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LTC3890-2

38902f

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the

maximum junction temperature rating for the LTC3890-2

to be exceeded. The INTVCC current, which is dominated

by the gate charge current, may be supplied by either the

VIN LDO or the EXTVCC LDO. When the voltage on the

EXTVCC pin is less than 4.7V, the VIN LDO is enabled. Power

dissipation for the IC in this case is highest and is equal

to VIN • IINTVCC. 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 3 of the

Electrical Characteristics. For example, the LTC3890E-2

INTVCC current is limited to less than 32mA from a 40V

supply when not using the EXTVCC supply at a 70°C ambi-

ent temperature:

TJ = 70°C + (32mA)(40V)(43°C/W) = 125°C

To prevent the maximum junction temperature from be-

ing exceeded, the input supply current must be checked

while operating in forced continuous mode (PLLIN/MODE

= INTVCC) at maximum VIN.

When the voltage applied to EXTVCC rises above 4.7V, the

VIN LDO is turned off and the EXTVCC LDO is enabled. The

EXTVCC LDO remains on as long as the voltage applied to

EXTVCC remains above 4.5V. The EXTVCC LDO attempts

to regulate the INTVCC voltage to 5.1V, so while EXTVCC

is less than 5.1V, the LDO is in dropout and the INTVCC

voltage is approximately equal to EXTVCC. When EXTVCC

is greater than 5.1V, up to an absolute maximum of 14V,

INTVCC is regulated to 5.1V.

Using the EXTVCC LDO allows the MOSFET driver and

control power to be derived from one of the LTC3890-2’s

switching regulator outputs (4.7V ≤ VOUT ≤ 14V) during

normal operation and from the VIN LDO when the output

is out of regulation (e.g., start-up, short-circuit). If more

current is required through the EXTVCC LDO than is speci-

APPLICATIONS INFORMATION

fied, an external Schottky diode can be added between the

EXTVCC and INTVCC pins. In this case, do not apply more

than 6V to the EXTVCC pin and make sure that EXTVCC ≤ VIN.

Significant efficiency and thermal gains can be realized

by powering INTVCC from the output, since the VIN cur-

rent resulting from the driver and control currents will be

scaled by a factor of (Duty Cycle)/(Switcher Efficiency).

For 5V to 14V regulator outputs, this means connecting

the EXTVCC pin directly to VOUT. Tying the EXTVCC pin to

an 8.5V supply reduces the junction temperature in the

previous example from 125°C to:

TJ = 70°C + (32mA)(8.5V)(43°C/W) = 82°C

However, for 3.3V and other low voltage outputs, additional

circuitry is required to derive INTVCCpowerfromtheoutput.

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC Grounded.ThiswillcauseINTVCC tobepowered

from the internal 5.1V regulator resulting in an efficiency

penalty of up to 10% at high input voltages.

2. EXTVCC Connected Directly to VOUT. This is the normal

connection for a 5V to 14V regulator and provides the

highest efficiency.

3. EXTVCC Connected to an External Supply. If an external

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

used to power EXTVCCprovidingitiscompatiblewiththe

MOSFET gate drive requirements. Ensure that EXTVCC

< VIN.

4. EXTVCCConnectedtoanOutput-DerivedBoostNetwork.

For 3.3V and other low voltage regulators, efficiency

gains can still be realized by connecting EXTVCC to an

output-derived voltage that has been boosted to greater

than 4.7V. This can be done with the capacitive charge

pump shown in Figure 9. Ensure that EXTVCC < VIN.

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