LTC3418EUHF 查看數據表（PDF） - Linear Technology

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LTC3418EUHF

8A, 4MHz, Monolithic Synchronous Step-Down Regulator

Linear Technology 

LTC3418

APPLICATIONS INFORMATION

Different core materials and shapes will change the size/cur-

rent and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price vs size requirements and any

radiated field/EMI requirements. New designs for surface

mount inductors are available from Coiltronics, Coilcraft,

Toko and Sumida.

CIN and COUT Selection

The input capacitance, CIN, is needed to filter the trapezoidal

wave current at the source of the top MOSFET. To prevent

large voltage transients from occurring, a low ESR input

capacitor sized for the maximum RMS current should be

used. The maximum RMS current is given by:

IRMS

= IOUT(MAX )

VOUT

VIN

VIN – 1

VOUT

This formula has a maximum at VIN = 2VOUT, where IRMS =

IOUT/2. This simple worst-case condition is commonly used

for design because even significant deviations do not offer

much relief. Note that ripple current ratings from capacitor

manufacturers are often based on only 2000 hours of life

which makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to meet

size or height requirements in the design.

The selection of COUT is determined by the effective series

resistance (ESR) that is required to minimize voltage ripple

and load step transients as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

the load transient response as described in a later section.

The output ripple, ΔVOUT, is determined by:

VOUT

IL



ESR



+

1

8fCOUT







The output ripple is highest at maximum input voltage

since ΔIL increases with input voltage. Multiple capacitors

placed in parallel may be needed to meet the ESR and

10

RMS current handling requirements. Dry tantalum, special

polymer, aluminum electrolytic and ceramic capacitors are

all available in surface mount packages. Special polymer

capacitors offer very low ESR but have lower capacitance

density than other types. Tantalum capacitors have the

highest capacitance density but it is important to only use

types that have been surge tested for use in switching

power supplies. Aluminum electrolytic capacitors have

significantly higher ESR, but can be used in cost-sensitive

applications provided that consideration is given to ripple

current ratings and long term reliability. Ceramic capaci-

tors have excellent low ESR characteristics but can have a

high voltage coefficient and audible piezoelectric effects.

The high Q of ceramic capacitors with trace inductance

can also lead to significant ringing.

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them ideal

for switching regulator applications. However, care must

be taken when these capacitors are used at the input and

output. When a ceramic capacitor is used at the input and

the power is supplied by a wall adapter through long wires,

a load step at the output can induce ringing at the input,

VIN. At best, this ringing can couple to the output and be

mistaken as loop instability. At worst, a sudden inrush

of current through the long wires can potentially cause a

voltage spike at VIN large enough to damage the part.

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics have the best temperature and voltage charac-

teristics of all the ceramics for a given value and size.

Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following equation:

VOUT

=

0.8

1+

R2

R1





The resistive divider allows pin VFB to sense a fraction of

the output voltage as shown in Figure 1.

3418fb

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