ISL6353 Datasheet, PDF (19/30 Pages) Intersil Corporation – Multiphase PWM Regulator for VR12 DDR Memory Systems

ISL6353
The sensed current is used for current monitoring and overcurrent
protection.
Phase1 Phase2 Phase3
Rsum
Rsum
Rsum
ISUM+
L
L
L
DCR
DCR
DCR
Rntcs
Rp
Rntc
Ro
Ro
Ro
Cn Vcn
Ri ^ISUM-
Io
FIGURE 10. DCR CURRENT-SENSING NETWORK
Figure 10 shows the inductor DCR current-sensing network for a
3-phase regulator. Inductor current flows through the DCR and
creates a voltage drop. Each inductor has two resistors 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 compensate for the increase
in inductor DCR as temperature increases.
The inductor output pads are electrically shorted in the schematic,
but have some parasitic impedance in the actual board layout,
which is why the signals cannot simply be shorted together for the
current-sense summing network. A resistor from 1Ω~10Ω for R_o is
recommended to create quality signals. Since the 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 represented at
capacitor C_n. Equations 4 through 8 describe the
frequency-domain relationship between total inductor current
I_o(s) and the C_n voltage V_Cn(s):
⎛
⎞
V_Cn(s)
=
⎜
⎜
⎜
⎝
-----------^R----ⁿ----^t--^c--ⁿ----^e---^t-----------
R_n
t
c
net
+
R_{-----s--u----m---}
N
×
D_-----NC----R_{--⎟⎟⎟}
⎠
× I_o(s) × A_cs(s)
(EQ. 4)
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
(EQ. 5)
(EQ. 6)
ω_L
=
D_-----C_----R_--
L
ω_sns
=
--------------------------¹----------------------------
-^R----ⁿ---^t--^c---ⁿ---^e---^t---^×-----^R---------s---^{N--u--------m------}
R_ntcn
e
t
+
R_{-----s--u----m---}
N
×
C_n
(EQ. 7)
(EQ. 8)
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 a reading of the inductor DC current. The NTC R_ntc
values decreases as its temperature increases. Proper selections
of R_sum, R_ntcs, R_p and R_ntc parameters ensure that V_Cn
represents the total inductor 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. A higher ratio of V_cn to the inductor DCR
voltage is recommended so the current monitor and OCP 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 component
values may need to be fine tuned on actual boards. To help fine
tune the network apply a full load condition to the regulator and
record the IMON pin voltage reading immediately; then record the
IMON voltage reading again when the board has reached thermal
steady state. A good NTC network can limit the IMON voltage drift
to within 1% over the temperature range. If droop is used for the
ISL6353 based regulator the output voltage can be used for this
test rather than IMON. DDR memory regulators typically do not
operate with droop enabled. The Intersil evaluation board layout
and current-sensing network parameters can be referred to in
order to help minimize engineering time.
V_Cn(s) needs to represent real-time I_o(s) for the controller to
achieve best OCP and IMON response. The transfer function
A_cs(s) has a pole ω_sns and a zero ω_L. ω_L and ω_sns should be
matched so A_cs(s) is unity gain at all frequencies. By forcing ω_L
equal to ω_sns and solving for the solution, Equation 9 gives Cn
value.
C_n
=
-----------------------------^L------------------------------
-^R----ⁿ---^t--^c---ⁿ---^e---^t---^×------^R--------^s---^N--u-------m------- × DCR
R_nt
cn
e
t
+
-^R----^s--^u----^m---
N
(EQ. 9)
For example, given N = 3, R_sum = 3.65kΩ, R_p = 11kΩ,
R_ntcs = 2.61kΩ, R_ntc = 10kΩ, DCR = 0.29mΩ and L = 0.22µH,
Equation 9 gives C_n = 0.79µF.
C_n is the capacitor used to match the inductor time constant.
Sometimes it takes the parallel combination of two or more
capacitors to get the desired value. To verify the capacitor value
is correct a repetitive load can be placed on the output voltage
and the IMON voltage can be monitored. The capacitor in parallel
with the IMON resistor needs to be removed for this test. The
19
September 15, 2011
FN6897.0