參數(shù)資料
型號(hào)： MAX1714
廠商： Maxim Integrated Products, Inc.
英文描述： High-Speed Step-Down Controller for Notebook Computers
中文描述： 高速、降壓型控制器，用于筆記本電腦
文件頁(yè)數(shù)： 17/24頁(yè)
文件大?。?/td> 443K
代理商： MAX1714
M
High-Speed Step-Down Controller
for Notebook Computers
______________________________________________________________________________________
17
the current-limit tolerance. A 1μA minimum divider current
is recommended.
Output Capacitor Selection
The output filter capacitor must have low enough effective
series resistance (ESR) to meet output ripple and load-
transient requirements, yet have high enough ESR to sat-
isfy stability requirements. Also, the capacitance value
must be high enough to absorb the inductor energy
going from a full-load to no-load condition without tripping
the overvoltage protection circuit.
In CPU V
CORE
converters and other applications where
the output is subject to violent load transients, the output
capacitor’s size 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
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple:
The actual microfarad 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 tantalums,
OS-CONs, and other electrolytics).
When using low-capacity filter capacitors such as ceram-
ic or polymer types, capacitor size is usually determined
by the capacity needed to prevent V
SAG
and 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 (also, see the V
SAG
and V
SOAR
equa-
tion in the
Transient Response
section).
Output Capacitor Stability Considerations
Stability is determined by the value of the ESR zero rela-
tive to the switching frequency. The point of instability is
given by the following equation:
f
ESR
For a typical 300kHz application, the ESR zero frequency
must be well below 95kHz, preferably below 50kHz.
Tantalum and OS-CON capacitors in widespread use at
the time of publication have typical ESR zero frequencies
of 25kHz. In the design example used for inductor selec-
tion, the ESR needed to support 50mVp-p ripple is
60mV/2.7A = 22m
. Two 470μF/4V Kemet T510 low-ESR
tantalum capacitors in parallel provide 22m
max ESR.
Their typical combined ESR results in a zero at 27kHz,
well within the bounds of stability.
Don’t put high-value ceramic capacitors directly across
the feedback sense point without taking precautions to
ensure stability. Large ceramic capacitors can have a
high ESR zero frequency and cause erratic, unstable
operation. However, it’s easy to add enough series resis-
tance by placing the capacitors a couple of inches
downstream from the feedback sense point, which
should be as close as possible to the inductor (see the
All-Ceramic-Capacitor Application
section).
Unstable operation manifests itself in two related but dis-
tinctly different ways: double-pulsing and fast-feedback
loop instability.
Double-pulsing occurs due to noise on the output or
because the ESR is so low that there isn’t enough volt-
age 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, resulting
in nothing worse than increased output ripple. However,
it can indicate the possible presence of loop instability,
which is caused by insufficient ESR.
Loop instability can result in oscillations at the output
after line or load perturbations that can trip the overvolt-
age protection latch or cause the output voltage to fall
below the tolerance limit.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully
observe the output voltage ripple envelope for over-
shoot and ringing. It can help to simultaneously monitor
the inductor current with an AC current probe. Don’t
allow more than one cycle of ringing after the initial
step-response under- or overshoot.
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents.
Nontantalum chemistries (ceramic, aluminum, or OS-
CON) are preferred due to their resistance to power-up
surge currents.
I
I
V
V
-V
V
RMS
LOAD
OUT
IN
OUT
IN
=
(
)
f
π
where f
1
2
R
C
ESR
ESR
F
=
=
π
R
Vp p
LIR I
ESR
LOAD MAX
≤
-
(
)
R
V
I
ESR
DIP
(
LOAD MAX
≤
)
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