LTC1625IGN datasheet(12/24 Pages) LINER

12 / 24 page

LTC1625

APPLICATIONS INFORMATION

Ferrite designs have very low core loss 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 rapidly 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!

Molypermalloy (from Magnetics, Inc.) is a very good, low

loss core material for toroids, but it is more expensive than

ferrite. A reasonable compromise from the same manu-

facturer is Kool M

µ. Toroids are very space efficient,

especially when you can use several layers of wire.

Because they generally lack a bobbin, mounting is more

difficult. However, designs for surface mount are available

which do not increase the height significantly.

Schottky Diode Selection

The Schottky diode D1 shown in Figure 1 conducts during

the dead time between the conduction of the power

MOSFETs. This prevents the body diode of the bottom

MOSFET from turning on and storing charge during the

dead time, which could cost as much as 1% in efficiency.

A 1A Schottky diode is generally a good size for 3A to 5A

regulators. The diode may be omitted if the efficiency loss

can be tolerated.

CIN and COUT Selection

In continuous mode, the drain current of the top MOSFET

is approximately a square wave of duty cycle VOUT/VIN. To

prevent large input voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS current is given by:

RMS

O MAX

OUT

≅−









()

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

= IO(MAX)/2. This simple worst-case condition is com-

monly 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. This makes it advisable to further derate

the capacitor or to choose a capacitor rated at a higher

temperature than required. Several capacitors may also be

placed in parallel to meet size or height requirements in the

design.

The selection of COUT is primarily determined by the ESR

required to minimize voltage ripple. The output ripple

∆VOUT is approximately bounded by:

∆∆

I ESR

OUT

≤+









()( )(

)

Since

∆IL increases with input voltage, the output ripple is

highest at maximum input voltage. Typically, once the ESR

requirement is satisfied the capacitance is adequate for

filtering and has the required RMS current rating.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest product of

ESR and size of any aluminum electrolytic at a somewhat

higher price.

In surface mount applications, multiple capacitors may

have to be placed in parallel to meet the ESR requirement.

Aluminum electrolytic and dry tantalum capacitors are

both available in surface mount packages. In the case of

tantalum, it is critical that the capacitors have been surge

tested for use in switching power supplies. An excellent

choice is the AVX TPS series of surface mount tantalum,

available in case heights ranging from 2mm to 4mm. Other

capacitor types include Sanyo OS-CON, Nichicon PL se-

ries, and Sprague 593D and 595D series. Consult the

manufacturer for other specific recommendations.

INTVCC Regulator

An internal P-channel low dropout regulator produces the

5.2V supply which powers the drivers and internal cir-

cuitry within the LTC1625. The INTVCC pin can supply up

to 50mA and must be bypassed to ground with a minimum

of 4.7

µF tantalum or low ESR electrolytic capacitance.

Good bypassing is necessary to supply the high transient

currents required by the MOSFET gate drivers.

LTC1625IGN Datenblatt(PDF) 12 Page - Linear Technology

LTC1625IGN Datenblatt(HTML) 12 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datenblatt Download

Link URL