Rainbow Electronics MAX8707 - Manual (Page 19)

Transient-Droop Amplifier
The MAX8707 controller includes a transient-droop
transconductance amplifier to handle the instantaneous
load transients typical of CPU applications. The tran-
sient-droop amplifier sets the correct voltage-positioning
slope during a load transient, complimenting the slower
steady-state voltage-positioning amplifier. The current-
sense inputs differentially sense the voltage across the
CSP_ and CSN_ current-sense element (inductor’s DCR
or current-sense resistor). The transconductance amplifi-
er’s output connects to the regulator’s transient-response
input (TRC), so the resistance between TRC and the ref-
erence voltage (REF) determines the transient voltage-
positioning gain as defined in the Multiphase,
Fixed-Frequency Design Procedure section.
If voltage positioning is not required, R DROOP is
defined by the maximum output-voltage sag with the
worst-case transient load (
∆V
OUT /
∆I
OUT ) and is sub-
ject to stability requirements. TRC is high impedance in
shutdown.
Differential Remote Sense
The multiphase controllers include differential, remote-
sense inputs to eliminate the effects of voltage drops
down the PC board traces and through the processor’s
power pins.
The MAX8707 GNDS amplifier adds an offset directly to
the target voltage, adjusting the output voltage to coun-
teract the voltage drop in the ground path. Connect the
feedback sense (FBS), voltage-positioning resistor
(R
VPS
), and ground-sense (GNDS) inputs directly to the
processor’s core supply remote- sense outputs.
Integrator Amplifier
An integrator amplifier forces the DC average of the
VPS voltage to equal the target voltage. This transcon-
ductance amplifier integrates the feedback voltage and
provides a fine adjustment to the regulation voltage
(
Figure 2), allowing accurate DC output-voltage regula-
tion regardless of the output ripple voltage. The integra-
tor amplifier has the ability to shift the output voltage by
±100mV (typ). The differential input voltage range is at
least
±60mV total, including DC offset and AC ripple.
The integration time constant can be set easily with an
external compensation capacitor at the CCV pin. Use a
470pF x (4 /
η
PH ) or greater ceramic capacitor.
The MAX8707 disables the integrator by connecting the
amplifier inputs together at the beginning of all transi-
tions done in pulse-skipping mode (SKIP = high). The
integrator remains disabled until 20µs after the transition
is completed (the internal target settles) and the output
is in regulation (edge detected on the error comparator).
When voltage positioning is disabled (VPS = FBS), the
transient droop must be less than the
±80mV minimum
adjustment range of the integrator amplifier to guaran-
tee proper DC output-voltage accuracy.
Offset Amplifier
The multiphase controllers include a fifth amplifier used
to add small offsets to the voltage-positioned load line.
The offset amplifier sums directly with the target volt-
age, making the offset gain independent of the DAC
code. This amplifier has the ability to offset the output
by
±100mV. The offset is adjusted using resistive volt-
age-dividers at the OFS input. For inputs from 0 to 0.8V,
the offset amplifier adds a negative offset to the output
that is equal to 1/8th the voltage appearing at the OFS
input (V
OFFSET
= -0.125 x V
OFS
). For inputs from 1.2V
MAX8707
Multiphase, Fixed-Frequency Controller for
AMD Hammer CPU Core Power Supplies
______________________________________________________________________________________ 19
MAX8707
AMD HAMMER
COMPONENTS
DESIGNATION
Circuit of Figure 1
Input Voltage Range 7V to 24V
VID Output Voltage (D4–D0) 1.50V (D4–D0 = 00010)
SUSV Suspend Voltage
(SUS = High)
0.80V
Maximum Load Current 80A
Number of Phases ( η
TOTAL
)
4 phases
(1) MAX8705 + (2) MAX8702
Inductor (Per Phase)
0.56µH, 1.6m Ω
Panasonic ETQP4LR56WFC
Switching Frequency
(Per Phase)
300kHz (OSC = REF)
High-Side MOSFET
(N
H , Per Phase)
Siliconix (1) Si7892DP
Low-Side MOSFET
(N
L
, Per Phase)
Siliconix (2) Si7356DP
Total Input Capacitance (C IN )
(8) 10µF, 25V
TDK C3225X7R1E106M
Taiyo Yuden
TMK325BJ106MN
Total Output Capacitance
(C
OUT )
(6) 330µF, 2.5V, 9m Ω
Sanyo 2R5TPE330M9
Current-Sense Resistor
(R
SENSE )
1.0m Ω
Panasonic ERJM1WTJ1M0U
Table 1. Component Selection for
Standard Multiphase Applications