LTC1775CS datasheet(13/24 Pages) LINER

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LTC1775

Schottky diode must be placed next to the synchronous

switch to minimize this effect. One also might consider

using a power switch with an integrated Schottky diode, or

omitting the diode altogether in high current applications.

CIN and COUT Selection

In continuous mode, the drain current of the top MOSFET

is approximately a square wave of duty cycle VOUT/VIN. To

prevent large input voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS current is given by:

RMS

O MAX

OUT

≅−









()

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

= IO(MAX)/2. This simple worst-case condition is com-

monly used for design because even significant deviations

do not offer much relief. Note that ripple current ratings

from capacitor manufacturers 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 also be

placed in parallel to meet size or height requirements in the

design.

The selection of COUT is primarily determined by the ESR

required to minimize voltage ripple. The output ripple

∆VOUT is approximately bounded by:

∆∆

I ESR

OUT

≤+









()( )(

)

Since

∆IL increases with input voltage, the output ripple is

highest at maximum input voltage. Typically, once the ESR

requirement is satisfied the capacitance is adequate for

filtering and has the required RMS current rating.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON (organic semiconductor

dielectric) capacitor available from Sanyo has the lowest

product of ESR and size of any aluminum electrolytic at a

somewhat higher price. An additional ceramic capacitor in

parallel with OS-CON capacitors is recommended to re-

duce the effect of their lead inductance.

In surface mount applications, multiple capacitors placed

in parallel may be required to meet the ESR, RMS current

handling and load step requirements. Dry tantalum, spe-

cial polymer and aluminum electrolytic capacitors are

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 to only use

types that have been surge tested for use in switching

power supplies. Several excellent surge-tested choices

are the AVX TPS and TPSV or the KEMET T510 series.

Aluminum electrolytic capacitors have significantly higher

ESR, but can be used in cost-driven applications providing

that consideration is given to ripple current ratings and

long term reliability. Other capacitor types include Nichicon

PL, NEC Neocap, Panasonic SP and Sprague 595D series.

INTVCC Regulator

An internal P-channel low dropout regulator produces the

5.2V supply which powers the drivers and internal cir-

cuitry within the LTC1775. The INTVCC pin can supply a

maximum RMS current of 50mA and must be bypassed to

ground with a minimum of 4.7

µF tantalum or low ESR

electrolytic capacitance. Good bypassing is necessary to

supply the high transient currents required by the MOSFET

gate drivers.

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the LTC1775

to exceed its maximum junction temperature rating. Most

of the supply current drives the MOSFET gates unless an

external EXTVCC source is used. The junction temperature

can be estimated from the equations given in Note 2 of the

Electrical Characteristics. For example, the LTC1775CGN

is limited to less than 14mA from a 30V supply:

TJ = 70°C + (14mA)(30V)(130°C/W) = 125°C

To prevent the maximum junction temperature from being

exceeded, the input supply current must be checked when

operating in continuous mode at high VIN. Relief can be

provided by using the EXTVCC pin to provide the gate drive

current.

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