ISL62883C Datasheet, PDF (26/43 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62883C
For example, the OCP threshold is 60µA for 3-phase
solution. We will design I_droop to be 40.9µA at full load,
so the OCP trip level is 1.5x of the full load current.
For inductor DCR sensing, Equation 28 gives the DC
relationship of V_cn(s) and I_o(s).
⎛
⎞
V_Cn
=
⎜
⎜
⎜
⎝
-----------^R-----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_nt
cn
e
t
+
-^R----^s---^u---^m---
N
×
D_-----CN_-----R_{---⎟⎟⎟}
⎠
× I_o
(EQ. 28)
Substitution of Equation 28 into Equation 1 gives
Equation 29:
I_droop
=
--²---
R_i
×
-----------^R-----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_nt
cn
et
+
-^R----^s---^u---^m---
N
×
D_-----C_-----R_---
N
×
I_o
(EQ.29)
Therefore:
R_i
=
--------------²---^R-----ⁿ---^t--^c---ⁿ---^e----^t---^×-----^D----^C-----^R------^×-----^I--^o--------------
N × _⎝⎛R_ntcnet + -^R----^s-_N--^u---^m---_⎠⎞ × I_droop
(EQ.30)
Substitution of Equation 20 and application of the OCP
condition in Equation 30 gives Equation 31:
R_i
=
-----------²----^×------⁽-----^{RR--------nn--------tt----cc------ss--------++----------RR--------nn-------t}-^{t--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.31)
where I_omax is the full load current, 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 = 0.88mΩ, I_omax = 51A and I_droopmax = 40.9µA,
Equation 31 gives R_i = 606Ω.
For resistor sensing, Equation 32 gives the DC
relationship of V_cn(s) and I_o(s).
V_Cn
=
-^R----^s---^e---ⁿ--
N
×
I_o
(EQ.32)
Substitution of Equation 32 into Equation 1 gives
Equation 33:
I_droop
=
--²---
R_i
×
R_{-----s---e---n--}
N
×
I_o
(EQ.33)
Therefore
R_i
=
-²---^R-----^s---^e---ⁿ-----^×----^I--^o--
N × I_droop
(EQ.34)
Substitution of Equation 34 and application of the OCP
condition in Equation 30 gives Equation 35:
R_i = -^2N---^R---_×--^s---_I^e--_d-ⁿ--_r--_o-^×--_o--^I--_po---_m-^m----_aa---_x-^x-
(EQ.35)
where I_omax is the full load current, I_droopmax is the
corresponding droop current. For example, given N = 3,
R_sen = 1mΩ, I_omax = 51A and I_droopmax = 40.9µA,
Equation 35 gives R_i = 831Ω.
A resistor from COMP to GND can adjust the internal OCP
threshold, providing another dimension of fine-tune
flexibility. Table 4 shows the detail. It is recommended to
scale I_droop such that the default OCP threshold gives
approximately the desired OCP level, then use R_comp to
fine tune the OCP level if necessary.
Load Line Slope
Refer to Figure 12.
For inductor DCR sensing, substitution of Equation 29
into Equation 2 gives the load line slope expression:
LL
=
-^V----^d---^r--^o----^o---^p-
I_o
=
2_----R_{-----d---r--o----o---p-}
R_i
×
-----------^R-----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_nt
cn
et
+
-^R----^s---^u---^m---
N
×
D_-----C_-----R_---
N
(EQ.36)
For resistor sensing, substitution of Equation 33 into
Equation 2 gives the load line slope expression:
LL
=
-^V----^d---^r--^o----^o---^p-
I_o
=
2_----R_{-----s---e---n-----}×_----R_{-----d---r---o---o---p-}
N × R_i
(EQ.37)
Substitution of Equation 30 and rewriting Equation 36,
or substitution of Equation 34 and rewriting Equation 37
give the same result in Equation 38:
R_droop
=
-------^I-^o--------
I_droop
×
L
L
(EQ. 38)
One can use the full load condition to calculate R_droop.
For example, given I_omax = 51A, I_droopmax = 40.9µA
and LL = 1.9mΩ, Equation 38 gives R_droop = 2.37kΩ.
It is recommended to start with the R_droop value
calculated by Equation 38, and fine tune it on the actual
board to get accurate load line slope. One should record
the output voltage readings at no load and at full load for
load line slope calculation. Reading the output voltage at
lighter load instead of full load will increase the
measurement error.
Current Monitor
Refer to Equation 18 for the IMON pin current
expression.
Refer to Figures 1 and 2, the IMON pin current flows
through R_imon. The voltage across R_imon is expressed in
Equation 39:
V_Rimon = 3 × I_droop × R_imon
(EQ.39)
Rewriting Equation 38 gives Equation 40:
I_droop
=
--------^I--^o--------
R_droop
×
LL
(EQ.40)
Substitution of Equation 40 into Equation 39 gives
Equation 41:
V_Rimon
=
3_----I_--o-----×_----L_----L_--
R_droop
×
R_im
on
(EQ.41)
Rewriting Equation 41 and application of full load
condition gives Equation 42:
R_imon
=
V_{-----R----i--m----o----n----}×_-----R_{----d----r--o---o----p-}
3I_o × LL
(EQ.42)
For example, given LL = 1.9mΩ, R_droop = 2.37kΩ,
V_Rimon = 999mV at I_omax = 51A, Equation 42 gives
R_imon = 8.14kΩ.
26
FN7557.1
March 18, 2010