LTC7803 datasheet(11/36 Pages) AD

11 / 36 page

LTC7803

Rev 0

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OPERATION

Main Control Loop

The LTC7803 uses a constant frequency, peak current

mode step-down architecture. During normal operation,

the external top MOSFET is turned on when the clock for

that channel sets the RS latch, and is turned off when the

main current comparator, ICMP, resets the RS latch. The

peak inductor current at which ICMP trips and resets the

latch is controlled by the voltage on the ITH pin, which is

the output of the error amplifier, EA. The error amplifier

compares the output voltage feedback signal at the VFB

pin (which is generated with an external resistor divider

connected across the output voltage, VOUT, to ground)

to the internal 0.800V reference voltage. When the load

current increases, it causes a slight decrease in VFB rela-

tive to the reference, which causes the EA to increase the

ITH voltage until the average inductor current matches

the new load current.

After the top MOSFET is turned off each cycle, the bot-

tom MOSFET is turned on until either the inductor current

goes to far negative (or starts to reverse in Burst or pulse

skipping mode as indicated by the current comparator IR)

or the beginning of the next clock cycle.

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTVCC pin.

When the EXTVCC pin is tied to a voltage less than 4.7V,

the VIN LDO (low dropout linear regulator) supplies 5.15V

from VIN to INTVCC. If EXTVCC is taken above 4.7V, the VIN

LDO is turned off and an EXTVCC LDO is turned on. Once

enabled, the EXTVCC LDO supplies 5.15V from EXTVCC to

INTVCC. Using the EXTVCC pin allows the INTVCC power to

be derived from a high efficiency external source.

Boost Supply and Dropout (BOOST and SW pins)

The top MOSFET driver is biased from the floating boot-

strap capacitor, CB, which normally recharges during each

cycle through an external low leakage Schottky diode or

PN Junction diode, DB, diode when the top MOSFET turns

off. If the input voltage, VIN, decreases to a voltage close

to VOUT, the loop may enter dropout and attempt to turn

on the top MOSFET continuously. The LTC7803 has an

internal charge pump that allows the top MOSFET to be

turned on continuously at 100% duty cycle.

Shutdown and Start-Up (RUN, TRACK/SS Pins)

The LTC7803 can be shutdown using the RUN pin. Pulling

this pin below 1.1V shuts down the main control loop.

Pulling the RUN pin below 0.7V disables the controller and

most internal circuits, including the INTVCC LDOs. In this

state, the LTC7803 draws only 1μA of quiescent current.

The RUN pin needs to be externally pulled up or driven

directly by logic. It can also be implemented as an under-

voltage lockout (UVLO) by connecting it to the output of an

external resistor divider network off VIN (see Applications

Information section).

The start-up of the controller’s output voltage VOUT is

controlled by the voltage on the TRACK/SS pin. When

the voltage on the TRACK/SS pin is less than the 0.8V

internal reference, the LTC7803 regulates the VFB volt-

age to the TRACK/SS pin voltage instead of the 0.8V

reference. This allows the TRACK/SS pin to be used to

program a soft-start by connecting an external capaci-

tor from the TRACK/SS pin to SGND. An internal 12.5μA

pull-up current charges this capacitor creating a voltage

ramp on the TRACK/SS pin. As the TRACK/SS voltage

rises linearly from 0V to 0.8V (and beyond), the output

voltage VOUT rises smoothly from zero to its final value.

Alternatively the TRACK/SS pin can be used to cause the

start-up of VOUT to track that of another supply. Typically,

this requires connecting to the TRACK/SS pin an exter-

nal resistor divider from the other supply to ground (see

Applications Information section).

Light Load Current Operation: Burst Mode Operation,

Pulse-Skipping or Forced Continuous Mode (MODE

Pin)

The LTC7803 can be enabled to enter high efficiency

Burst Mode operation, constant frequency pulse-skipping

mode, or forced continuous conduction mode at low load

currents.

To select Burst Mode operation, tie the MODE pin to GND.

To select forced continuous operation, tie the MODE pin

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