LTC7802EUFDM datasheet(22/34 Pages) AD

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LTC7802

Rev. 0

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on the EXTVCC pin voltage. INTVCC powers the MOSFET

gate drivers and most of the internal circuitry. The VIN

LDO and the EXTVCC LDO each regulate INTVCC to 5.1V

and can provide a peak current of at least 100mA.

The INTVCC pin must be bypassed to ground with a min-

imum of 4.7μF ceramic capacitor, placed as close as

possible to the pin. An additional 1μF ceramic capacitor

placed directly adjacent to the INTVCC and GND pins is

also highly recommended to supply the high frequency

transient currents required by the MOSFET gate drivers.

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the max-

imum junction temperature rating for the LTC7802 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 dissi-

pation for the IC in this case 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 equa-

tions given in Note 2 of the Electrical Characteristics. For

example, the LTC7802 INTVCC current is limited to less

than 35mA from a 36V supply when not using the EXTVCC

supply at a 70°C ambient temperature:

TJ = 70°C + (35mA)(36V)(43°C/W) = 125°C

To prevent the maximum junction temperature from being

exceeded, the input supply current must be checked

while operating in continuous conduction mode (MODE

= INTVCC) at maximum VIN.

When the voltage applied to EXTVCC rises above 4.7V (typ-

ical), 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 approximately

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 30V), INTVCC is regulated

to 5.1V. Using the EXTVCC LDO allows the MOSFET

driver and control power to be derived from one of the

LTC7802’s switching regulator outputs (4.8V ≤ VOUT ≤

30V) during normal operation and from the VIN LDO when

the output is out of regulation (e.g., start-up, short-cir-

cuit). If more current is required through the EXTVCC LDO

than is specified, 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.

Significant efficiency and thermal gains can be realized

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

rent resulting from the driver and control currents will be

scaled by a factor of VOUT/(VIN • Efficiency). For 5V to 30V

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 + (35mA)(8.5V)(43°C/W) = 83°C

However, for 3.3V and other low voltage outputs, addi-

tional circuitry is required to derive INTVCC power from

the output.

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC grounded. This will cause INTVCC to be pow-

ered from the internal VIN LDO resulting in an efficiency

penalty of up to 10% or more at high input voltages.

2. EXTVCC connected directly to one of the regulator out-

puts. This is the normal connection for an application

with an output in the range of 5V to 30V and provides

the highest efficiency. If both outputs are in the 5V to

30V range, connect EXTVCC to the lesser of the two

outputs to maximize efficiency.

3. EXTVCC connected to an external supply. If an external

supply is available, it may be used to power EXTVCC

provided that it is compatible with the MOSFET gate

drive requirements. This supply may be higher or lower

than VIN; however, a lower EXTVCC voltage results in

higher efficiency.

4. EXTVCC connected to an output-derived boost or

charge pump. For regulators where both outputs are

below 5V, efficiency gains can still be realized by con-

necting EXTVCC to an output-derived voltage that has

been boosted to greater than 4.8V.

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