ISL62883HRTZ-T Datasheet, PDF (22/37 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs

ISL62883, ISL62883B
Substitution of Equation 28 into Equation 1 gives Equation 29:
I_droop
=
-²---
R_i
×
-----------^R----ⁿ----^t--^c--ⁿ----^e---^t-----------
R_n
t
c
net
+
-^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------^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. 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 = -^2N---^R---_×--^s--_I^e--_d-ⁿ--_r---_o^×---_o--^I-_po--
(EQ. 34)
Substitution of Equation 34 and application of the OCP condition
in Equation 30 gives:
R_i
=
2_----R_{-----s--e---n-----}×_-----I_{--o---m-----a---x-}
N × I_droopmax
(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 3 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 9.
For inductor DCR sensing, substitution of Equation 29 into
Equation 2 gives the load line slope expression:
LL = -^V---^d---^r--^o----^o---^p- = -²---^R-----^d---^r--^o---^o---^p-- × -----------^R----ⁿ---^t---^c--ⁿ----^e---^t----------- × ^D-----^C----^R--
I_o
R_i
R_n
t
c
net
+
R_{-----s--u----m---}
N
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-n----}××_-----RR_{----di---r--o---o----p-}
(EQ. 37)
Substitution of Equation 30 and rewriting Equation 36, or
substitution of Equation 34 and rewriting Equation 37 gives the
same result in Equation 38:
R_droop
=
------^I--^o------- × LL
I_droop
(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--------- × LL
R_droop
(EQ. 40)
Substitution of Equation 40 into Equation 39 gives Equation 41:
V_Rimon
=
3_----I_--o-----×_----L_----L_-
R_droop
×
R_imon
(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 = 963mV at I_omax = 51A, Equation 42 gives
R_imon = 7.85kΩ.
A capacitor C_imon can be paralleled with R_imon to filter the IMON
pin voltage. The R_imonC_imon time constant is the user’s choice. It
is recommended to have a time constant long enough such that
switching frequency ripples are removed.
Compensator
Figure 15 shows the desired load transient response waveforms.
Figure 21 shows the equivalent circuit of a voltage regulator (VR)
with the droop function. A VR is equivalent to a voltage source
(= VID) and output impedance Z_out(s). If Z_out(s) is equal to the
load line slope LL, i.e. constant output impedance, in the entire
frequency range, V_o will have square response when I_o has a
square change.
22
FN6891.4
June 21, 2011