LTC3672B-1_15 Datasheet, PDF(8/12 Page) Linear Technology – Monolithic Fixed-Output 400mA Buck Regulator with Dual

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC3672B-1_15 Datasheet, PDF (8/12 Pages) Linear Technology – Monolithic Fixed-Output 400mA Buck Regulator with Dual

◁

LTC3672B-1

OPERATION

LOW DROPOUT LINEAR REGULATORS (LDOS)

The LTC3672B-1 contains two 150mA ï¬xed-output LDO

regulators. LDO1 takes power from the VIN1 pin and regu-

lates a 1.2V output at the LDO1 pin. By connecting VIN1

to the buck regulatorâs 1.8V output, overall conversion

efï¬ciency can be improved, because the bulk of the step-

down will be done by the buck regulator at higher efï¬ciency

than what the LDO can do on its own. For example, for

the case of deriving a 1.2V output from a 3.6V input (e.g.

Lithium-Ion battery nominal voltage), using an LDO to

do all of the step-down results in an efï¬ciency of at most

1.2V/3.6V = 33.3%, using the fact that the upper-bound on

any linear regulatorâs efï¬ciency is output voltage divided by

input voltage. Feeding the LDO from the output of the buck

regulator, with a typical buck efï¬ciency of 85%, raises the

ceiling on overall efï¬ciency to 85% â¢ 1.2V/1.8V = 56.6%.

This can increase battery life by up to 70%!

LDO2 takes power straight from VIN and regulates a 2.8V

output at the LDO2 pin.

For stability, each LDO output must be bypassed to ground

with a minimum 1Î¼F ceramic capacitor.

APPLICATIONS INFORMATION

BUCK REGULATOR INDUCTOR SELECTION

Many different sizes and shapes of inductors are avail-

able from numerous manufacturers. Choosing the right

inductor from such a large selection of devices can be

overwhelming, but following a few basic guidelines will

make the selection process much simpler.

The buck regulator is designed to work with inductors in

the range of 2.2Î¼H to 10Î¼H. A 4.7Î¼H inductor is a good

starting point. Larger value inductors reduce ripple cur-

rent, which improves output ripple voltage. Lower value

inductors result in higher ripple current and improved

transient response time. To maximize efï¬ciency, choose

an inductor with a low DC resistance. Choose an inductor

with a DC current rating at least 1.5 times larger than the

maximum load current to ensure that the inductor does

not saturate during normal operation. If output short circuit

is a possible condition, the inductor should be rated to

handle the maximum peak current speciï¬ed for the step-

down converters.

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 Permalloyâ¢ materials

are small and donât radiate much energy, but generally

cost more than powdered iron core inductors with similar

electrical characteristics. Inductors that are very thin or

have a very small volume typically have much higher core

and DCR losses, and will not give the best efï¬ciency. The

choice of which style inductor to use often depends more

on the price vs size, performance, and any radiated EMI

requirements than on what the buck regulator needs to

operate.

Table 1 shows several inductors that work well with the

buck regulator. These inductors offer a good compromise

in current rating, DCR and physical size. Consult each

manufacturer for detailed information on their entire

selection of inductors.

3672B1f

▷