ISL62881_14 Datasheet, PDF (21/35 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62881, ISL62881B
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 = 14A and I_droopmax = 14µA, Equation 23 gives R_i = 2kΩ.
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
×
D
CR
(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- = ²----^R-----^s--^e----ⁿ----^×-----^R----^d---^r--^o---^o----^p-
I_o
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------- × LL
I_droop
(EQ. 26)
One can use the full load condition to calculate R_droop. For
example, given I_omax = 14A, I_droopmax = 14µA and LL = 7mΩ,
Equation 26 gives R_droop = 7kΩ.
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)
Substitution of Equation 28 into Equation 27 gives Equation 29:
V_Rimon
=
3_----I_--o-----×_----L_----L_-
R_droop
×
R_imo
n
(EQ. 29)
Rewriting Equation 29 and application of full load condition gives
Equation 30:
R_imon
=
V_{----R----i-m-----o----n----}×_-----R_{----d---r--o---o----p-}
3I_o × LL
(EQ. 30)
For example, given LL = 7mΩ, R_droop = 7kΩ, V_Rimon = 963mV at
I_omax = 14A, Equation 30 gives R_imon = 22.9kΩ.
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 14 shows the desired load transient response waveforms.
Figure 20 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.
Zout(s) = LL
iO
VID
VR
+
LOAD
VO
-
FIGURE 20. VOLTAGE REGULATOR EQUIVALENT CIRCUIT
A VR with active droop function is a dual-loop system consisting
of a voltage loop and a droop loop which is a current loop.
However, neither loop alone is sufficient to describe the entire
system. The spreadsheet shows two loop gain transfer functions,
T1(s) and T2(s), that describe the entire system. Figure 21
conceptually shows T1(s) measurement set-up and Figure 22
conceptually shows T2(s) measurement set-up. The VR senses
the inductor current, multiplies it by a gain of the load line slope,
then adds it on top of the sensed output voltage and feeds it to
the compensator. T(1) is measured after the summing node, and
T2(s) is measured in the voltage loop before the summing node.
The spreadsheet gives both T1(s) and T2(s) plots. However, only
T2(s) can be actually measured on an ISL62881 regulator.
T1(s) is the total loop gain of the voltage loop and the droop loop.
It always has a higher crossover frequency than T2(s) and has
more meaning of system stability.
21
FN6924.3
June 16, 2011