Datasheet

LTM4620A
14
4620afb
For more information www.linear.com/LTM4620A
APPLICATIONS INFORMATION
Burst Mode Operation
The LTM4620A is capable of Burst Mode operation on
each regulator in which the power MOSFETs operate in
-
termittently based on load demand, thus saving quiescent
current. For applications where maximizing the efficiency
at very light loads is a high priority, Burst Mode operation
should be applied. Burst Mode operation is enabled with
the MODE/PLLIN pin floating. During this operation, the
peak current of the inductor is set to approximately one
third of the maximum peak current value in normal opera
-
tion even though the voltage at the COMP pin indicates
a
lower value. The voltage at the COMP pin drops when
the inductor’s average current is greater than the load
requirement. As the COMP voltage drops below 0.5V, the
BURST comparator trips, causing the internal sleep line
to go high and turn off both power MOSFETs.
In sleep mode, the internal circuitry is partially turned off,
reducing the quiescent current to about 450µA for each
output. The load current is now being supplied from the
output capacitors. When the output voltage drops, caus
-
ing COMP to rise above 0.5V, the internal sleep line goes
low,
and the LTM4620A resumes normal operation. The
next
oscillator cycle will turn on the top power MOSFET
and the switching cycle repeats. Either regulator can be
configured for Burst Mode operation.
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 LTM4620A to skip cycles at low output loads, thus
increasing efficiency by reducing switching loss. Tying
the MODE/PLLIN pin to INTV
CC
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, induc
-
tor 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 continu
-
ous mode is disabled and inductor current is prevented
from
reversing until the LTM4620A’s output voltage is in
regulation. Either regulator can be configured for force
continuous mode.
Multiphase Operation
For output loads that demand more than 13A of current,
two outputs in LTM4620A or even multiple LTM4620As
can be paralleled to run out of phase to provide more
output current without increasing input and output volt
-
age ripples. The MODE/PLLIN pin allows the LTM4620A
to synchronize to an external clock (between 400kHz and
780kHz) and the internal phase-locked-loop allows the
LTM4620A to lock onto an 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 PHMODE
pin to INTV
CC
, 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 PHMODE pin
of each LTM4620A channel 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
22