LTC3621 datasheet(9/20 Pages) AD

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LTC3644/LTC3644-2

Rev. 0

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OPERATION

The LTC3644/LTC3644-2 is a quad high efficiency

monolithic step-down regulator, which uses a constant

frequency, peak current mode architecture. It operates

through a wide VIN range and regulates with ultralow

quiescent current. The operation frequency is set at either

1MHz or 2.25MHz and can be synchronized to an external

oscillator ±50% of the inherent frequency. To suit a variety

of applications, the selectable MODE/SYNC pin allows the

user to trade off output ripple for efficiency.

For each channel, the output voltage is set by an external

divider returned to the FB pin. An error amplifier compares

the divided output voltage with a reference voltage of

0.6V and adjusts the peak inductor current accordingly.

Overvoltage and undervoltage comparators pull the

PGOOD output low if the output voltage is not within 7.5%

of the programmed value. The PGOOD output goes high

immediately after achieving regulation and goes low 32

clock cycles after falling out of regulation.

Main Control Loop

During normal operation, the top power switch (P-channel

MOSFET)isturnedonatthebeginningofaclockcycle.The

inductorcurrentisallowedtorampuptoapeaklevel.Once

the level is reached, the top power switch is turned off and

the bottom switch (N-channel MOSFET) is turned on until

the next clock cycle. The peak current level is controlled

by the internally compensated ITH voltage, which is the

output of the error amplifier. This amplifier compares the

FB voltage to the 0.6V internal reference. When the load

current increases, the FB voltage decreases slightly below

the reference, which causes the error amplifier to increase

the ITH voltage until the average inductor current matches

the new load current.

The main control loop is shut down by pulling the RUN

pin to ground.

Low Current Operation

Twodiscontinuousconductionmodes(DCM)areavailable

to control the operation of the LTC3644 at low currents.

Both modes, Burst Mode operation and pulse-skipping

mode, automatically switch from continuous operation to

the selected mode when the load current is low.

Tooptimizeefficiency,BurstModeoperationcanbeselected

by tying the MODE/SYNC pin to INTVCC. In Burst Mode

operation, the peak inductor current is set to be at least

550mA, even if the output of the error amplifier demands

less.Thus,whentheswitcherisonatrelativelylightoutput

loads, FB voltage will rise and cause the ITH voltage to

drop. Once the ITH voltage goes below 0.2V, the switcher

goes into sleep mode with both power switches off. The

switchers remain in this sleep state until the external load

pulls the output voltage below its regulation point. When

all channels are in sleep mode, the part draws an ultralow

10µA of quiescent current from SVIN.

To minimize VOUT ripple, pulse-skipping mode can be

selected by grounding the MODE/SYNC pin. In LTC3644,

pulse-skipping mode is implemented similarly to Burst

Mode operation with the peak inductor current set to be at

least 90mA. This results in lower ripple than in Burst Mode

operation with the trade-off of slightly lower efficiency.

Forced Continuous Mode Operation

The LTC3644 also has the ability to operate in the forced

continuous mode by setting the MODE/SYNC voltage

between1VandVINTVCC–1.2V.Inforcedcontinuousmode,

the switcher switches cycle by cycle regardless of what

the output load current is. If forced continuous mode is

selected, the minimum peak current is set to be –250mA

in order to ensure that the part can operate continuously

at zero output load.

High Duty Cycle/Dropout Operation

Whentheinputsupplyvoltagedecreasestowardstheoutput

voltage, the duty cycle increases and slope compensation

is required to maintain the fixed switching frequency. The

LTC3644 has internal circuitry to accurately maintain the

peak current limit (ILIM) of 2.2A even at high duty cycles.

As the duty cycle approaches 100%, the LTC3644 enters

dropoutoperation.Duringdropout,thetopPMOSswitchis

turnedoncontinuously,andallactivecircuitryiskeptalive.

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