LTC7802EUFDM-3.3 datasheet(23/34 Pages) AD

23 / 34 page

LTC7802-3.3

Rev. 0

For more information www.analog.com

Single Output 2-Phase Operation

For high power 3.3V output applications, the two channels

can be operated in a 2-phase single output configuration.

The channels switch 180° out-of-phase, which reduces

the required output capacitance in addition to the required

input capacitance and power supply induced noise. To

configure the LTC7802-3.3for 2-phase operation, tie VFB2

to INTVCC, ITH2 to ground, and RUN2 to RUN1.

The RUN1, VOUT1, ITH1, TRACK/SS1 pins are then used

to control both channels, but each channel uses its own

ICMP and IR comparators to monitor their respective

inductor currents. Figure 10 is a typical application con-

figured for single output 2-phase operation.

INTVCC Regulators

The LTC7802-3.3 features two separate internal low

dropout linear regulators (LDOs) that supply power at

the INTVCC pin from either the VIN pin or the EXTVCC pin

depending 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

minimum 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 maxi-

mum junction temperature rating for the LTC7802-3.3 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

equations given in Note 2 of the Electrical Characteristics.

For example, the LTC7802-3.3 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

(typical), the VIN LDO is turned off and the EXTVCC LDO is

enabled. The EXTVCC LDO remains on as long as the volt-

age 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 con-

trol power to be derived from one of the LTC7802-3.3’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-circuit). If more

current is required through the EXTVCC LDO than is speci-

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.

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.

APPLICATIONS INFORMATION

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