LTC1628C datasheet(17/32 Pages) LINER

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LTC1628/LTC1628-PG

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 lower storage capacity per unit volume than other

capacitor types. These capacitors offer a very cost-effec-

tive output capacitor solution and are an ideal choice when

combined with a controller having high loop bandwidth.

Tantalum capacitors 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 ranging from 2mm to 4mm. Aluminum

electrolytic capacitors can be used in cost-driven applica-

tions providing that consideration is given to ripple current

ratings, temperature and long term reliability. A typical

application will require several to many aluminum electro-

lytic capacitors in parallel. A combination of the above

mentioned capacitors will often result in maximizing per-

formance and minimizing overall cost. Other capacitor

types include Nichicon PL series, NEC Neocap, Pansonic

SP and Sprague 595D series. Consult manufacturers for

other specific recommendations.

INTVCC Regulator

An internal P-channel low dropout regulator produces 5V

at the INTVCC pin from the VIN supply pin. INTVCC powers

the drivers and internal circuitry within the LTC1628. The

INTVCC pin regulator can supply a peak 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 bypassing is necessary to supply

the high transient currents required by the MOSFET gate

drivers and to prevent interaction between channels.

Higher input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the LTC1628 to be

exceeded. The system supply current is normally domi-

nated by the gate charge current. Additional external

loading of the INTVCC and 3.3V linear regulators also

needs to be taken into account for the power dissipation

calculations. The total INTVCC current can be supplied by

either the 5V 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 5V linear regulator. Power dissipation for the IC in

this case is highest: (VIN)(IINTVCC), and overall efficiency

is lowered. The gate charge current is dependent on

operating frequency as discussed in the Efficiency Consid-

erations section. The junction temperature can be esti-

mated by using the equations given in Note 2 of the

Electrical Characteristics. For example, the LTC1628 VIN

current is limited to less than 24mA from a 24V supply

when not using the EXTVCC pin as follows:

TJ = 70°C + (24mA)(24V)(95°C/W) = 125°C

Use of the EXTVCC input pin reduces the junction tempera-

ture to:

TJ = 70°C + (24mA)(5V)(95°C/W) = 81°C

Dissipation should be calculated to also include any added

current drawn from the internal 3.3V linear regulator. To

prevent maximum junction temperature from being ex-

ceeded, the input supply current must be checked operat-

ing in continuous mode at maximum VIN.

EXTVCC Connection

The LTC1628 contains an internal P-channel MOSFET

switch connected between the EXTVCC and INTVCC pins.

When the voltage applied to EXTVCC rises above 4.7V, the

internal regulator is turned off and the switch closes,

connecting the EXTVCC pin to the INTVCC pin thereby

supplying internal power. The switch remains closed as

long as the voltage applied to EXTVCC remains above 4.5V.

This allows the MOSFET driver and control power to be

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