lx1676 Microsemi Corporation, lx1676 Datasheet - Page 13

• Manufacturer Part Number: lx1676
• Description: Mobile Amd Athlontm Vrm Controller
• Manufacturer: Microsemi Corporation

Copyright © 2000
Revision: 1.0, 4/12/2005
O
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:
where
value and ESR is the Effective Series Resistance of the
output capacitor.
maximum output current.
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:
and
The inductance value can be calculated by:
O
performance specifications.
(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
UTPUT
UTPUT
The output inductor should be selected to meet the
∆I is the inductor ripple current, L is the output inductor
∆I should typically be in the range of 20% to 40% of the
.
The output capacitor is sized to meet ripple and transient
TM
I
C
NDUCTOR
APACITOR
∆I
T
L
V
RISE
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
=
RIPPLE
=
T
FALL
V
V
=
IN
IN
L
=
×
−
=
−
∆I
ESR
L
(
V
Higher inductance results in
L
V
V
×
OUT
Effective Series Resistance
OUT
IN
V
×
−
∆I
∆I
OUT
I
V
×
RIPPLE
×
OUT
f
D
f
D
s
)
s
A P P L I C A T I O N N O T E
Integrated Products Division
®
Microsemi
microprocessor power supply, the output capacitor is
usually selected from ESR instead of capacitance or RMS
current capability.
requirements usually has a larger capacitance and current
capability than strictly needed
The allowed ESR can be found by:
maximum load current step change, and VEX is the
allowed output voltage excursion in the transient.
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.
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.
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.
I
to filter the pulsating current generated by the buck
converter to reduce interference to other circuits connected
to the same 5V rail.
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:
The maximum RMS value of 0.25I
25% or 75%.
Mobile AMD Athlon™ VRM Controller
NPUT
Where IRIPPLE is the inductor ripple current, ∆I is the
Electrolytic capacitors can be used for the output
There is frequently strong pressure to use the least
The input capacitor and the input inductor, if used, are
Where I
C
APACITOR
L
P
is the inductor current and d is the duty cycle.
The OS-CON series from Sanyo generally
RODUCTION
I
RMS
ESR
=
A capacitor that satisfies the ESR
×
I
L
(
I
In addition, the input capacitor
RIPPLE
d(
D
0
5 .
ATA
+
−
∆I
d)
)
S
for d < 0.5
<
L
HEET
V
will occur when d =
EX
LX1676
Page 13