LTC7803 datasheet(24/36 Pages) AD

24 / 36 page

LTC7819

Rev. 0

For more information www.analog.com

Single Output Multi-Phase Operation

For high power applications, two or more channels can

be operated in a multi-phase single output configuration.

The channels switch 120° out-of-phase, which reduces

the required output capacitance in addition to the required

input capacitance and power supply induced noise.

Channel 1 is always the master channel in a multi-phase

application. Channels 2 or 3 are designated as slave chan-

nels that follow channel 1 if their respective VFB pins are

tied to INTVCC.

To configure for single output 3-phase operation, tie both

VFB2 and VFB3 to INTVCC, ITH2 and ITH3 to ground, and

RUN2 and RUN3 to RUN1. The RUN1, VFB1, ITH1, and

TRACK/SS1 pins then control all three channels, but each

channel uses its own ICMP and IR comparators to moni-

tor their respective inductor currents. Figure 10 is a typical

application configured for single output 3-phase operation.

Similarly, to configure two channels for single output

2-phase with the third channel as a separate independent

output, tie either VFB2 or VFB3 to INTVCC. That channel

will then follow channel 1 as described above, and should

have its ITH pin tied to ground and its RUN pin tied to

RUN1. The third channel can be used as an independent

single-phase output.

INTVCC Regulators

The LTC7819 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

maximum junction temperature rating for the LTC7819 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 4.8V, 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 LTC7819 INTVCC current is limited to less

than 46mA from a 36V supply when not using the EXTVCC

supply at a 70°C ambient temperature:

TJ = 70°C + (46mA)(36V)(33°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 4.8V, 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 LTC7819’s switch-

ing 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 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

APPLICATIONS INFORMATION

LTC7803 Datasheet(PDF) 24 Page - Analog Devices

LTC7803 Datasheet(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL