LTM4644MPY datasheet(11/36 Pages) LINER

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LTM4644/LTM4644-1

4644fe

For more information www.linear.com/LTM4644

APPLICATIONS INFORMATION

Input Decoupling Capacitors

The LTM4644 module should be connected to a low ac-

impedance DC source. For each regulator channel, a 10µF

input ceramic capacitor is recommended for RMS ripple

current decoupling. A bulk input capacitor is only needed

when the input source impedance is compromised by long

inductive leads, traces or not enough source capacitance.

The bulk capacitor can be an electrolytic aluminum capaci-

tor or polymer capacitor.

Without considering the inductor ripple current, the RMS

current of the input capacitor can be estimated as:

ICIN(RMS) =

IOUT(MAX)

η%

• D •(1−D)

where η%istheestimatedefficiencyofthepowermodule.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth design,

only single piece of low ESR output ceramic capacitor is

required for each regulator channel to achieve low output

voltagerippleandverygoodtransientresponse.Additional

output filtering may be required by the system designer,

if further reduction of output ripples or dynamic transient

spikes is required. Table 7 shows a matrix of different

output voltages and output capacitors to minimize the

voltage droop and overshoot during a 2A load step tran-

sient. Multiphase operation will reduce effective output

ripple as a function of the number of phases. Application

Note77discussesthisnoisereductionversusoutputripple

current cancellation, but the output capacitance will be

more a function of stability and transient response. The

LTpowerCAD™DesignToolisavailabletodownloadonline

for output ripple, stability and transient response analysis

and calculating the output ripple reduction as the number

of phases implemented increases by N times.

Discontinuous Conduction Mode (DCM)

In applications where low output ripple and high efficiency

at intermediate current are desired, discontinuous con-

duction mode (DCM) should be used by connecting the

MODE pin to SGND. At light loads the internal current

comparator may remain tripped for several cycles and

force the top MOSFET to stay off for several cycles, thus

skipping cycles. The inductor current does not reverse

in this mode.

Force Continuous Conduction Mode (CCM)

In applications where fixed frequency operation is more

critical than low current efficiency, and where the lowest

output ripple is desired, forced continuous conduction

modeoperationshouldbeused.Forcedcontinuousopera-

tion can be enabled by tying the MODE pin to INTVCC. 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

MOSFETalwaysturnsonwitheachoscillatorpulse.During

start-up, forced continuous mode is disabled and inductor

current is prevented from reversing until the LTM4644’s

output voltage is in regulation.

Operating Frequency

The operating frequency of the LTM4644 is optimized to

achievethecompactpackagesizeandtheminimumoutput

ripplevoltagewhilestillkeepinghighefficiency.Thedefault

operating frequency is internally set to 1MHz. In most ap-

plications, no additional frequency adjusting is required.

If any operating frequency other than 1MHz is required

by application, the µModule regulator can be externally

synchronized to a clock from 700kHz to 1.3MHz.

Frequency Synchronization and Clock In

The power module has a phase-locked loop comprised

of an internal voltage controlled oscillator and a phase

detector. This allows all internal top MOSFET turn-on to

be locked to the rising edge of the same external clock.

The external clock frequency range must be within ±30%

around the 1MHz set frequency. A pulse detection circuit

is used to detect a clock on the CLKIN pin to turn on the

phase-locked loop. The pulse width of the clock has to

be at least 400ns. The clock high level must be above 2V

and clock low level below 0.3V. During the start-up of

the regulator, the phase-locked loop function is disabled.

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