENT-SENSING NETWORK
Figure 22 shows the inductor DCR current-sensing network for a
3-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
ignores it.
The summed inductor current information is presented to the
capacitor C_n. Equations 17 thru 21 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_n
t
c
ne
t
+
-^R----^s---^u---^m---
N

D_-----CN_-----R_{---}

 I_os  A_css
(EQ. 17)
R_ntcnet
=
_---R_{----n----t--c---s----+-----}R_{----n----t-c----}_---_-----R_----p--
R_ntcs + R_ntc + R_p
A_css
=
----¹----+------^-----^s----L-----
1 + -_----_s-^s--_n---_s-
(EQ. 18)
(EQ. 19)
_L
=
D_-----C_-----R_---
L
(EQ. 20)
_sns
=
---------------------------¹-----------------------------
-^R----ⁿ---^t--^c---ⁿ----^e---^t---^------^R--------^s--^N----^u-------^m------
R_n
t
cn
e
t
+
-^R----^s---^u---^m---
N

C_n
(EQ. 21)
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
value decrease as its temperature decreases. Proper selection of
R_sum, R_ntcs, R_p and R_ntc parameters ensures that V_Cn
represents 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 a 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 w_sns and a zero w_L. One needs to match w_L and w_sns so
A_cs(s) is unity gain at all frequencies. By forcing w_L equal to w_sns
and solving for the solution, Equation 22 gives Cn value.
C_n
=
------------------------------^L--------------------------------
-^R----ⁿ---^t--^c---ⁿ----^e---^t---^------^R--------^s--^N----^u-------^m------
R_ntc
n
et
+
-^R----^s---^u---^m---
N

D
C
R
(EQ. 22)
For example, given N = 3, R_sum = 3.65kΩ, R_p = 11kΩ,
R_ntcs = 2.61kΩ, R_ntc = 10kΩ, DCR = 0.88mΩ and L = 0.36µH,
Equation 22 gives C_n = 0.406µF.
Assuming the compensator design is correct, Figure 23 shows the
expected load transient response waveforms if C_n is correctly
selected. When the load current I_core has a square change, the
output voltage V_core also has a square response.
If C_n value is too large or too small, V_Cn(s) does not accurately
represent real-time I_o(s) and worsens the transient response.
Figure 24 shows the load transient response when C_n is too
small. V_core sags excessively upon load insertion and may create
a system failure. Figure 25 shows the transient response when
28
FN8270.1
March 8, 2012