lx1673 Microsemi Corporation, lx1673 Datasheet - Page 9

• Manufacturer Part Number: lx1673
• Description: High Frequency Pwm Regulator
• Manufacturer: Microsemi Corporation

Copyright © 2004
Rev 1.0, 3/18/2005
O
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:
and ESR is the Effective Series Resistance of the output
capacitor.
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
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.
UTPUT
UTPUT
The output inductor should be selected to meet the
where
∆I is the inductor ripple current, L is the output inductor value
∆I should typically be in the range of 20% to 40% of the
and
.
The inductance value can be calculated by
The output capacitor is sized to meet ripple and transient
The inductor-current rise and fall times are:
TM
I
C
NDUCTOR
APACITOR
∆I
L
T
V
RISE
=
RIPPLE
=
T
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
In an advanced microprocessor power
FALL
V
V
=
IN
IN
L
=
×
−
=
−
∆I
ESR
L
(
V
L
V
V
×
OUT
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
supply, the output capacitor is usually selected for 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:
load current step change, and V
excursion in the transient.
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.
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.
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
current requirements. The RMS current is:
maximum value occurs when d = 50%, then I
input and output voltages in the range of 2 to 3V, the required RMS
current is very close to 0.5I
output voltage rises and how large the inductor current is required
to charge the output capacitor. The output voltage will follow the
voltage at the SS pin if the required inductor current does not
exceed the maximum allowable current for the inductor.
I
S
NPUT
OFT
There is frequently strong pressure to use the least expensive
The input capacitor and the input inductor, if used, are to filter
Where I
The value of the soft-start capacitor determines how fast the
Where I
Electrolytic capacitors can be used for the output capacitor, but
-S
High Frequency PWM Regulator
C
TART
APACITOR
RIPPLE
L
is the inductor current and d is the duty cycle. The
C
P
APACITOR
RODUCTION
is the inductor ripple current, ∆I is the maximum
ESR
I
RMS
×
L
(
.
I
=
RIPPLE
I
L
D
EX
ATA
+
d(1
∆I
is the allowed output voltage
−
)
S
d)
<
HEET
V
EX
RMS
=0.5I
L
. For a 5V
LX1673
Page 9