Rainbow Electronics MAX8707 User Manual
Page 31
MAX8707
Multiphase, Fixed-Frequency Controller for
AMD Hammer CPU Core Power Supplies
______________________________________________________________________________________
31
Current Limit
Peak Inductor Current Limit (ILIM(PK))
The MAX8707 overcurrent protection employs a peak
current-sensing algorithm that uses either current-
sense resistors or the inductor’s DCR as the current-
sense element (see the Current Sense section). Since
the controller limits the peak inductor current, the maxi-
mum average load current is less than the peak cur-
rent-limit threshold by an amount equal to half the
inductor ripple current. Therefore, the maximum load
capability is a function of the current-sense resistance,
inductor value, switching frequency, and input voltage.
When combined with the undervoltage-protection cir-
cuit, this current-limit method is highly effective.
The peak current-limit threshold is set with a single exter-
nal resistor between ILIM(PK) and analog ground, where
the resistor is determined by the following equation:
where R
SENSE
is the resistance value of the current-
sense element (inductors’ DCR or current-sense resis-
tor), R
TRC
is the resistance between TRC and REF, and
I
PKLIMIT
is the desired peak current limit (per phase).
The peak current-limit-threshold voltage adjustment
range is from 20mV to 80mV.
The peak current-limit circuit also prevents excessive
reverse inductor currents when V
OUT
is sinking current.
The negative current-limit threshold is equivalent to the
positive current limit, and tracks the positive current limit
when R
ILIM(PK)
or R
TRC
are adjusted. When a phase
drops below the negative current limit, the controller acti-
vates an on-time pulse at the next clock edge, regard-
less of the error-amplifier state, until the inductor current
rises above the negative current-limit threshold.
Average Inductor Current-Limit
(ILIM(AVE))
The MAX8707 also uses the accurate CRSP to CRSN cur-
rent-sense voltage to limit the average current per phase.
When the average current-limit threshold is exceeded,
the controller internally reduces the peak inductor cur-
rent-limit threshold (ILIM(PK)) until the average current
remains within the programmed limits. When the accurate
current sensing is disabled (CRSP = V
CC
), the average
current-limit circuit is disabled.
The average current-limit threshold defaults to 25mV if
ILIM(AVE) is connected to V
CC
. In adjustable mode, the
average current-limit threshold voltage is precisely
1/20th the voltage difference between ILIM(AVE) and
the reference:
The logic threshold for switchover to the 25mV default
value is approximately V
CC
- 1V. The average current-
limit circuit also prevents against excessive reverse
inductor current when V
OUT
is sinking current. The neg-
ative current-limit threshold is equivalent to the positive
current limit, and tracks the positive current limit when
V
LAVE
is adjusted.
Output-Capacitor Selection
The output filter capacitor must have low enough effec-
tive series resistance (ESR) to meet output ripple and
load-transient requirements. In CPU V
CORE
converters
and other applications where the output is subject to
large load transients, the output capacitor’s size typi-
cally depends on how much ESR is needed to prevent
the output from dipping too low under a load transient.
Ignoring the sag due to finite capacitance:
In non-CPU applications, the output capacitor’s size
often depends on how much ESR is needed to maintain
an acceptable level of output ripple voltage. The output
ripple voltage of a step-down controller equals the total
inductor ripple current multiplied by the output capaci-
tor’s ESR. When operating multiphase systems out-of-
phase, the peak inductor currents of each phase are
staggered, resulting in lower output ripple voltage
(V
RIPPLE
) by reducing the total inductor ripple current.
For nonoverlapping, multiphase operation (V
IN
≥ η
PH
x
V
OUT
), the maximum ESR to meet the output-ripple-
voltage requirement is:
where
η
PH
is the total number of active phases, and f
SW
is the switching frequency per phase. The actual capac-
itance value required relates to the physical size need-
ed to achieve low ESR, as well as to the chemistry of the
capacitor technology. Thus, the capacitor selection is
usually limited by ESR and voltage rating rather than by
capacitance value (this is true of polymer types).
R
V
f
L
V
V
V
V
ESR
IN SW
IN
PH
OUT
OUT
RIPPLE
≤
−
(
)
η
(
)
(
)
R
R
V
I
ESR
PCB
STEP
LOAD MAX
+
≤
∆
V
V
V
LAVE
REF
ILIM AVE
=
−
(
)
20
R
V
R
I
R
ILIM PK
TRC
PKLIMIT
SENSE
(
)
=
Ч
8