3412EFE 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

生产厂家

3412EFE

2.5A, 4MHz, Monolithic Synchronous Step-Down Regulator

Linear Technology 

LTC3412

APPLICATIO S I FOR ATIO

Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. High efficiency converters generally cannot af-

ford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite, mollypermalloy,

or Kool Mμ® cores. Actual core loss is independent of core

size for a fixed inductor value but it is very dependent on

the inductance selected. As the inductance increases, core

losses decrease. Unfortunately, increased inductance re-

quires more turns of wire and therefore copper losses will

increase.

Ferrite designs have very low core losses and are preferred

at high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates “hard,” which means that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy mate-

rials are small and don’t radiate 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 trapezoi-

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

large ripple voltage, a low ESR input capacitor sized for the

maximum RMS current should be used. 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

10

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 tempera-

ture than required. Several capacitors may also be paral-

leled 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:

ΔVOUT

≤

ΔIL

⎛

⎝⎜

ESR

+

1

8fC OUT

⎞

⎠⎟

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 RMS

current handling requirements. Dry tantalum, special poly-

mer, 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

3412fb

Share Link: