LT3757 datasheet(23/38 Pages) LINER

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LTC3784

3784fb

For more information www.linear.com/LTC3784

applicaTions inForMaTion

Significant thermal gains can be realized by powering

INTVCC from an external supply. Tying the EXTVCC pin

to a 5V supply reduces the junction temperature in the

previous example from 125°C to 77°C in a QFN package:

TJ = 70°C + (32mA)(5V)(43°C/W) = 77°C

and from 125°C to 74°C in an SSOP package:

TJ = 70°C + (15mA)(5V)(80°C/W) = 77°C

The following list summarizes possible connections for

EXTVCC:

EXTVCCGrounded.ThiswillcauseINTVCCtobepowered

fromtheinternal5.4Vregulatorresultinginanefficiency

penalty at high input voltages.

EXTVCC Connected to an External Supply. If an external

supply is available in the 5V to 14V range, it may be

used to provide power. Ensure that EXTVCC is always

lower than or equal to VBIAS.

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CB connected to the BOOST

pins supply the gate drive voltages for the topside MOS-

FETs. Capacitor CBintheBlockDiagramischargedthough

external diode DB from INTVCC when the SW pin is low.

When one of the topside MOSFETs is to be turned on, the

driver places the CB voltage across the gate and source

of the desired MOSFET. This enhances the MOSFET and

turns on the topside switch. The switch node voltage, SW,

rises to VOUT and the BOOST pin follows. With the topside

MOSFET on, the boost voltage is above the output voltage:

VBOOST = VOUT + VINTVCC. The value of the boost capacitor

CBneedstobe100timesthatofthetotalinputcapacitance

of the topside MOSFET(s). The reverse breakdown of the

external Schottky diode must be greater than VOUT(MAX).

The external diode DB can be a Schottky diode or silicon

diode, but in either case it should have low leakage and fast

recovery. Pay close attention to the reverse leakage at high

temperatures, where it generally increases substantially.

Each of the topside MOSFET drivers includes an internal

charge pump that delivers current to the bootstrap capaci-

tor from the BOOST pin. This charge current maintains

the bias voltage required to keep the top MOSFET on

continuously during dropout/overvoltage conditions. The

Schottky/silicon diodes selected for the topside drivers

shouldhaveareverseleakagelessthantheavailableoutput

current the charge pump can supply. Curves displaying

the available charge pump current under different operat-

ing conditions can be found in the Typical Performance

Characteristics section.

A leaky diode DB in the boost converter can not only

prevent the top MOSFET from fully turning on but it can

also completely discharge the bootstrap capacitor CB and

create a current path from the input voltage to the BOOST

pin to INTVCC. This can cause INTVCC to rise if the diode

leakage exceeds the current consumption on INTVCC.

This is particularly a concern in Burst Mode operation

where the load on INTVCC can be very small. The external

Schottky or silicon diode should be carefully chosen such

that INTVCC never gets charged up much higher than its

normal regulation voltage.

Fault Conditions: Overtemperature Protection

At higher temperatures, or in cases where the internal

power dissipation causes excessive self heating on-chip

(such as an INTVCC short to ground), the overtemperature

shutdown circuitry will shut down the LTC3784. When the

junction temperature exceeds approximately 170°C, the

overtemperaturecircuitrydisablestheINTVCCLDO,causing

the INTVCC supply to collapse and effectively shut down

the entire LTC3784 chip. Once the junction temperature

drops back to approximately 155°C, the INTVCCLDOturns

back on. Long term overstress (TJ > 125°C) should be

avoided as it can degrade the performance or shorten

the life of the part.

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