ISL62883C Datasheet, PDF (27/43 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62883C
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 18 shows the desired load transient response
waveforms. Figure 24 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 24. VOLTAGE REGULATOR EQUIVALENT
CIRCUIT
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 27 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 25 conceptually
shows T1(s) measurement set-up and Figure 26
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
ISL62883C 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.
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.
Q1
Vin
GATE ^Q2
DRIVER
L
Vo
Cout
io
LOAD LINE SLOPE
20Ω
MOD.
EA
COMP
VID
LOOP GAIN = ^{CHANNEL B}
CHANNEL A
ISOLATION
TRANSFORMER
CHANNEL A
CHANNEL B
NETWORK
ANALYZER _{EXCITATION OUTPUT}
FIGURE 25. LOOP GAIN T1(s) MEASUREMENT SET-UP
Q1
VIN
GATE Q2
DRIVER
L
VO
COUT
IO
LOAD LINE SLOPE
20 ^Ω
MOD.
EA
COMP
VID
LOOP GAIN= ^{CHANNEL B}
CHANNEL A
ISOLATION
TRANSFORMER
CHANNEL A
CHANNEL B
NETWORK
ANALYZER EXCITATION OUTPUT
FIGURE 26. LOOP GAIN T2(s) MEASUREMENT SET-UP
27
FN7557.1
March 18, 2010