LTM4627_15 datasheet(12/32 Pages) LINER

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LTM4627

4627fc

For more information www.linear.com/LTM4627

Output Capacitors

The LTM4627 is designed for low output voltage ripple

noise. The bulk output capacitors defined as COUT are

chosen with low enough effective series resistance (ESR)

to meet the output voltage ripple and transient require-

ments. COUT can be a low ESR tantalum capacitor, low

ESR Polymer capacitor or ceramic capacitors. The typical

outputcapacitancerangeisfrom200µFto800µF.Additional

output filtering may be required by the system designer

if further reduction of output ripple or dynamic transient

spikesisrequired.Table4showsamatrixofdifferentoutput

voltages and output capacitors to minimize the voltage

droop and overshoot during a 7.5A/µs transient. The table

optimizes total equivalent ESR and total bulk capacitance

to optimize the transient performance. Stability criteria

are considered in the Table 4 matrix, and LTpowerCAD is

available for stability analysis. Multiphase operation will

reduceeffectiveoutputrippleasafunctionofthenumberof

phases.ApplicationNote77discussesthisnoisereduction

versus output ripple current cancellation, but the output

capacitance should be considered carefully as a function

of stability and transient response. LTpowerCAD can be

usedtocalculatetheoutputripplereductionasthenumber

of implemented phases increases by N times.

Burst Mode Operation

The LTM4627 is capable of Burst Mode operation in which

the power MOSFETs operate intermittently based on load

demand, thus saving quiescent current. For applications

where maximizing the efficiency at very light loads is a

high priority, Burst Mode operation should be applied. To

enableBurstModeoperation,simplyfloattheMODE_PLLIN

pin. During Burst Mode operation, the peak current of the

inductorissettoapproximately30%ofthemaximumpeak

current value in normal operation even though the voltage

at the COMP pin indicates a lower value. The voltage at the

COMP pin drops when the inductor’s average current is

greater than the load requirement. As the COMP voltage

drops below 0.5V, the burst comparator trips, causing

the internal sleep line to go high and turn off both power

MOSFETs.

In sleep mode, the internal circuitry is partially turned

off, reducing the quiescent current. The load current is

now being supplied from the output capacitors. When the

output voltage drops, causing COMP to rise, the internal

sleep line goes low, and the LTM4627 resumes normal

operation. The next oscillator cycle will turn on the top

power MOSFET and the switching cycle repeats.

Pulse-Skipping Mode Operation

In applications where low output ripple and high effi-

ciencyatintermediatecurrentsaredesired,pulse-skipping

mode should be used. Pulse-skipping operation allows

the LTM4627 to skip cycles at low output loads, thus

increasing efficiency by reducing switching loss. Tying

the MODE_PLLIN pin to INTVCC enables pulse-skipping

operation. With pulse-skipping mode at light load, the

internalcurrentcomparatormayremaintrippedforseveral

cycles, thus skipping operation cycles. This mode has

lower ripple than Burst Mode operation and maintains a

higher frequency operation than Burst Mode operation.

Forced Continuous Operation

In applications where fixed frequency operation is more

critical than low current efficiency, and where the lowest

output ripple is desired, forced continuous operation

should be used. Forced continuous operation can be

enabled by tying the MODE_PLLIN pin to ground. In this

mode, inductor current is allowed to reverse during low

output loads, the COMP voltage is in control of the current

comparator threshold throughout, and the top MOSFET

alwaysturnsonwitheachoscillatorpulse.Duringstart-up,

forced continuous mode is disabled and inductor current

is prevented from reversing until the LTM4627’s output

voltage is in regulation.

APPLICATIONS INFORMATION

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