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| 型号： | LX1681CDM |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 230 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, SOIC-8 |
| 文件页数： | 6/9页 |
| 文件大小： | 145K |
| 代理商： | LX1681CDM |
V O L T AGE -M ODE PWM C ONTROLLERS
LX1681/1682
PRODUCT DA T ABOOK 1996/1997
Copyright 1999
Rev. 1.0 5/99
6
P RODUCTION D AT A S HEET
APPLICA TION INFORMA TION
OUTPUT INDUCTOR
The output inductor should be selected to meet the requirements
of the output voltage ripple in steady-state operation and the
inductor current slew-rate during transient.
The peak-to-peak output voltage ripple is:
V
RIPPLE = ESR * IRIPPLE
where
I
RIPPLE =
*
I
RIPPLE is the inductor ripple current, L is the output inductor
value and ESR is the Effective Series Resistance of the output
capacitor.
I
RIPPLE should typically be in the range of 20% to 40% of the
maximum output current. Higher inductance results in lower
output voltage ripple, allowing slightly higher ESR to satisfy the
transient specification. Higher inductance also slows the induc-
tor current slew rate in response to the load-current step change,
I, resulting in more output-capacitor voltage droop. The
inductor-current rise and fall times are:
T
RISE = L * I/(VIN – VOUT )
and
T
FALL = L * I/VOUT
When using electrolytic capacitors, the capacitor voltage
droop is usually negligible, due to the large capacitance.
OUTPUT CAPACITOR
The output capacitor is sized to meet ripple and transient
performance specifications. Effective Series Resistance (ESR) is
a critical parameter. When a step load current occurs, the output
voltage will have a step that equals the product of the ESR and
the current step,
I. In an advanced microprocessor power
supply, the output capacitor is usually selected for ESR instead
of capacitance or RMS current capability.
A capacitor that
satisfies the ESR requirement usually has a larger capacitance and
current capability than strictly needed. The allowed ESR can be
found by:
ESR * (I
RIPPLE + I ) < VEX
where I
RIPPLE is the inductor ripple current, I is the maximum
load current step change, and V
EX is the allowed output voltage
excursion in the transient.
(V
IN - VOUT )
f
SW * L
V
OUT
V
IN
OUTPUT CAPACITOR (continued)
Electrolytic capacitors can be used for the output capacitor,
but are less stable with age than tantalum capacitors. As they age,
their ESR degrades, reducing the system performance and
increasing the risk of failure. It is recommended that multiple
parallel capacitors be used, so that, as ESR increases with age,
overall performance will still meet the processor’s requirements.
There is frequently strong pressure to use the least expensive
components possible, however, this could lead to degraded
long-term reliability, especially in the case of filter capacitors.
Linfinity’s demonstration boards use Sanyo MV-GX filter capaci-
tors, which are aluminum electrolytic, and have demonstrated
reliability. The Oscon series from Sanyo generally provides the
very best performance in terms of long term ESR stability and
general reliability, but at a substantial cost penalty. The MV-GX
series provides excellent ESR performance at a reasonable cost.
Beware of off-brand, very low-cost filter capacitors, which have
been shown to degrade in both ESR and general electrolytic
characteristics over time.
INPUT CAPACITOR
The input capacitor and the input inductor are to filter the
pulsating current generated by the buck converter to reduce
interference to other circuits connected to the same 5V rail. In
addition, the input capacitor provides local de-coupling the buck
converter. The capacitor should be rated to handle the RMS
current requirement. The RMS current is:
I
RMS = IL √ d(1-d)
where I
L is the inductor current and the d is the duty cycle. The
maximum value, when d = 50%, I
RMS = 0.5IL.
For 5V input and
output in the range of 2 to 3V, the required RMS current is very
close to 0.5I
L.
SOFT-START CAPACITOR
The value of the soft-start capacitor determines how fast the
output voltage rises and how large the inductor current is
required to charge the output capacitor. The output voltage will
follow the voltage at SS pin if the required inductor current does
not exceed the maximum current in the inductor.
The SS pin voltage can be expressed as:
V
SS = VSET (1-e
-t/RssCss)
where V
SET is the reference voltage.
R
SS and CSS are soft start
resistor and capacitor. The required inductor current for the
output capacitor to follow the SS-pin voltage equals the required
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