Microsemi

Linfinity Microelectronics Division

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

Page 6

Copyright

 2000

Rev. 1.0, 2001-09-19

LX1684

Voltage-Mode PWM Controller

L I N F IN IT Y

D I V I S I O N

APPLI CATION I

APPLI CATION I

APPLI CATION I

APPLI CATION I N FO RM ATION

NF O R M A TION

NF O R M A TION

NF O R M A 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:

RIPPLE

RIPPLE

I

ESR

V

×

=

where

()

IN

OUT

OUT

IN

RIPPLE

V

V

V

-

V

I

×

×

=

L

value 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. The inductor-

current rise and fall times are:

()

OUT

IN

V

V

−

∆

×

=

I

L

and

OUT

V

I

L

∆

×

=

When using electrolytic capacitors, the capacitor voltage droop

is usually negligible, due to the large capacitance.

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:

()

EX

V

I

ESR

<

∆

+

I

load current step change, and V

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 increases 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.

Linfinity’s demonstration boards use Sanyo MV-GX filter

capacitors,

which

are

aluminum

electrolytic,

and

have

demonstrated reliability. The Oscon 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 MV-GX 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.

INPUT CAPACITOR

The input capacitor and the input inductor are to filter the

pulsating current generated by the buck converter to reduce

interference to other circuits connected to the same 12V rail. In

addition, the input capacitor provides local de-coupling the buck

converter. The capacitor should be rated to handle the RMS

current requirement. The RMS current is:

)

1

(

d

d

I

I

L

RMS

−

=

where I

maximum value, when d = 50%, I

output in the range of 3V, the required RMS current is very close

to 0.43I

SOFT-START CAPACITOR

The value of the soft-start capacitor determines how fast the

output voltage rises and how large the inductor current is required

to charge the output capacitor. The output voltage will follow the

voltage at SS pin if the required inductor current does not exceed

the maximum current in the inductor. The SS pin voltage can be

expressed as:

)

1

(

/

SS

SS

C

R

t

SET

SS

e

V

V

−

−

=

where V

resistor and capacitor. The required inductor current for the output

capacitor to follow the SS-pin voltage equals the required

capacitor current plus the load current. The soft-start capacitor

should be selected so that the overall inductor current does not

exceed its maximum.