LX1676

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 13

Mobile AMD Athlon

™ VRM Controller

Copyright

© 2000

Revision: 1.0, 4/12/2005

TM

®

APPLI CATI ON NOTE

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

RIPPLE

I

ESR

V

×

=

where

s

D

L

V

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

V

∆I

L

T

−

×

=

and

OUT

FALL

V

∆I

L

T

×

=

.

The inductance value can be calculated by:

s

D

∆I

V

V

L

OUT

IN

f

×

−

=

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

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.

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

I

L

RMS

−

=

for d < 0.5

25% or 75%.