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LTC3774

3774fc

For more information www.linear.com/LTC3774

maximum RMS current of one channel must be used. The

maximum RMS capacitor current is given by:

CIN Required IRMS ≈

IMAX

VIN

VOUT

(

) VIN – VOUT

(

)

⎡⎣

⎤⎦

1/2

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

offermuchrelief.Notethatcapacitormanufacturers’ripple

current ratings 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 be paralleled to meet

size or height requirements in the design. Due to the high

operating frequency of the LTC3774, ceramic capacitors

can also be used for CIN. Always consult the manufacturer

if there is any question.

Ceramic capacitors are becoming very popular for small

designsbutseveralcautionsshouldbeobserved.X7R,X5R

and Y5V are examples of a few of the ceramic materials

used as the dielectric layer, and these different dielectrics

have very different effect on the capacitance value due to

thevoltageandtemperatureconditionsapplied.Physically,

if the capacitance value changes due to applied voltage

change, there is a concomitant piezo effect which results

in radiating sound! A load that draws varying current at

an audible rate may cause an attendant varying input volt-

age on a ceramic capacitor, resulting in an audible signal.

A secondary issue relates to the energy flowing back into

a ceramic capacitor whose capacitance value is being

reducedbytheincreasingcharge.Thevoltagecanincrease

ataconsiderablyhigherratethantheconstantcurrentbeing

supplied because the capacitance value is decreasing as

thevoltageisincreasing!Nevertheless,ceramiccapacitors,

when properly selected and used, can provide the lowest

overall loss due to their extremely low ESR.

A small (0.1µF to 1µF) bypass capacitor, CIN, between the

chip VIN pin and ground, placed close to the LTC3774, is

also suggested. A 2.2Ω to 10Ω resistor placed between

CIN and VIN pin provides further isolation.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically once the ESR require-

ment is satisfied the capacitance is adequate for filtering.

The steady-state output ripple (∆VOUT) is determined by:

∆VOUT ≈ ∆IRIPPLE ESR+

8fCOUT

⎛

⎝

⎜

⎞

⎠

⎟

where f = operating frequency, COUT = output capacitance

and ∆IRIPPLE = ripple current in the inductor. The output

ripple is highest at maximum input voltage since ∆IRIPPLE

increases with input voltage. The output ripple will be less

than 50mV at maximum VIN with ∆IRIPPLE = 0.4IOUT(MAX)

assuming:

COUT required ESR < N • RSENSE

and

COUT >

( ) RSENSE

(

)

TheemergenceofverylowESRcapacitorsinsmall,surface

mount packages makes very small physical implementa-

tions possible. The ability to externally compensate the

switching regulator loop using the ITH pin allows a much

wider selection of output capacitor types. The impedance

characteristic of each capacitor type is significantly differ-

ent than an ideal capacitor and therefore requires accurate

modelingorbenchevaluationduringdesign.Manufacturers

suchasNichicon,NipponChemi-ConandSanyoshouldbe

consideredforhighperformancethrough-holecapacitors.

TheOS-CONsemiconductordielectriccapacitorsavailable

from Sanyo and the Panasonic SP surface mount types

have a good (ESR)(size) product.

OncetheESRrequirementforCOUThasbeenmet,theRMS

currentratinggenerallyfarexceedstheIRIPPLE(P-P)require-

ment. Ceramic capacitors from AVX, Taiyo Yuden, Murata

and TDK offer high capacitance value and very low ESR,

especially applicable for low output voltage applications.

In surface mount applications, multiple capacitors may

have to be paralleled to meet the ESR or RMS current

handling requirements of the application. Aluminum

electrolytic and dry tantalum capacitors are both available

in surface mount configurations. New special polymer

surface mount capacitors offer very low ESR also but

have much lower capacitive density per unit volume. In

APPLICATIONS INFORMATION

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