LTC4449 datasheet(25/48 Pages) AD

25 / 48 page

LTC7872

Rev. 0

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significantly different from that of an ideal capacitor and

therefore requires accurate modeling or bench evalua-

tion during design. Manufacturers such as Nichicon,

Nippon Chemi-Con and Sanyo should be considered for

high performance through-hole capacitors. The OS-CON

semiconductor dielectric capacitors available from Sanyo

and the Panasonic SP surface mount types have a good

(ESR)(size) product.

Once the ESR requirement for COUT has been met, the

RMS current rating generally far exceeds the IRIPPLE(P–P)

requirement. 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 elec-

trolytic and dry tantalum capacitors are both available in

surface mount configurations. New special polymer sur-

face mount capacitors offer very low ESR also but have

much lower capacitive density per unit volume. In the

case of tantalum, it is critical that the capacitors are surge

tested for use in switching power supplies. Several excel-

lent choices are the AVX TPS, AVX TPSV, the KEMET T510

series of surface mount tantalums or the Panasonic SP

series of surface mount special polymer capacitors avail-

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

capacitor types include Sanyo POSCAP, Sanyo OS-CON,

Nichicon PL series and Sprague 595D series. Consult the

manufacturers for other specific recommendations.

CHIGH Capacitor Selection for Boost Operation

Contributions of ESR (equivalent series resistance), ESL

(equivalent series inductance) and the bulk capacitance

must be considered when choosing the correct combina-

tion of output capacitors for a boost converter application.

The choice of component(s) begins with the maximum

acceptable ripple voltage (expressed as a percentage of

the output voltage), and how this ripple should be divided

between the ESR step and the charging/discharging ∆V.

For the purpose of simplicity we will choose 2% for the

maximum output ripple, to be divided equally between the

ESR step and the charging/discharging ∆V. This percentage

in radiating sound! A load that draws varying current at an

audible rate may cause an attendant varying input voltage

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

reduced by the increasing charge. The voltage can increase

at a considerably higher rate than the constant current

being supplied because the capacitance value is decreasing

as the voltage is increasing! Nevertheless, ceramic capaci-

tors, when properly selected and used, can provide the

lowest overall loss due to their extremely low ESR.

A small (0.1μF to 1μF) capacitor, CIN, placed close to

the LTC7872 between the VIN pin and ground, bypasses

switching noise to ground. A 2.2Ω to 10Ω resistor, placed

between CIN and VHIGH pins decouples the VHIGH pin from

switching noise.

The selection of COUT at VOUT is driven by the required

effective series resistance (ESR). Typically once the ESR

requirement is satisfied the capacitance is adequate for

filtering. The steady-state output ripple (∆VOUT) is deter-

mined by:

ΔVOUT ≈ ΔIRIPPLE 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 (VHIGH). The output ripple will

be less than 50mV at maximum VIN with ∆IRIPPLE = 0.4

IOUT(MAX) assuming:

COUT required ESR < N • RSENSE

and

COUT >

( ) RSENSE

(

)

The emergence of very low ESR capacitors in small,

surface mount packages makes very small physical

implementations possible. The ability to externally com-

pensate 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

APPLICATIONS INFORMATION

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