LTC3728LEGN-1-TRPBF 查看數據表（PDF） - Linear Technology

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LTC3728LEGN-1-TRPBF

Dual, 550kHz, 2-Phase Synchronous Regulator

Linear Technology 

LTC3728L-1

OPERATION (Refer to Functional Diagram)

as Burst Mode operation, but does provide a lower noise,

constant frequency operating mode down to approximately

1% of the designed maximum output current.

Continuous Current (PWM) Operation

Tying the FCB pin to ground will force continuous current

operation. This is the least efficient operating mode, but

may be desirable in certain applications. The output can

source or sink current in this mode. When sinking current

while in forced continuous operation, the controller will

cause current to flow back into the input filter capacitor.

If large enough, this element will prevent the input sup-

ply from boosting to unacceptably high levels; see COUT

Selection in the Applications Information Section.

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 al-

lows the INTVCC power to be derived from a high efficiency

external source such as the output of the regulator 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 10μs and the pin is allowed to be pulled

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

Foldback Current

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

current of each switching regulator. Foldback current limit-

ing is activated when the output voltage falls below 70%

of its nominal level. If a short is present, a safe, low output

current is provided due to the 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-1 family of dual high ef-

ficiency DC/DC controllers brings the considerable benefits

of 2-phase operation to portable applications for the first

time. Notebook computers, PDAs, handheld terminals and

automotive electronics will all benefit from the lower input

filtering requirement, reduced electromagnetic interference

(EMI) and increased efficiency associated with 2-phase

operation.

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

family, constant-frequency dual switching regulators

operated both channels in phase (i.e., single-phase

operation). 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 reduction 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.

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