ISL62881C_14 Datasheet, PDF (22/37 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62881C, ISL62881D
Substitution of Equation 28 into Equation 27 gives
Equation 29:
V_Rimon
=
3_----I_--o-----×_----L_----L_--
R_droop
×
R_im
on
(EQ. 29)
Rewriting Equation 29 and application of full load
condition gives Equation 30:
R_imon = ^V-----^R----ⁱ--^m--₃--^o--_I--^no----^××-----^RL----_L^d----^r--^o---^o----^p-
(EQ. 30)
For example, given LL = 7mΩ, R_droop = 3.08kΩ,
V_Rimon = 999mV at I_omax = 22A, Equation 30 gives
R_imon = 6.66kΩ.
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.
VID
Zout(s) = LL
iO
VR
+
LOAD ^VO
-
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.
T2(s) is the voltage loop gain with closed droop loop. It
has more meaning of output voltage response.
Design the compensator to get stable T1(s) and T2(s)
with sufficient phase margin, and output impedance
equal or smaller than the load line slope.
L
VO
Q1
VIN
GATE Q2
DRIVER
COUT
IO
LOAD LINE SLOPE
-
MOD
EA
COMP ⁺
VID
20Ω
CHANNEL B
LOOP GAIN =
CHANNEL A
+
+
ISOLATION
TRANSFORMER
CHANNEL A
NETWORK
ANALYZER
CHANNEL B
EXCITATION OUTPUT
FIGURE 21. LOOP GAIN T1(s) MEASUREMENT SET-UP
Q1
VIN
GATE ^Q2
DRIVER
L
VO
COUT
IO
FIGURE 20. VOLTAGE REGULATOR EQUIVALENT
Intersil provides a Microsoft Excel-based spreadsheet to
help design the compensator and the current sensing
network, so the VR achieves constant output impedance
as a stable system. Figure 23 shows a screenshot of the
spreadsheet.
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 ISL62881C regulator.
LOAD LINE SLOPE
MOD
COMP
-
EA
+
VID
+
+
20Ω
CHANNEL B
LOOP GAIN =
CHANNEL A
ISOLATION
TRANSFORMER
CHANNEL A
CHANNEL B
NETWORK
ANALYZER _{EXCITATION OUTPUT}
FIGURE 22. LOOP GAIN T2(s) MEASUREMENT SET-UP
22
FN7596.0
March 8, 2010