ISL6277HRZ Datasheet, PDF (30/37 Pages) Intersil Corporation – Multiphase PWM Regulator for AMD Fusion™ Mobile CPUs Using SVI 2.0

ISL6277
Resistor Current-Sensing Network
PHASE1 PHASE2 PHASE3
L
L
L
DCR
DCR
DCR
RSEN RSEN RSEN
RSUM
RSUM
RSUM
RO
RO
RO
VCN⁺ CN
-
RI
ISUM+
ISUM-
IO
FIGURE 28. RESISTOR CURRENT-SENSING NETWORK
Figure 28 shows the 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 23 thru 25 give the
V_Cn(s) expression.
V_Cns
=
-^R----^s---^e---ⁿ--
N

I_o
s


A_R
sen
s
(EQ. 23)
A_Rsens
=
-----------¹-----------
1
+
------^s------
_sns
(EQ. 24)
_Rsen
=
--------------¹--------------
R_{-----s---u---m---}
N

C_n
(EQ. 25)
Transfer function A_Rsen(s) always has unity gain at DC.
Current-sensing resistor R_sen value does not have significant
variation over-temperature, so there is no need for the NTC
network.
The recommended values are R_sum = 1kΩ and C_n = 5600pF.
Overcurrent Protection
Refer to Equation 1 on page 17 and Figures 22, 26 and 28;
resistor R_i sets the droop current, I_droop. Tables 1 and 2 show the
internal OCP threshold. It is recommended to design I_droop
without using the R_comp resistor.
For example, the OCP threshold is 1.5V on the IMON pin. This
translates to 45μA of ISUM current or 56.25μA of droop current.
I_droop is designed to be 45μA at full load, so the OCP trip level is
1.25x of the full load current.
For inductor DCR sensing, Equation 26 gives the DC relationship
of V_cn(s) and I_o(s):


V_Cn
=




------------^R----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_n
t
cne
t
+
-^R----^s---^u---^m---
N

D_-----CN_-----R_{---}

 I_o
(EQ. 26)
Substitution of Equation 26 into Equation 1 gives Equation 27:
I_droop
=
--¹---
R_i

-----------^R-----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_n
t
cne
t
+
-^R----^s---^u---^m---
N

D_-----C_-----R_---
N

I_o
(EQ. 27)
Therefore:
R_i
=
---------------^R-----ⁿ---^t--^c---ⁿ----^e---^t---^-----^D----^C-----^R------^-----^I--^o----------------
N



R_n
tc
n
et
+
-^R----^s-_N--^u---^m---_

I_droop
(EQ. 28)
Substitution of Equation 18 and application of the OCP condition
in Equation 28 gives Equation 29:
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_droopmax
(EQ. 29)
where I_omax is the full load current and I_droopmax is the
corresponding droop current. For example, given N = 3,
R_sum = 3.65kΩ, R_p = 11kΩ, R_ntcs = 2.61kΩ, R_ntc = 10kΩ,
DCR = 0.9mΩ, I_omax = 65A and I_droopmax = 45μA. Equation 29
gives R_i = 359Ω.
For resistor sensing, Equation 30 gives the DC relationship of
V_cn(s) and I_o(s).
V_Cn
=
-^R----^s---^e---ⁿ--
N

I_o
(EQ. 30)
Substitution of Equation 30 into Equation 1 gives Equation 31:
I_droop
=
--¹---
R_i

R_{-----s---e---n--}
N

I_o
(EQ. 31)
Therefore:
R_i
=
--^R----^s---^e----ⁿ----^-----^I--^o--
N  I_droop
(EQ. 32)
Substitution of Equation 32 and application of the OCP condition
in Equation 28 gives Equation 33:
R_i = _N--^R----^s_---^e--_I--^nd---_r-^-_o----_o^I--^o-_p--^m-_m----^a-_a--^x--_x-
(EQ. 33)
where I_omax is the full load current and I_droopmax is the
corresponding droop current. For example, given N = 3,
R_sen = 1mΩ, I_omax = 65A and I_droopmax = 45µA, Equation 33
gives R_i = 481Ω.
30
FN8270.1
March 8, 2012