ISL62881C_14 Datasheet, PDF (21/37 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62881C, ISL62881D
Transfer function A_Rsen(s) always has unity gain at DC.
Current-sensing resistor R_sen value will 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
Referring to Equation 1 and Figures 12, 13 and 19,
resistor R_i sets the droop current I_droop. Table 3 shows
the internal OCP threshold. It is recommended to design
I_droop without using the R_comp resistor.
For example, the OCP threshold is 60µA. We will design
I_droop to be 50µA at full load, so the OCP trip level is 1.2x
of the full load current.
For inductor DCR sensing, Equation 16 gives the DC
relationship of V_cn(s) and I_o(s).
V_Cn
=
⎛
⎜
⎝
-----------^R-----ⁿ---^t--^c---ⁿ---^e----^t----------
R_ntcnet + R_sum
×
D
C
⎞
R⎟
⎠
× I_o
(EQ. 16)
Substitution of Equation 16 into Equation 1 gives:
I_droop
=
--²---
R_i
×
-----------^R-----ⁿ---^t--^c---ⁿ---^e----^t----------
R_ntcnet + R_sum
×
DCR
×
I_o
(EQ. 17)
Therefore:
R_i
=
--------²----^R----ⁿ----^t-^c---ⁿ----^e---^t---^×-----^D-----^C----^R------^×-----^I--^o---------
(R_ntcnet + R_sum) × I_droop
(EQ. 18)
Substitution of Equation 8 and application of the OCP
condition in Equation 18 gives:
R_i
=
-----²-----^×-----^(-----RR---------^n-n------^tt----^cc------^ss--------++----------^RR---------^nn-------tt---^c-c------⁾--+----^×-----^R-----R----^p-----^p------×-----^D-----^C-----^R------^×----^I--^o----^m----^a----^x-----
⎛
⎜
⎝
(_---R_{-----n---t--c---s----+-----}R_{----n----t--c---})_---×_-----R_----p--
R_ntcs + R_ntc + R_p
+
⎞
R _{s u m⎠⎟}
×
I_droopmax
(EQ. 19)
where I_omax is the full load current, I_droopmax is the
corresponding droop current. For example, given
R_sum = 1.82kΩ, R_p = 11kΩ, R_ntcs = 2.61kΩ, R_ntc = 10kΩ,
DCR = 1.3mΩ, I_omax = 22A and I_droopmax = 50µA,
Equation 19 gives R_i = 873Ω.
For resistor sensing, Equation 20 gives the DC
relationship of V_cn(s) and I_o(s).
V_Cn = R_sen × I_o
(EQ. 20)
Substitution of Equation 20 into Equation 1 gives
Equation 21:
I_droop
=
--²---
R_i
×
R_s
e
n
×
I_o
(EQ. 21)
Therefore:
R_i
=
2_----R_{-----s---e---n-----}×_----I_--o--
I_droop
(EQ. 22)
Substitution of Equation 22 and application of the OCP
condition in Equation 18 gives:
R_i
=
2_----R_{-----s---e---n-----}×_----I_{--o----m----a----x-}
I_droopmax
(EQ. 23)
where I_omax is the full load current, I_droopmax is the
corresponding droop current. For example, given
R_sen = 1mΩ, I_omax = 22A and I_droopmax = 50µA,
Equation 23 gives R_i = 880Ω.
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 12.
For inductor DCR sensing, substitution of Equation 17
into Equation 2 gives the load line slope expression in
Equation 24.
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_ntcnet + R_sum
×
DCR
(EQ. 24)
For resistor sensing, substitution of Equation 21 into
Equation 2 gives the load line slope expression in
Equation 25:
LL
=
-^V----^d---^r--^o---^o----^p-
I_o
=
2_----R_{-----s---e---n-----}×_----R_{-----d---r---o---o---p-}
R_i
(EQ. 25)
Substitution of Equation 18 and rewriting Equation 24,
or substitution of Equation 22 and rewriting Equation 25
gives the same result in Equation 26:
R_droop
=
-------^I--^o-------
I_droop
×
L
L
(EQ. 26)
One can use the full load condition to calculate R_droop.
For example, given I_omax = 22A, I_droopmax = 50µA and
LL = 7mΩ, Equation 26 gives R_droop = 3.08kΩ.
It is recommended to start with the R_droop value
calculated by Equation 26, 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
Referring to Equation 6 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 shown in
Equation 27:
V_Rimon = 3 × I_droop × R_imon
(EQ. 27)
Rewriting Equation 26 gives Equation 28:
I_droop
=
--------^I--^o--------- × LL
R_droop
(EQ. 28)
21
FN7596.0
March 8, 2010