LX1681CDM データシートの表示（PDF） - Microsemi Corporation

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LX1681CDM

VOLTAGE - MODE PWM CONTROLLERS

Microsemi Corporation 

PRODUCT DATABOOK 1996/1997

LX1681/1682

VOLTAGE-MODE PWM CONTROLLERS

PRODUCTION DATA SHEET

APPLICATION INFORMATION

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 = ESR * I

RIPPLE

RIPPLE

where

I=

RIPPLE

(VIN - VOUT )

fSW * L

*

VOUT

V

IN

I is the inductor ripple current, L is the output inductor

RIPPLE

value and ESR is the Effective Series Resistance of the output

capacitor.

IRIPPLE 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 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

RISE

=

L

*

∆I/(V

IN

–

V

OUT

)

and

TFALL = L * ∆I/VOUT

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:

ESR * (IRIPPLE + ∆I ) < VEX

where IRIPPLE is the inductor ripple current, ∆I is the maximum

load current step change, and V is the allowed output voltage

EX

excursion in the transient.

OUTPUT CAPACITOR (continued)

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 capaci-

tors, 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 5V 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:

I = I √ d(1-d)

RMS L

where I is the inductor current and the d is the duty cycle. The

L

maximum value, when d = 50%, IRMS = 0.5IL. For 5V input and

output in the range of 2 to 3V, the required RMS current is very

close to 0.5IL.

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:

VSS = VSET (1-e-t/RssCss)

where VSET is the reference voltage. RSS and CSS are soft start

resistor and capacitor. The required inductor current for the

output capacitor to follow the SS-pin voltage equals the required

6

Copyright © 1999

Rev. 1.0 5/99

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