LX1671
Multiple Output LoadSHARE™ PWM
I N T E G R A T E D P R O D U C T S
PRODUCTION DATA SHEET
APPLICATION NOTE
OUTPUT INDUCTOR
OUTPUT CAPACITOR
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:
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
requirements usually has a larger capacitance and current capability
than strictly needed. The allowed ESR can be found by:
VRIPPLE = ESR× IRIPPLE
where
VIN − VOUT
D
∆I =
×
L
f s
ESR×
(
IRIPPLE + ∆I < VEX
)
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.
∆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
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.
The inductor-current rise and fall times are:
∆I
TRISE = L×
(
V − VOUT
)
IN
and
∆I
TFALL = L×
VOUT
.The inductance value can be calculated by
VIN − VOUT
∆I
D
L =
×
f s
Copyright © 2000
Rev. 1.0a, 2004-06-14
Microsemi
Page 16
Integrated Products, Power Management
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
MICROSEMI [ Microsemi ]
