LTC3707IGN datasheet(17/32 Pages) LINER

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LTC3707

3707fb

APPLICATIONS INFORMATION

that the output capacitance does not signiﬁcantly discharge

during the operating frequency period due to ripple current.

The choice of using smaller output capacitance increases

the ripple voltage due to the discharging term but can be

compensated for by using capacitors of very low ESR to

maintain the ripple voltage at or below 50mV. The ITH pin

OPTI-LOOP compensation components can be optimized

to provide stable, high performance transient response

regardless of the output capacitors selected.

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. Special

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-effective

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

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 combination of the

above mentioned capacitors will often result in maximizing

performance 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 pow-

ers the drivers and internal circuitry within the LTC3707.

The INTVCC pin regulator can supply a peak current of

40mA and must be bypassed to ground with a minimum

of 4.7μF tantalum, 10μF special polymer, or low ESR type

electrolytic capacitor. A 1μF ceramic capacitor placed di-

rectly 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 LTC3707 to be

exceeded.Thesystemsupplycurrentisnormallydominated

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 high-

est: (VIN)(IINTVCC), and overall efﬁciency is lowered. The

gate charge current is dependent on operating frequency

as discussed in the Efﬁciency Considerations section.

The junction temperature can be estimated by using the

equations given in Note 2 of the Electrical Characteristics.

For example, the LTC3707 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 tem-

perature 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

exceeded, the input supply current must be checked

operating in continuous mode at maximum VIN.

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