ISL6267 Datasheet, PDF (24/33 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion Mobile CPUs

ISL6267
MOSFET r_DS(ON) voltage drop, which is proportional to the
inductor current. A phase comparator inside the controller
monitors the phase voltage during on-time of the low-side
MOSFET and compares it with a threshold to determine the zero
crossing point of the inductor current. If the inductor current has
not reached zero when the low-side MOSFET turns off, it will flow
through the low-side MOSFET body diode, causing the phase
node to have a larger voltage drop until it decays to zero. If the
inductor current has crossed zero and reversed the direction
when the low-side MOSFET turns off, it will flow through the
high-side MOSFET body diode, causing the phase node to have a
spike until it decays to zero. The controller continues monitoring
the phase voltage after turning off the low-side MOSFET. To
minimize the body diode-related loss, the controller also adjusts
the phase comparator threshold voltage accordingly in iterative
steps such that the low-side MOSFET body diode conducts for
approximately 40ns.
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 20. DCR CURRENT-SENSING NETWORK
Figure 20 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 18 thru 22 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_ntcn
e
t
+
-^R----^s--^u----^m---
N
×
D_-----NC----R_{--⎟⎟⎟}
⎠
× I_o(s) × A_cs(s)
(EQ. 18)
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
+
------^s------
ω_sns
ω_L
=
D_-----C_----R_--
L
(EQ. 19)
(EQ. 20)
(EQ. 21)
ω_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
where N is the number of phases.
(EQ. 22)
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 23 gives Cn value.
24
January 31, 2011
FN7801.0