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| 型号： | LX1676-CPWTR |
| 厂商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分类： | 稳压器 |
| 英文描述： | 1.5 A SWITCHING CONTROLLER, 1100 kHz SWITCHING FREQ-MAX, PDSO38 |
| 封装： | PLASTIC, TSSOP-38 |
| 文件页数： | 5/16页 |
| 文件大小： | 387K |
| 代理商： | LX1676-CPWTR |
LX1676
PRELIMINARY DATA SHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 13
WWW
.Microse
m
i
.CO
M
Mobile AMD Athlon
VRM Controller
I N T E GR A T ED
PRODUCT S
Copyright
2000
Revision: 0.4b, 9/14/2004
APPLICATION NOTE
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:
RIPPLE
I
ESR
V
×
=
where
s
D
L
V
I
OUT
IN
f
×
=
I is the inductor ripple current, L is the output inductor
value and ESR is the Effective Series Resistance of the
output capacitor.
I 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 inductor current slew rate in response to the load-
current step change, I, resulting in more output-capacitor
voltage droop. When using electrolytic capacitors, the
capacitor voltage droop is usually negligible, due to the
large capacitance
The inductor-current rise and fall times are:
()
OUT
IN
RISE
V
I
L
T
×
=
and
OUT
FALL
V
I
L
T
×
=
.
The inductance value can be calculated by:
s
D
I
V
L
OUT
IN
f
×
=
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 from ESR instead of capacitance or RMS
current capability.
A capacitor that satisfies the ESR
requirements usually has a larger capacitance and current
capability than strictly needed
The allowed ESR can be found by:
()
EX
RIPPLE
V
I
ESR
<
+
×
Where IRIPPLE is the inductor ripple current, I is the
maximum load current step change, and VEX is the
allowed output voltage excursion in the transient.
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 increase 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. Microsemi’s demonstration boards use
the CDE Polymer AL-EL (ESRE) filter capacitors, which
are aluminum electrolytic, and have demonstrated
reliability.
The OS-CON 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 CDE Polymer AL-EL (ESRE) filter
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, if used, 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 for the buck converter. The
capacitor should be rated to handle the RMS input current
requirement. The RMS input current is:
d)
5
.
0
d(
I
L
RMS
=
for d < 0.5
Where IL is the inductor current and d is the duty cycle.
The maximum RMS value of 0.25IL will occur when d =
25% or 75%.
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LL
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AA
TT
IIOO
NN
SS
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