LT3757 datasheet(22/36 Pages) LINER

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LTC3787

3787fc

APPLICATIONS INFORMATION

EXTVCC remains above 4.55V. The EXTVCC LDO attempts

to regulate the INTVCC voltage to 5.4V, so while EXTVCC

is less than 5.4V, the LDO is in dropout and the INTVCC

voltage is approximately equal to EXTVCC. When EXTVCC

is greater than 5.4V, up to an absolute maximum of 6V,

INTVCC is regulated to 5.4V.

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 79°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)(90°C/W) = 77°C

If more current is required through the EXTVCC LDO than

is specified, an external Schottky diode can be added be-

tween the EXTVCC and INTVCC pins. Make sure that in all

cases EXTVCC ≤ VBIAS (even at start-up and shutdown).

The following list summarizes possible connections for

EXTVCC:

EXTVCC Grounded.ThiswillcauseINTVCCtobepowered

from the internal 5.4V regulator resulting in an efficiency

penalty at high input voltages.

EXTVCC Connected to an External Supply. If an external

supply is available in the 5V to 6V range, it may be used

to provide power. Ensure that EXTVCC is always lower

than VBIAS.

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CB connected to the BOOST

pins supply the gate drive voltages for the topside

MOSFETs. Capacitor CB in the Block Diagram is charged

though 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 volt-

age, 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 CB needs to be 100 times that of the

total input capacitance 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

should have a reverse leakage less than the available output

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 LTC3787. When the

junction temperature exceeds approximately 170°C, the

overtemperature circuitry disables the INTVCCLDO,causing

the INTVCC supply to collapse and effectively shut down

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