RC5054AM 查看數據表（PDF） - Fairchild Semiconductor

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RC5054AM

Programmable Synchronous DC-DC Converter Controller for Low Voltage Microprocessors

Fairchild Semiconductor 

PRODUCT SPECIFICATION

RC5054A

Component Selection Guidelines

Output Capacitor Selection

An output capacitor is required to filter the output and supply

the load transient current. The filtering requirements are a

function of the switching frequency and the ripple current.

The load transient requirements are a function of the slew

rate (di/dt) and the magnitude of the transient load current.

These requirements are generally met with a mix of capaci-

tors and careful layout.

Modern microprocessors produce transient load rates above

1A/ns. High frequency capacitors initially supply the tran-

sient and slow the current load rate seen by the bulk capaci-

tors. The bulk filter capacitor values are generally

determined by the ESR (effective series resistance) and volt-

age rating requirements rather than actual capacitance

requirements.

High frequency decoupling capacitors should be placed as

close to the power pins of the load as physically possible. Be

careful not to add inductance in the circuit board wiring that

could cancel the usefulness of these low inductance compo-

nents. Consult with the manufacturer of the load on specific

decoupling requirements. For example, Intel recommends

that the high frequency decoupling for the Pentium Pro be

composed of at least forty (40) 1µF ceramic capacitors in the

1206 surface-mount package.

Use only specialized low-ESR capacitors intended for

switching-regulator applications for the bulk capacitors. The

bulk capacitor’s ESR will determine the output ripple voltage

and the initial voltage drop after a high slew-rate transient.

An aluminum electrolytic capacitor’s ESR value is related to

the case size with lower ESR available in larger case sizes.

However, the equivalent series inductance (ESL) of these

capacitors increases with case size and can reduce the useful-

ness of the capacitor to high slew-rate transient loading.

Unfortunately, ESL is not a specified parameter. Work with

your capacitor supplier and measure the capacitor’s imped-

ance with frequency to select a suitable component. In most

cases, multiple electrolytic capacitors of small case size per-

form better than a single large case capacitor.

Output Inductor Selection

The output inductor is selected to meet the output voltage

ripple requirements and minimize the converter’s response

time to the load transient. The inductor value determines the

converter’s ripple current and the ripple voltage is a function

of the ripple current. The ripple voltage and current are

approximated by the following equations:

∆I

=

V-----I--NF----S-–----×V-----LO----U---T--

•

-V----O----U---T--

VIN

∆VOUT = ∆I × ESR

Increasing the value of inductance reduces the ripple current

and voltage. However, the large inductance values reduce the

converter’s response time to a load transient.

One of the parameters limiting the converter’s response to

a load transient is the time required to change the inductor

current. Given a sufficiently fast control loop design, the

RC5054A will provide either 0% or 100% duty cycle in

response to a load transient. The response time is the time

required to slew the inductor current from an initial current

value to the transient current level. During this interval the

difference between the inductor current and the transient

current level must be supplied by the output capacitor. Mini-

mizing the response time can minimize the output capaci-

tance required.

The response time to a transient is different for the applica-

tion of load and the removal of load. The following equations

give the approximate response time interval for application

and removal of a transient load:

tRISE

=

---L-----×-----I--T---R----A---N-----

VIN – VOUT

tFALL

=

L------×----I---T---R---A----N--

VOUT

where: ITRAN is the transient load current step, tRISE is the

response time to the application of load, and tFALL is the

response time to the removal of load. With a +5V input source,

the worst case response time can be either at the application

or removal of load and dependent upon the DACOUT setting.

Be sure to check both of these equations at the minimum and

maximum output levels for the worst case response time.

Input Capacitor Selection

Use a mix of input bypass capacitors to control the voltage

overshoot across the MOSFETs. Use small ceramic capaci-

tors for high frequency decoupling and bulk capacitors to

supply the current needed each time Q1 turns on. Place the

small ceramic capacitors physically close to the MOSFETs

and between the drain of Q1 and the source of Q2.

The important parameters for the bulk input capacitor are the

voltage rating and the RMS current rating. For reliable oper-

ation, select the bulk capacitor with voltage and current ratings

above the maximum input voltage and largest RMS current

required by the circuit. The capacitor voltage rating should be

at least 1.25 times greater than the maximum input voltage

and a voltage rating of 1.5 times is a conservative guideline.

The RMS current rating requirement for the input capacitor

of a buck regulator is approximately 1/2 the DC load current.

For a through hole design, several electrolytic capacitors

(Panasonic HFQ series or Nichicon PL series or Sanyo MVGX

or equivalent) may be needed. For surface mount designs,

solid tantalum capacitors can be used, but caution must be

exercised with regard to the capacitor surge current rating.

These capacitors must be capable of handling the surge-cur-

rent at power-up. The TPS series available from AVX, and

the 593D series from Sprague are both surge current tested.

9

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