LX1668

PRODUCT DA T ABOOK 1996/1997

Copyright © 1999

Rev. 1.0

4/99

10

APPLICA TION INFORMA TION

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:

V

where

I

*

I

value and ESR is the Effective Series Resistance of the output

capacitor.

I

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 induc-

tor current slew rate in response to the load-current step change,

∆I, resulting in more output-capacitor voltage droop. The

inductor-current rise and fall times are:

T

and

T

When using electrolytic capacitors, the capacitor voltage

droop is usually negligible, due to the large capacitance.

For higher current applications, such as Pentium II proces-

sors, a 2.5µH inductor is recommended for the best combination

of fast response and manageable ripple voltage.

For lower

current applications, such as Pentium and other Socket 7

processors, a 5µH inductor is sufficient. The effect of different

inductor values is shown in Figure 7 above.

Notice how, with a smaller inductor, transient response time

is improved, but at the expense of much greater ripple.

INPUT INDUCTOR

In order to supply faster transient load changes, a smaller output

inductor is needed. However, reducing the size of the output

inductor will result in a higher ripple voltage on the input supply,

as shown in Figure 8 above. This noise on the 5V rail can affect

other system components, such as graphics cards. It is recom-

mended that a 1 – 1.5µH inductor, L2, is used on input to the

regulator, to filter the ripple on the 5V supply. Ensure that this

inductor has the same current rating as the output inductor.

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 for ESR instead

of capacitance or RMS current capability.

A capacitor that

satisfies the ESR requirement usually has a larger capacitance and

current capability than strictly needed. The allowed ESR can be

found by:

ESR * (I

where I

load current step change, and V

(V

f

V

OUT

V

IN

FIGURE 7 — Effect Of Different Inductor Values

L

Input Ripple Voltage

Output Voltage

(2.8V Set Point)

Input Current

L