LTC1735CF datasheet(15/32 Pages) LINER

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LTC1735

1735fc

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, ESR, RMS current han-

dling and load step specifications may require multiple

capacitors in parallel. Aluminum electrolytic, dry tantalum

and special polymer capacitors are available in surface

mount packages. Special polymer surface mount capaci-

tors 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 capaci-

tors offer the highest capacitance 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 rang-

ing from 1.5mm to 4.1mm. Aluminum electrolytic capaci-

tors can be used in cost-driven applications, provided that

consideration is given to ripple current ratings, tempera-

ture and long-term reliability. A typical application will

require several to many aluminum electrolytic capacitors

in parallel. A combination of the above mentioned capaci-

tors 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.

EXTVCC Connection

The LTC1735 contains an internal P-channel MOSFET

switch connected between the EXTVCC and INTVCC pins.

Whenever the EXTVCC pin is above 4.7V, the internal 5.2V

regulator shuts off, the switch closes and INTVCC power is

supplied via EXTVCC until EXTVCC drops below 4.5V. This

allows the MOSFET gate drive and control power to be

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