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LTM4643

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APPLICATIONS INFORMATION

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 LTM4643’s

output voltage is in regulation.

Operating Frequency

The operating frequency of the LTM4643 is optimized to

achievethecompactpackagesizeandtheminimumoutput

ripplevoltagewhilestillkeepinghighefficiency.Thedefault

operating frequency is internally set to 1.2MHz. In most

applications,noadditionalfrequencyadjustingisrequired.

If any operating frequency other than 1.2MHz is required

by application, the µModule regulator can be externally

synchronized to a clock from 850kHz to 1.5MHz.

Please note, a minimum switching frequency is required

for given VIN, VOUT operating conditions to keep a maxi-

mum peak-to-peak inductor ripple current below 2A for

the LTM4643. The peak-to-peak inductor ripple current

can be calculated as:

ΔIPK−PK =

VOUT

FS(MHz)

•

VIN – VOUT

VOUT

The maximum 2A peak-to-peak inductor ripple current

is enforced due to the nature of the valley current mode

control to maintain output voltage regulation at no load.

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 1.2MHz 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 100ns. 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.

Multichannel Parallel Operation

For loads that demand more than 3A of output current,

the LTM4643 multiple regulator channels can be easily

paralleled to provide more output current without increas-

ing input and output voltage ripples. The LTM4643 has

preset built-in phase shift between each two of the four

regulator channels which is suitable to employ a 2+2, 3+1

or 4 channels parallel operation. Table 2 gives the phase

difference between regulator channels.

Table 2. Phase Difference Between Regulator Channels

CHANNEL

CH1

CH2

CH3

CH4

Phase Difference

180°

90°

180°

Figure 1 shows a 2+2 and a 4-channels parallel concept

schematic for clock phasing.

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.

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