LTC3637 datasheet(13/26 Pages) LINER

13 / 26 page

LTC3637

3637fa

For more information www.linear.com/LTC3637

APPLICATIONS INFORMATION

depends on the price versus size requirements and any

radiated field/EMI requirements. New designs for surface

mount inductors are available from Würth, Coilcraft, TDK,

Toko, and Sumida.

CIN and COUT Selection

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

current at the source of the top high side MOSFET. CIN

should be sized to provide the energy required to charge

the inductor without causing a large decrease in input

voltage (

∆VIN). The relationship between CIN and ∆VIN

is given by:

CIN >

L •IPEAK2

2 • VIN • ∆VIN

It is recommended to use a larger value for CIN than

calculated by the above equation since capacitance de-

creases with applied voltage. In general, a 4.7µF X7R

ceramiccapacitorisagoodchoiceforCINinmostLTC3637

applications.

To minimize large ripple voltage, a low ESR input capaci-

tor sized for the maximum RMS current should be used.

RMS current is given by:

IRMS =IOUT(MAX) •

VOUT

VIN

•

VIN

VOUT

– 1

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

IOUT/2.Thissimpleworst-caseconditioniscommonlyused

for design because even significant deviations do not offer

much relief. Note that ripple current ratings from capacitor

manufacturers are often based only on 2000 hours of life

which makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required.Severalcapacitorsmayalsobeparalleledtomeet

size or height requirements in the design.

The output capacitor, COUT, filters the inductor’s ripple

current and stores energy to satisfy the load current when

the LTC3637 is in sleep. The output ripple has a lower limit

of VOUT/160 due to the 5mV typical hysteresis of the feed-

back comparator. The time delay of the comparator adds

an additional ripple voltage that is a function of the load

current. During this delay time, the LTC3637 continues to

switch and supply current to the output. The output ripple

can be approximated by:

∆VOUT ≈

IPEAK

–ILOAD











 •

4 • 10–6

COUT

VOUT

160

The output ripple is a maximum at no load and approaches

lower limit of VOUT/160 at full load. Choose the output

capacitor COUT to limit the output voltage ripple ∆VOUT

using the following equation:

COUT ≥

IPEAK • 2 •10–6

∆VOUT –

VOUT

160

The value of the output capacitor must be large enough

to accept the energy stored in the inductor without a large

change in output voltage during a single switching cycle.

Setting this voltage step equal to 1% of the output voltage,

the output capacitor must be:

COUT > 50 •L •

IPEAK

VOUT













Typically, a capacitor that satisfies the voltage ripple

requirement is adequate to filter the inductor ripple. To

avoid overheating, the output capacitor must also be sized

to handle the ripple current generated by the inductor.

The worst-case ripple current in the output capacitor is

given by IRMS = IPEAK/2. Multiple capacitors placed in

parallel may be needed to meet the ESR and 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 only to 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 capacitors have excellent low ESR

characteristics but can have high voltage coefficient and

audible piezoelectric effects. The high quality factor (Q)

LTC3637 Datasheet(PDF) 13 Page - Linear Technology

LTC3637 Datasheet(HTML) 13 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 25 26

Datasheet Download

Link URL