LTC3728L datasheet(11/32 Pages) LINER

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LTC3728L/LTC3728LX

3728lxfa

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 left open, an internal 5V low

dropout linear regulator supplies INTVCC power. If EXTVCC

is taken above 4.7V, the 5V regulator is turned off and an

internal switch is turned on connecting EXTVCC to INTVCC.

This allows the INTVCC power to be derived from a high

efficiency external source such as the output of the regu-

lator itself or a secondary winding, as described in the

Applications Information section.

Output Overvoltage Protection

An overvoltage comparator, OV, guards against transient

overshoots (>7.5%) as well as other more serious condi-

tions that may overvoltage the output. In this case, the top

MOSFET is turned off and the bottom MOSFET is turned on

until the overvoltage condition is cleared.

Power Good (PGOOD) Pin

The PGOOD pin is connected to an open drain of an internal

MOSFET. The MOSFET turns on and pulls the pin low when

either output is not within

±7.5% of the nominal output

level as determined by the resistive feedback divider.

When both outputs meet the

±7.5% requirement, the

MOSFET is turned off within 10

µs and the pin is allowed to

be pulled up by an external resistor to a source of up to 7V.

Foldback Current, Short-Circuit Detection

and Short-Circuit Latchoff

The RUN/SS capacitors are used initially to limit the inrush

current of each switching regulator. After the controller

has been started and been given adequate time to charge

up the output capacitors and provide full load current, the

RUN/SS capacitor is used in a short-circuit time-out

circuit. If the output voltage falls to less than 70% of its

nominal output voltage, the RUN/SS capacitor begins

discharging on the assumption that the output is in an

overcurrent and/or short-circuit condition. If the condi-

tion lasts for a long enough period as determined by the

size of the RUN/SS capacitor, the controller will be shut

down until the RUN/SS pin(s) voltage(s) are recycled.

This built-in latchoff can be overridden by providing a

µA pull-up at a compliance of 5V to the RUN/SS pin(s).

This current shortens the soft start period but also pre-

vents net discharge of the RUN/SS capacitor(s) during an

overcurrent and/or short-circuit condition. Foldback cur-

rent limiting is also activated when the output voltage falls

below 70% of its nominal level whether or not the short-

circuit latchoff circuit is enabled. Even if a short is present

and the short-circuit latchoff is not enabled, a safe, low

output current is provided due to internal current foldback

and actual power wasted is low due to the efficient nature

of the current mode switching regulator.

THEORY AND BENEFITS OF 2-PHASE OPERATION

The LTC1628 and the LTC3728L family of dual high

efficiency DC/DC controllers brings the considerable ben-

efits of 2-phase operation to portable applications for the

first time. Notebook computers, PDAs, handheld termi-

nals and automotive electronics will all benefit from the

lower input filtering requirement, reduced electromag-

netic interference (EMI) and increased efficiency associ-

ated with 2-phase operation.

Why the need for 2-phase operation? Up until the 2-phase

family, constant-frequency dual switching regulators op-

erated both channels in phase (i.e., single-phase opera-

tion). This means that both switches turned on at the same

time, causing current pulses of up to twice the amplitude

of those for one regulator to be drawn from the input

capacitor and battery. These large amplitude current pulses

increased the total RMS current flowing from the input

capacitor, requiring the use of more expensive input

capacitors and increasing both EMI and losses in the input

capacitor and battery.

With 2-phase operation, the two channels of the dual-

switching regulator are operated 180 degrees out of

phase. This effectively interleaves the current pulses

drawn by the switches, greatly reducing the overlap time

where they add together.

The result is a significant reduc-

tion in total RMS input current, which in turn allows less

expensive input capacitors to be used, reduces shielding

requirements for EMI and improves real world operating

efficiency.

(Refer to Functional Diagram)

OPERATIO

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