Rainbow Electronics MAX8707 User Manual
Page 27
results in high light-load efficiency. Trade-offs in PFM
noise vs. light-load efficiency are made by varying the
inductor value. Generally, low inductor values produce
a broader efficiency vs. load curve, while higher values
result in higher full-load efficiency (assuming that the
coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values
include larger physical size and degraded load-tran-
sient response (especially at low input-voltage levels).
Current Sense
The output current of each phase is sensed differential-
ly. Each phase of the MAX8707 has an independent
return path for fully differential current-sense. A low off-
set voltage and high-gain (10V/V) differential current
amplifier at each phase allow low-resistance current-
sense resistors to be used to minimize power dissipa-
tion. Sensing the current at the output of each phase
offers advantages, including less noise sensitivity, more
accurate current sharing between phases, and the flexi-
bility of using either a current-sense resistor or the DC
resistance of the output inductor.
Using the DC resistance (R
DCR
) of the output inductor
allows higher efficiency. In this configuration, the initial
tolerance and temperature coefficient of the inductor’s
DCR must be accounted for in the output-voltage
droop-error budget. This current-sense method uses an
RC filtering network to extract the current information
from the output inductor (
Figure
7). The time constant
of the RC network should match the inductor’s time
constant (L/R
DCR
):
where C
SENSE
is the sense capacitor and R
EQ
is the
equivalent sense resistance. To minimize the current-
sense error due to the current-sense inputs’ bias cur-
rent (I
CSP
_ and I
CSN
_), choose R
EQ
less than 2k
Ω and
use the above equation to determine the sense capaci-
tance (C
SENSE
). Choose capacitors with 5% tolerance
and resistors with 1% tolerance specifications.
Temperature compensation is recommended for this
current-sense method.
When using a current-sense resistor for accurate out-
put-voltage positioning (CRSP to CRSN for the
MAX8707), differential RC-filter circuits should be used
to cancel the equivalent series inductance of the cur-
rent-sense resistor (
Figure
7). Similar to inductor DCR-
sensing methods, the RC filter’s time constant should
match the L/R time constant formed by the current-
sense resistor’s parasitic inductance:
where L
ESL
is the equivalent series inductance of the
current-sense resistor, R
SENSE
is the current-sense
resistance value, C
SENSE
is the compensation capaci-
tor, and R
EQ
is the equivalent compensation resistance.
Current Balance
The fixed-frequency, multiphase, current-mode archi-
tecture automatically forces the individual phases to
remain current balanced. After the oscillator triggers an
on-time, the controller does not terminate the on-time
until the amplified differential current-sense voltage
reaches the integrated threshold voltage (V
REF
- V
TRC
).
This control scheme regulates the peak inductor cur-
rent of each phase, forcing them to remain properly
balanced. Therefore, the average inductor-current vari-
ation depends mainly on the variation in the current-
sense element and inductance value.
Peak/Average Current Limit
The MAX8707 current-limit circuit employs a fast peak
inductor current-sensing algorithm. Once the current-
sense signal (CSP to CSN) of the active phase exceeds
the peak current-limit threshold, the PWM controller ter-
minates the on-time. The MAX8707 also includes a
slower average current sense that uses a current-sense
resistor between CRSP and CRSN to accurately limit
the inductor current. When this average current-sense
threshold is exceeded, the current-limit circuit lowers
the peak current-limit threshold, effectively lowering the
average inductor current. See the Current Limit section
in the Design Procedure section.
L
R
R
C
ESL
SENSE
EQ
SENSE
=
L
R
R
C
DCR
EQ
SENSE
=
MAX8707
Multiphase, Fixed-Frequency Controller for
AMD Hammer CPU Core Power Supplies
______________________________________________________________________________________
27
ON-TIME
TIME
0
INDUCTOR CURRENT
t
ON(SKIP)
=
V
OUT
V
IN
f
SW
I
IDLE
I
LOAD
≈ I
LOAD(SKIP)
2
Figure
6. Pulse-Skipping/Discontinuous Crossover Point