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LTC1628CG-SYNC

High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

Linear Technology 

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OPERATIO (Refer to Functional Diagram)

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, 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

>5µA 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.

LTC1628-SYNC

THEORY AND BENEFITS OF 2-PHASE OPERATION

The LTC1628-SYNC dual high efficiency DC/DC controller

brings the considerable benefits of 2-phase operation to

portable applications for the first time. Notebook comput-

ers, 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 LTC1628

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.

Figure 3 compares the input waveforms for a representa-

tive single-phase dual switching regulator to the new

LTC1628-SYNC 2-phase dual switching regulator. An

actual measurement of the RMS input current under these

conditions shows that 2-phase operation dropped the

input current from 2.53ARMS to 1.55ARMS. While this is an

impressive reduction in itself, remember that the power

losses are proportional to IRMS2, meaning that the actual

power wasted is reduced by a factor of 2.66. The reduced

input ripple voltage also means less power is lost in the

input power path, which could include batteries, switches,

trace/connector resistances and protection circuitry. Im-

provements in both conducted and radiated EMI also

directly accrue as a result of the reduced RMS input

current and voltage.

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