LTM4641
APPLICATIONS INFORMATION—POWER SUPPLY FEATURES
The output capacitors C
and C
must
Pulse-Skipping ꢁode vs Forced Continuous ꢁode
OUT(BULK)
OUT(MLCC)
be chosen with low enough effective series resistance
(ESR) to meet the output voltage ripple requirements
and provide localized bypassing for the load. Although
the LTM4641 provides fast transient response, the output
voltage at the POL is reliant on nearby charge stored in a
In applications where high DC/DC conversion efficiency
at light-load currents is highly desired—when the input
voltage source is a battery, for example—pulse-skipping
modeoperationshouldbeemployed.Pulse-skippingmode
operation prevents power flow from the output capacitors
reservoir of ceramic capacitors C
to minimize
OUT(MLCC)
to the input source. Be aware, however, due to M ’s re-
BOT
sag and overshoot in the initial microseconds of a high
dI/dt transient load step-up and step-down, respectively.
sulting asynchronous operation at light load, applications
employing pulse-skipping mode may necessitate more
If used, C
can be comprised of low ESR tantalum
OUT(BULK)
output capacitance and/or a higher OV
setting than
PGM
or low ESR polymer capacitor(s); these capacitors then
serve as a local reservoir to replenish the MLCCs during
operation in forced continuous mode would.
Pulse-skipping mode is activated by connecting FCB to
transient load events. It is also possible to use C
OUT(MLCC)
INTV . Forced continuous operation is activated by con-
only, however, the use of feedforward capacitors, C ,
CC
FF
necting FCB to SGND.
shouldthenbeinstalledintheremote-sensefeedbackpath,
to obtain an optimized transient response (see Figure 5
feedback connections).
Be aware that in pulse-skipping mode and ultralight loads
(say, less than 20mA out), the V voltage may appear as
ING
a sawtooth waveform as a result of being charge-pumped
The C
ceramic capacitors should be at least
OUT(MLCC)
at a slower rate, to conserve energy.
X5R-type material. X5R-type and X7R-type MLCCs are
recommended when operating PCB temperatures are not
more than 85°C and 125°C, respectively. Both materials
are renown in the industry for having a relatively low ca-
pacitance change over their respective temperature range
of operation ( 15%). However, X5R and X7R MLCCs do
exhibit significant loss of capacitance with applied DC
voltage and are subject to aging effects, and this must
be taken into account in any system design. Refer to the
capacitor manufacturer’s specifications for details.
SeeAppendixEformoreinformationonhowpulse-skipping
mode works.
Soft-Start, Rail-Tracking and Start-Up Into Pre-Bias
The TRACK/SS pin can be used to either soft-start the
output of the LTM4641 regulator, or make LTM4641’s
output voltage track another rail coincidentally or ratio-
metrically. When RUN or HYST is low, the TRACK/SS
pin is discharged. When RUN and HYST are released,
TRACK/SS sources a microamp of current.
The typical output capacitance range is between 200μF
to 800μF. The system designer should use discretion in
determining whether additional output filtering may be
needed, if further reduction of output ripple—or output
voltage deviation during dynamic load or line transient
events—is required.
When a soft-start capacitor, C , is applied to the pin, the
SS
currentsourceisresponsibleforgeneratinganoutputvolt-
age turn-on time of 0.6ms per nanofarad of capacitance.
The power stage is high impedance (M
and M
TOP
BOT
are off) until the TRACK/SS pin voltage exceeds V , the
FB
In Table 9, guidelines are provided for output capacitor
selection, for various operating conditions. The table
optimizes total equivalent ESR and total bulk capacitance
for the transient load step performance. Stability criteria
is considered. The Linear Technology LTpowerCAD™ de-
sign tool is available for transient simulation and stability
analysis, if desired.
remote-sense differential amplifier’s output voltage. This
allowspower-upintopre-biasedoutputvoltageconditions
without sinking of current from the output capacitors.
WhenTRACK/SSexceedsthecontrolIC’s600mVbandgap
voltage, V is regulated at 600mV and V
reaches its
FB
OUT
nominal output voltage.
4641f
24
Linear [ Linear ]
