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LTC3728L-1

3728l1fc

APPLICATIONS INFORMATION

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,

Panasonic SP, NEC Neocap, Cornell Dubilier ESRE 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 IC.

The INTVCC pin regulator can supply a peak current of

50mA 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 IC to be exceeded.

The system supply current is normally dominated 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 in-

ternal 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 IC VIN current is thermally limited to less

than 67mA from a 24V supply when not using the EXTVCC

pin as follows:

TJ = 70°C + (67mA)(24V)(34°C/W) = 125°C

Use of the EXTVCC input pin reduces the junction tem-

perature to:

TJ = 70°C + (67mA)(5V)(34°C/W) = 81°C

The absolute maximum rating for the INTVCC Pin is 40mA.

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.

EXTVCC Connection

The IC contains an internal P-channel MOSFET switch

connected between the EXTVCC and INTVCC pins. When

the voltage applied to EXTVCCrises above4.7V, the internal

regulator is turned off and the switch closes, connecting

the EXTVCCpin to the INTVCCpin 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 derived from the

output during normal operation (4.7V < VOUT < 7V) and

from the internal regulator when the output is out of regu-

lation (start-up, short-circuit). If more current is required

through the EXTVCC switch than is speciﬁed, an external

Schottky diode can be added between the EXTVCC and

INTVCC pins. Do not apply greater than 7V to the EXTVCC

pin and ensure that EXTVCC < VIN.

Signiﬁcant efﬁciency gains can be realized by powering

INTVCC from the output, since the VIN current resulting

from the driver and control currents will be scaled by a

factor of (Duty Cycle)/(Efﬁciency). For 5V regulators this

supply means connecting the EXTVCC pin directly to VOUT.

However, for 3.3V and other lower voltage regulators,

additional circuitry is required to derive INTVCC power

from the output.

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC Left Open (or Grounded). This will cause INTVCC

to be powered from the internal 5V regulator resulting in an

efﬁciency penalty of up to 10% at high input voltages.

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