Rainbow Electronics MAX8707 User Manual
Page 32
MAX8707
Multiphase, Fixed-Frequency Controller for
AMD Hammer CPU Core Power Supplies
32
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The capacitance value required is determined primarily
by the output transient-response requirements. Low
inductor values allow the inductor current to slew faster,
replenishing the charge removed from or added to the
output filter capacitors by a sudden load step.
Therefore, the amount of output soar when the load is
removed is a function of the output voltage and induc-
tor value. The minimum output capacitance required to
prevent overshoot (V
SOAR
) due to stored inductor ener-
gy can be calculated as:
where
η
PH
is the total number of active phases. When
using low-capacity ceramic filter capacitors, capacitor
size is usually determined by the capacity needed to
prevent V
SOAR
from causing problems during load
transients. Generally, once enough capacitance is
added to meet the overshoot requirement, undershoot
at the rising load edge is no longer a problem.
Input Capacitor Selection
The input capacitor must meet the ripple-current
requirement (I
RMS
) imposed by the switching currents.
The multiphase controllers operate out-of-phase, which
reduces the RMS input current by dividing the input cur-
rent between several staggered stages. For duty cycles
less than 100%/
η
PH
per phase, the I
RMS
requirements
can be determined by the following equation:
where
η
PH
is the total number of out-of-phase switching
regulators. The worst-case RMS current requirement
occurs when operating with V
IN
= 2
η
PH
V
OUT
. At this
point, the above equation simplifies to I
RMS
= 0.5 x
I
LOAD
/
η
PH
.
For most applications, nontantalum chemistries (ceram-
ic, aluminum, or OS-CON) are preferred due to their
resistance to inrush surge currents, typical of systems
with a mechanical switch or connector in series with the
input. If the MAX8707 is operated as the second stage
of a two-stage power-conversion system, tantalum
input capacitors are acceptable. In either configuration,
choose an input capacitor that exhibits less than 10
°C
temperature rise at the RMS input current for optimal
circuit longevity.
Setting Voltage Positioning
Voltage positioning dynamically lowers the output volt-
age in response to the load current, reducing the out-
put capacitance and processor’s power-dissipation
requirements. The controller uses two transconduc-
tance amplifiers to set the transient and DC output volt-
age droop (
Figure
2). The transient-compensation
(TRC) amplifier determines how quickly the MAX8707
responds to the load transient. The slower voltage-posi-
tioning (VPS) amplifier adjusts the steady-state regula-
tion voltage as a function of the load. This adjustability
allows flexibility in the selected current-sense resistor
value or inductor DCR, and allows smaller current-
sense resistance to be used, reducing the overall
power dissipated.
Steady-State Voltage Positioning
Connect a resistor (R
VPS
) between VPS and FBS to set
the DC steady-state droop (load line) based on the
required DC voltage-positioning slope (R
DROOP
):
where the current-sense resistance (R
SENSE
) depends
on the current-sense method, and the voltage-position-
ing amplifier’s transconductance (G
M(VPS)
) is typically
200µS as defined in the Electrical Characteristics
table
.
When the MAX8707 CRS sensing is enabled, R
SENSE
is
defined as the accurate CRS current-sense resistance:
R
SENSE
= R
CRS
when CRS sensing is enabled.
When the MAX8707 CRS sensing is disabled, the con-
troller sums together the input signals of the current-
sense inputs (CSP_, CSN_). These inputs typically use
the inductors’ DC resistance (R
DCR
) to sense the cur-
rent, so R
SENSE
is defined as the average of the effec-
tive CS current-sense resistances (see the Current
Sense section):
R
SENSE
= R
DCR
when CRS sensing is disabled.
When the inductors’ DCR (R
DCR
) is used as the cur-
rent-sense elements (for lossless sensing), R
VPS
should
include an NTC thermistor to minimize the temperature
dependence of the voltage-positioning slope.
To avoid output-voltage errors caused by the voltage-
positioning current, a second transconductance ampli-
fier generates an equivalent current on the FBS input.
Accurate MAX8707 CRS sensing is disabled by con-
necting CRSP to V
CC
.
R
R
R
G
VPS
DROOP
SENSE
M VPS
=
(
)
I
I
V
V
V
V
RMS
LOAD
PH IN
PH OUT
IN
PH OUT
=
−
η
η
η
(
)
C
I
L
V
V
OUT
LOAD MAX
PH
OUT
SOAR
≥
(
)
(
)
∆
2
2
η