LTC3421

12

3421f

APPLICATIO S I FOR ATIO

Output Capacitor Selection

The output voltage ripple has two components to it. The

bulk value of the capacitor is set to reduce the ripple due

to charge into the capacitor each cycle. The maximum

ripple due to charge is given by:

V

IV

CV

f

RBULK

PIN

OUT

OUT

=

•

••

The ESR (equivalent series resistance) is usually the most

dominant factor for ripple in most power converters. The

ripple due to capacitor ESR is simply given by:

Low ESR capacitors should be used to minimize output

voltage ripple. For surface mount applications, AVX TPS

series tantalum capacitors, Sanyo POSCAP or Taiyo Yuden

ceramic capacitors are recommended. For through-hole

applications, Sanyo OS-CON capacitors offer low ESR in a

small package size.

In some layouts it may be necessary to place a 1

µFlowESR

possible.

Input Capacitor Selection

The input filter capacitor reduces peak currents drawn

from the input source and reduces input switching noise.

Since the IC can operate at voltages below 0.5V once the

output is regulated, the demand on the input capacitor is

much less. In most applications 1

µF per amp of peak input

current is recommended. Taiyo Yuden offers very low ESR

ceramic capacitors, for example the 1

µF in a 0603 case

(JMK107BJ105MA).

Table 2. Capacitor Vendor Information

SUPPLIER

PHONE

FAX

WEB SITE

AVX

(803) 448-9411 (803) 448-1943 www.avxcorp.com

Sanyo

(619) 661-6322 (619) 661-1055 www.sanyovideo.com

TDK

(847) 803-6100 (847) 803-6296 www.component.tdk.com

Murata

USA:

USA:

www.murata.com

(814) 237-1431 (814) 238-0490

(800) 831-9172

Taiyo Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com

Operating Frequency Selection

There are several considerations in selecting the operating

frequency of the converter. The first is, which are the sen-

sitive frequency bands that cannot tolerate any spectral

noise? The second consideration is the physical size of the

converter. As the operating frequency goes up, the induc-

tor and filter capacitors go down in value and size. The trade

off is in efficiency since the switching losses due to gate

charge are going up proportional with frequency.

Another operating frequency consideration is whether the

application can allow “pulse skipping.” In this mode, the

minimum on time of the converter cannot support the duty

cycle, so the converter ripple will go up and there will be

a low frequency component of the output ripple. In many

applications where physical size is the main criterion,

running the converter in this mode is acceptable. In appli-

cations where it is preferred not to enter this mode, the

maximum operating frequency is given by:

f

VV

Vt

Hz

MAX NOSKIP

OUT

IN

OUT

ON MIN

_

()

–

•

=

Thermal Considerations

To deliver the power that the LTC3421 is capable of, it is

imperative that a good thermal path be provided to

dissipate the heat generated within the package. This can

be accomplished by taking advantage of the large thermal

pad on the underside of the IC. It is recommended that mul-

tiple vias in the printed circuit board be used to conduct

heat away from the IC and into a copper plane with as much

area as possible. In the event that the junction tempera-

ture gets too high, the peak current limit will automatically

be decreased. If the junction temperature continues to rise,

the part will go into thermal shutdown, and all switching

will stop until the temperature drops.

The LTC3421 will maintain voltage regulation when the

input voltage is above the output voltage. This is achieved

by terminating the switching on the synchronous PMOS