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LTC3780

3780fe

APPLICATIONS INFORMATION

Schottky Diode (D1, D2) Selection

and Light Load Operation

The Schottky diodes D1 and D2 shown in Figure 1 conduct

during the dead time between the conduction of the power

MOSFET switches. They are intended to prevent the body

diode of synchronous switches B and D from turning on

and storing charge during the dead time. In particular, D2

signiﬁcantly reduces reverse recovery current between

switch D turn-off and switch C turn-on, which improves

converter efﬁciency and reduces switch C voltage stress.

In order for the diode to be effective, the inductance

between it and the synchronous switch must be as small

as possible, mandating that these components be placed

adjacently.

In buck mode, when the FCB pin voltage is 0.85 < VFCB

< 5V, the converter operates in skip-cycle mode. In this

mode, synchronous switch B remains off until the induc-

tor peak current exceeds one-ﬁfth of its maximum peak

current. As a result, D1 should be rated for about one-half

to one-third of the full load current.

In boost mode, when the FCB pin voltage is higher than

5.3V, the converter operates in discontinuous current mode.

In this mode, synchronous switch D remains off until the

inductor peak current exceeds one-ﬁfth of its maximum

peak current. As a result, D2 should be rated for about

one-third to one-fourth of the full load current.

In buck mode, when the FCB pin voltage is higher than 5.3V,

the converter operates in constant frequency discontinu-

ous current mode. In this mode, synchronous switch B

remains on until the inductor valley current is lower than

the sense voltage representing the minimum negative

inductor current level (VSENSE = –5mV). Both switch A

and B are off until next clock signal.

In boost mode, when the FCB pin voltage is 0.85 < VFCB

< 5.3V, the converter operates in Burst Mode operation.

In this mode, the controller clamps the peak inductor

current to approximately 20% of the maximum inductor

current. The output voltage ripple can increase during

Burst Mode operation.

INTVCC Regulator

An internal P-channel low dropout regulator produces 6V

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

the drivers and internal circuitry within the LTC3780. 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 directly adjacent

to the INTVCC and PGND IC pins is highly recommended.

Good bypassing is necessary to supply the high transient

current required by 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 LTC3780 to be

exceeded.Thesystemsupplycurrentisnormallydominated

by the gate charge current. Additional external loading of

the INTVCC also needs to be taken into account for the

power dissipation calculations. The total INTVCC current

can be supplied by either the 6V internal linear regulator

or by the EXTVCC input pin. When the voltage applied to

the EXTVCC pin is less than 5.7V, all of the INTVCC current

is supplied by the internal 6V linear regulator. Power dis-

sipation for the IC in this case is VIN • IINTVCC, and overall

efﬁciency is lowered. The junction temperature can be

estimated by using the equations given in Note 2 of the

Electrical Characteristics. For example, a typical application

operating in continuous current mode might draw 24mA

from a 24V supply when not using the EXTVCC pin:

TJ = 70°C + 24mA • 24V • 34°C/W = 90°C

Use of the EXTVCC input pin reduces the junction tem-

perature to:

TJ = 70°C + 24mA • 6V • 34°C/W = 75°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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