MAX8550-MAX8551 MAXIM [Maxim Integrated Products], MAX8550-MAX8551 Datasheet - Page 20

• Manufacturer Part Number: MAX8550-MAX8551
• Description: Integrated DDR Power-Supply Solutions for Desktops, Notebooks, and Graphic Cards
• Manufacturer: MAXIM [Maxim Integrated Products]

The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as to
the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value (this is true of tanta-
lums, OSCONs, polymers, and other electrolytics).
When using low-capacity filter capacitors, such as
ceramic capacitors, size is usually determined by the
capacity needed to prevent V
causing problems during load transients. Generally,
once enough capacitance is added to meet the over-
shoot requirement, undershoot at the rising load edge
is no longer a problem (see the V
tions in the Transient Response section). However, low-
capacity filter capacitors typically have high-ESR zeros
that can affect the overall stability (see the Stability
Requirements section).
For Quick-PWM controllers, stability is determined by
the value of the ESR zero relative to the switching fre-
quency. The boundary of instability is given by the fol-
lowing equation:
If C
in the Typical Applications Circuit of Figure 8, the f
remains the same as that of a single capacitor.
For a typical 600kHz application, the ESR zero frequen-
cy must be well below 190kHz, preferably below
100kHz. Two 150µF/4V Sanyo POS capacitors are used
to provide 12mΩ (max) of R
42kHz, well within the bounds of stability.
Do not put high-value ceramic capacitors directly
across the feedback sense point without taking precau-
tions to ensure stability. Large ceramic capacitors can
have a high-ESR zero frequency and cause erratic,
unstable operation. However, it is easy to add enough
series resistance by placing the capacitors a couple of
inches downstream from the feedback sense point,
which should be as close as possible to the inductor.
Unstable operation manifests itself in two related but
distinctly different ways: double pulsing and fast-feed-
back loop instability. Double pulsing occurs due to
noise on the output or because the ESR is so low that
there is not enough voltage ramp in the output voltage
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
where
20
OUT
______________________________________________________________________________________
:
consists of multiple same-value capacitors, as
f
ESR
=
f
ESR
2
π
ESR
×
≤
Stability Requirements
. This results in a zero at
R
SAG
f
ESR
SAG
SW
π
1
and V
and V
×
C
OUT
SOAR
SOAR
equa-
from
ESR
signal. This “fools” the error comparator into triggering
a new cycle immediately after the 400ns minimum off-
time period has expired.
Double pulsing is more annoying than harmful, result-
ing in nothing worse than increased output ripple.
However, it can indicate the possible presence of loop
instability due to insufficient ESR. Loop instability can
result in oscillations at the output after line or load
steps. Such perturbations are usually damped but can
cause the output voltage to rise above or fall below the
tolerance limits. The easiest method for checking stabil-
ity is to apply a very fast zero-to-max load transient and
carefully observe the output-voltage-ripple envelope for
overshoot and ringing. It can help to simultaneously
monitor the inductor current with an AC current probe.
Do not allow more than one cycle of ringing after the
initial step-response under/overshoot.
A minimum value of 60µF is needed to stabilize the VTT
output for load currents up to ±1.5A. This value of capaci-
tance limits the regulator’s unity-gain bandwidth frequen-
cy to about 700kHz (typ) to allow adequate phase margin
for stability. To keep the capacitor acting as a capacitor
within the regulator’s bandwidth, it is important that
ceramic caps with low ESR and ESL be used.
Since the gain bandwidth is also determined by the
transconductance of the output FETs, which increases
with load current, the output capacitor needs to be
greater than 60µF if the load current exceeds 1.5A, but
can be smaller than 60µF if the maximum load current
is less than 1.5A. As a rule, choose the minimum
capacitance and maximum ESR for the output capaci-
tor using the following:
RESR value is measured at the unity-gain-bandwidth
frequency given by approximately:
Once these conditions for stability are met, additional
capacitors including those of electrolytic and tantalum
types can be connected in parallel to the ceramic
capacitor (if desired) to further suppress noise or volt-
age ripple at the output.
VTT Output Capacitor Selection (LDO)
C
R
ESR MAX
OUT MIN
f
GBW
_
_
=
C
=
=
OUT
40
60
5
m
µ
Ω
×
F
×
×
I
LOAD
1 5
.
I
I
LOAD
1 5
LOAD
1 5
A
.
.
A
A