LTM4630 datasheet(13/34 Pages) LINER

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LTM4630

4630fa

For more information www.linear.com/LTM4630

APPLICATIONS INFORMATION

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 LTM4630 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. At light loads the internal current comparator

may remain tripped for several cycles and force the top

MOSFETtostayoffforseveralcycles,thusskippingcycles.

The inductor current does not reverse in this mode. This

mode will maintain higher effective frequencies thus lower

output ripple and lower noise than Burst Mode operation.

Eitherregulatorcanbeconfiguredforpulse-skippingmode.

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 GND. 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 LTM4630’s output

voltage is in regulation. Either regulator can be configured

for force continuous mode.

Multiphase Operation

For output loads that demand more than 18A of current,

two outputs in LTM4630 or even multiple LTM4630s can

be paralleled to run out of phase to provide more output

currentwithoutincreasinginputandoutputvoltageripples.

The MODE_PLLIN pin allows the LTM4630 to synchronize

to an external clock (between 400kHz and 780kHz) and

theinternalphase-locked-loopallowstheLTM4630tolock

onto incoming clock phase as well. The CLKOUT signal

can be connected to the MODE_PLLIN pin of the following

stage to line up both the frequency and the phase of the

entire system. Tying the PHASMD pin to INTVCC,SGND,or

(floating) generates a phase difference (between

MODE_PLLIN and CLKOUT) of 120 degrees, 60 degrees,

or 90 degrees respectively. A total of 12 phases can be

cascaded to run simultaneously with respect to each other

by programming the PHASMD pin of each LTM4630 chan-

nel to different levels. Figure 3 shows a 2-phase design,

4-phase design and a 6-phase design example for clock

phasing with the PHASMD table.

A multiphase power supply significantly reduces the

amount of ripple current in both the input and output ca-

pacitors. The RMS input ripple current is reduced by, and

the effective ripple frequency is multiplied by, the number

of phases used (assuming that the input voltage is greater

than the number of phases used times the output voltage).

The output ripple amplitude is also reduced by the number

of phases used when all of the outputs are tied together

to achieve a single high output current design.

The LTM4630 device is an inherently current mode con-

trolled device, so parallel modules will have very good

current sharing. This will balance the thermals on the

design. Figure 26 shows an example of parallel operation

and pin connection.

Input RMS Ripple Current Cancellation

Application Note 77 provides a detailed explanation of

multiphaseoperation.TheinputRMSripplecurrentcancel-

lationmathematicalderivationsarepresented,andagraph

isdisplayedrepresentingtheRMSripplecurrentreduction

as a function of the number of interleaved phases. Figure 4

shows this graph.

Frequency Selection and Phase-Lock Loop

(MODE_PLLIN and fSET Pins)

TheLTM4630deviceisoperatedoverarangeoffrequencies

toimprovepowerconversionefficiency.Itisrecommended

to operate the module at 500kHz over the output range for

the best efficiency and inductor current ripple

The LTM4630 switching frequency can be set with an

external resistor from the fSET pin to SGND. An accurate

10µA current source into the resistor will set a voltage

that programs the frequency or a DC voltage can be

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