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參數(shù)資料
| 型號： | MAX8632ETI |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | Quadruple 2-Input Positive-NAND Gates With Open-Drain Outputs 14-CDIP -55 to 125 |
| 中文描述： | 集成的DDR電源適用于臺式機(jī)，筆記本電腦解決方案和圖形卡 |
| 文件頁數(shù)： | 20/29頁 |
| 文件大?。?/td> | 512K |
| 代理商： | MAX8632ETI |
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M
Stability Requirements
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
OUT
consists of multiple same-value capacitors, as
in the Typical Applications Circuit of Figure 8, the f
ESR
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 12m
(max) of R
ESR
. This results in a zero at
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
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.
VTT Output Capacitor Selection (LDO)
A minimum value of 20μF is needed to stabilize the VTT
output. This value of capacitance limits the regulator’s
unity-gain bandwidth frequency to approximately 1.8MHz
(typ) to allow adequate phase margin for stability. To
keep the capacitor acting as a capacitor within the regu-
lator’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 may need to be
greater than 20μF if the load current exceeds 1.5A, but
can be smaller than 20μF if the maximum load current
is less than 1.5A. As a guideline, choose the minimum
capacitance and maximum ESR for the output capaci-
tor using the following:
R
ESR
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.
VTTR Output Capacitor Selection (LDO)
The VTTR buffer is a scaled-down version of the VTT
regulator, with much smaller output transconductance.
Its compensation cap can therefore be smaller, and its
ESR larger, than what is required for its larger counter-
part. For typical applications requiring load current up
to ±15mA, a ceramic cap with a minimum value of 1μF
is recommended (R
ESR
< 0.3
). Connect this cap
between VTTR and the analog ground plane.
VTTI Input Capacitor Selection (LDO)
Both the VTT and VTTR output stages are powered
from the same VTTI input. Their output voltages are ref-
erenced to the same REFIN input. The value of the VTTI
bypass capacitor is chosen to limit the amount of rip-
ple/noise at VTTI, or the amount of voltage dip during a
load transient. Typically VTTI is connected to the output
of the buck regulator, which already has a large bulk
.
f
C
I
A
GBW
OUT
LOAD
1 5
=
×
36
.
.
_
_
C
F
I
A
A
R
m
I
OUT MIN
LOAD
1 5
1 5
ESR MAX
LOAD
=
×
=
×
20
5
μ
:
×
f
f
where
f
R
C
ESR
SW
π
ESR
ESR
OUT
≤
=
×
π
1
2
Integrated DDR Power-Supply Solution for
Desktops, Notebooks, and Graphic Cards
20
______________________________________________________________________________________
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| MAX8632ETI+TGD2 | 制造商:Rochester Electronics LLC 功能描述: 制造商:Maxim Integrated Products 功能描述: |
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