Linear ANALOG DEVICES LTM4644EY PBF Series Manual page 11

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:
I
OUT(MAX)
I
=
CIN(RMS)
η%
where η% is the estimated efficiency of the power module.
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
voltage ripple and very good transient response. 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
Note 77 discusses this noise reduction versus output ripple
current cancellation, but the output capacitance will be
more a function of stability and transient response. The
LTpowerCAD™ Design Tool is available to download online
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
• D • (1−D)
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LTM4644/LTM4644-1
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
mode operation should be used. Forced continuous opera-
tion can be enabled by tying the MODE pin to INTV
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 always turns on with each oscillator pulse. 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
achieve the compact package size and the minimum output
ripple voltage while still keeping high efficiency. The default
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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. In
CC
Rev. G
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