ISL6353 Datasheet, PDF (20/30 Pages) Intersil Corporation – Multiphase PWM Regulator for VR12 DDR Memory Systems

ISL6353
IMON voltage should be approximately a square wave with little
or no overshoot. In regulators without droop control the capacitor
value can be selected to err on the high side to overdamp the
current sense input to the controller to avoid overshoots.
Resistor Current-Sensing Network
Phase1 Phase2 Phase3
L
L
L
DCR
DCR
DCR
Rsen Rsen Rsen
Rsum
Rsum
Rsum
Ro
Ro
Ro
ISUM+
Vcn
Cn
Ri ^ISUM-
Io
FIGURE 11. RESISTOR CURRENT-SENSING NETWORK
Figure 11 shows the precision resistor current-sensing network
for a 3-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 10 through 12 give
V_Cn(s) expressions:
V_Cn(s)
=
-^R----^s--^e---ⁿ--
N
×
I_o
(s)
×
A_Rs
en
(
s
)
(EQ. 10)
A_Rsen(s)
=
----------¹------------
1
+
------^s------
ω_sns
(EQ. 11)
ω_Rsen
=
-------------¹--------------
R_{-----s--u----m---}
N
×
C_n
(EQ. 12)
Transfer function A_Rsen(s) always has unity gain at DC. The
current-sensing resistor R_sen value will not have a significant
variation over temperature, so there is no need for the NTC
network.
Recommended values are R_sum = 1kΩ and C_n = 5600pF.
Overcurrent Protection
The ISL6353 implements overcurrent protection (OCP) by
comparing the average value of the measured current I_sense with
an internal current source reference. The OCP threshold is 60µA
for 3-phase, 2-phase and 1-phase PS0 operation. In PS1/2 mode
the OCP threshold is scaled based on the number of active
phases in PS1/2 mode divided by the number of active phases in
PS0 mode. For example, if the regulator operates in 3-phase
mode in PS0, 2-phase in PS1 mode and 1-phase in PS2 mode,
the OCP threshold will be 60µA in PS0 mode, 40µA in PS1 mode
and 20µA in PS2 mode. For a 2-phase design, the OCP threshold
is 60µA in PS0 mode and 30µA in PS1 and PS2 mode. The
ISL6353 declares a OCP fault when I_sense is above the threshold
for 120µs.
Referring to Equation 3 and Figure 10, resistor R_i sets the sensed
current I_sense. In general, I_sense can be set to 40µA at the
maximum load current expected in the design. The OCP trip level
will be 1.5 times the maximum load current with a threshold at
60µA. The OCP ratio can be set to something other than 1.5
times the maximum load current by setting
I_sense = 60µA/OCP_ratio.
For inductor DCR sensing, Equation 13 gives the DC relationship
of V_cn(s) and I_o(s).
⎛
⎞
V_Cn
=
⎜
⎜
⎜
⎝
-----------^R----ⁿ---^t---^c--ⁿ----^e---^t-----------
R_n
t
c
net
+
-^R----^s--^u----^m---
N
×
D_-----NC----R_{--⎟⎟⎟}
⎠
× I_o
(EQ. 13)
Substitution of Equation 13 into Equation 3 gives Equation 14:
II_sense
=
-¹---
R_i
×
-----------^R----ⁿ----^t--^c--ⁿ----^e---^t-----------
R_ntcnet
+
-^R----^s--^u----^m---
N
×
D_-----C_----R_--
N
×
I_o
(EQ. 14)
Therefore:
R_i
=
----------------^R----ⁿ---^t--^c---ⁿ---^e---^t----^×----^D-----^C----^R-----^×-----^I--^o----------------
N
×
⎛
⎝
R_n
t
c
net
+
-^R----^s-_N-^u----^m---_⎠⎞
×
I_sense
(EQ. 15)
Substitution of Equation 5 and application of the full load
condition in Equation 15 gives Equation 16:
R_i
=
----------------^(-----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_sensemax
(EQ. 16)
where I_omax is the full load current, and I_sensemax is the
corresponding sensed current based on the desired OCP to I_omax
ratio.
For resistor sensing, Equation 17 gives the DC relationship of
V_cn(s) and I_o(s).
V_Cn
=
-^R----^s--^e----ⁿ-
N
×
I_o
(EQ. 17)
Substitution of Equation 17 into Equation 3 gives Equation 18:
I_sense
=
-¹---
R_i
×
-^R----^s--^e----ⁿ-
N
×
I_o
(EQ. 18)
Therefore:
R_i
=
--^R----^s---^e---ⁿ----^×-----^I--^o---
N × I_sense
(EQ. 19)
Application of the full load condition gives Equation 20:
R_i
=
--^R----^s---^e---ⁿ----^×-----^I--^o---^m-----^a---^x---
N × I_sensemax
(EQ. 20)
where I_omax is the full load current, and I_sensemax is the
corresponding sensed current.
20
September 15, 2011
FN6897.0