LTC1735CS datasheet(14/32 Pages) LINER

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LTC1735

OUT

()

∆

where

∆I is the change in load current.

Manufacturers such as Nichicon, United Chemicon and

Sanyo can be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest (ESR)(size)

product of any aluminum electrolytic at a somewhat

higher price. An additional ceramic capacitor in parallel

with OS-CON capacitors is recommended to reduce the

inductance effects.

In surface mount applications multiple capacitors may

need to be used in parallel to meet the ESR, RMS current

handling and load step requirements of the application.

Aluminum electrolytic, dry tantalum and special polymer

capacitors are available in surface mount packages. Spe-

cial polymer surface mount capacitors offer very low ESR

but have much lower capacitive density per unit volume

than other capacitor types. These capacitors offer a very

cost-effective output capacitor solution and are an ideal

choice when combined with a controller having high loop

bandwidth. Tantalum capacitors offer the highest capaci-

tance density and are often used as output capacitors for

switching regulators having controlled soft-start. Several

excellent surge-tested choices are the AVX TPS, AVX

TPSV or the KEMET T510 series of surface mount

tantalums, available in case heights ranging from 2mm to

4mm. Aluminum electrolytic capacitors can be used in

cost-driven applications providing that consideration is

given to ripple current ratings, temperature and long-term

reliability. A typical application will require several to many

aluminum electrolytic capacitors in parallel. A combina-

tion of the above mentioned capacitors will often result in

maximizing performance and minimizing overall cost.

Other capacitor types include Nichicon PL series, NEC

Neocap, Panasonic SP and Sprague 595D series. Consult

manufacturers for other specific recommendations.

Like all components, capacitors are not ideal. Each ca-

pacitor has its own benefits and limitations. Combina-

tions of different capacitor types have proven to be a very

cost effective solution. Remember also to include high

frequency decoupling capacitors. They should be placed

as close as possible to the power pins of the load. Any

inductance present in the circuit board traces negates

their usefulness.

INTVCC Regulator

An internal P-channel low dropout regulator produces the

5.2V supply that powers the drivers and internal circuitry

within the LTC1735. The INTVCC pin can supply a maxi-

mum RMS current of 50mA and must be bypassed to

ground with a minimum of 4.7

µF tantalum, 10µF special

polymer or low ESR type electrolytic capacitor. A 1

µF

ceramic capacitor placed directly adjacent to the INTVCC

and PGND IC pins is highly recommended. Good bypass-

ing is required to supply the high transient currents

required by the MOSFET gate drivers.

Higher input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the LTC1735 to be

exceeded. The system supply current is normally domi-

nated by the gate charge current. Additional loading of

INTVCC also needs to be taken into account for the power

dissipation calculations. The total INTVCC current can be

supplied by either the 5.2V internal linear regulator or by

the EXTVCC input pin. When the voltage applied to the

EXTVCC pin is less than 4.7V, all of the INTVCC current is

supplied by the internal 5.2V linear regulator. Power

dissipation for the IC in this case is highest: (VIN)(IINTVCC)

and overall efficiency is lowered. The gate charge is

dependent on operating frequency as discussed in the

Efficiency Considerations section. The junction tempera-

ture can be estimated by using the equations given in

Note 2 of the Electrical Characteristics. For example, the

LTC1735CS is limited to less than 17mA from a 30V

supply when not using the EXTVCC pin as follows:

TJ = 70°C + (17mA)(30V)(110°C/W) = 126°C

Use of the EXTVCC input pin reduces the junction tempera-

ture to:

TJ = 70°C + (17mA)(5V)(110°C/W) = 79°C

To prevent maximum junction temperature from being

exceeded, the input supply current must be checked

operating in continuous mode at maximum VIN.

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