LTM4630IV View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTM4630IV Datasheet PDF : 34 Pages
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LTM4630
APPLICATIONS INFORMATION
Pulse-Skipping Mode Operation
In applications where low output ripple and high effi-
ciency at intermediate currents are desired, 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
MOSFET to stay off for several cycles, thus skipping cycles.
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.
Either regulator can be configured for pulse-skipping mode.
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
always turns on with each oscillator pulse. During start-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
current without increasing input and output voltage ripples.
The MODE_PLLIN pin allows the LTM4630 to synchronize
to an external clock (between 400kHz and 780kHz) and
the internal phase-locked-loop allows the LTM4630 to lock
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
multiphase operation. The input RMS ripple current cancel-
lation mathematical derivations are presented, and a graph
is displayed representing the RMS ripple current reduction
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)
The LTM4630 device is operated over a range of frequencies
to improve power conversion efficiency. It is recommended
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
For more information www.linear.com/LTM4630
4630fa
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