LTM4641
APPENDICES
operation (FCB logic high), the inductor ripple current at
light loads appears as an asymmetrical truncated triangle
waveform; inductor current does not go below 0A.
through a board-to-board connector; an inductive length
of cable (say, 50cm in length, or more); or, if the load is
highly inductive—then it is proactive to leave provision
in one’s layout for a pair of small filter capacitors, C
CMA
Appendix F. Adjusting the Fast Output Overvoltage
Comparator Threshold
and C
OSNS
. C
and C
should be placed directly from
CMB CMA
CMB
OSNS
+
–
V
to SGND and V
to SGND, respectively—as
close to the pins of the module as possible. Configured
The output overvoltage inception threshold (OV
volt-
PGM
in this manner, C
and C
can be used to attenuate
CMA
CMB
age) can be adjusted or tightened from its default value.
The following guidelines must be followed, however:
common mode noise in the remote-sense signal pin pair.
•ꢀ ItꢀisꢀnotꢀrecommendedꢀtoꢀchangeꢀtheꢀOV
voltage
Appendix E. Inspiration For Pulse-Skipping Mode
Operation
PGM
dynamically because the fast OOV comparator has no
glitch immunity beyond what is provided by OV ’s
PGM
canmake
When M
is turned on—for a duration of time propor-
TOP
internal47pFcapacitor, androutingofOV
it vulnerable to electrostatic noise.
PGM
tionaltoI current—inductorcurrentisrampedupwards,
ION
and energy is built up in the inductor’s B-field. Ultimately,
a “packet” of energy is transferred from the input capaci-
tors to the output capacitors. In forced continuous mode
•ꢀ Theꢀ15.6μsꢀtimeꢀconstantꢀfilterꢀformedꢀbyꢀOV
’s in-
PGM
ternal47pFcapacitoranddefault499kΩ||1MΩresistor-
dividernetworkshouldbemaintainedforpracticalvalues
operation (FCB logic low), M
and M
are operated in
TOP
BOT
of OV
voltage: 0.6V < V
< 0.9V. Capacitive
apurelysynchronousfashion, meaning:whenM
ison,
PGM
OVPGM
TOP
filtering of OV must not be applied indiscriminately.
M
is off—and vice versa. Observe that when M
is
PGM
TOP
BOT
The OV
voltage must come up very rapidly with the
PGM
turnedoff,theB-fieldintheinductorcannotinstantaneously
1V atstart-up, topreventaraceconditionthatwould
vanish: the collapsing B-field forces inductor current to
flow through M ’s on-die Schottky diode—resulting in
unwanted freewheeling diode power loss; M
on for lower power loss, instead. With M
REF
otherwise result in nuisance OOV detection and a faulty
BOT
latchoff event—so any externally applied capacitance
is turned
BOT
on, inductor
cannot be arbitrarily high. On the other hand, OV
PGM
BOT
must have some filtering from switching noise sources
and should be sufficiently insulated from any possible
current ramps downward as energy in its B-field wanes.
In steady-state forced continuous mode operation, the
inductor ripple current appears as a triangle waveform
whose average value equates to the load’s current. Forced
continuous mode operation (forcing synchronous opera-
dynamic activity on 1V . (See Figure 9.)
REF
•ꢀ Externalꢀresistor(s)ꢀappliedꢀbetweenꢀOV
and 1V
/
REF
PGM
SGNDshouldberelativelyhighimpedance, tominimize
loading on the 1V output. Then, small values of
tion of M
and M ) provides a mechanism for consis-
REF
TOP
BOT
C
achieveaconsistenttimeconstantasOVPGM’s
resistance-divider network is altered.
tent output voltage ripple, regardless of the load current.
However, in this mode of operation, at light load currents
(say, less than 2A out), observe that the inductor current
is periodically negative—which means some packets of
energythataretransferredfromtheinputcapacitorstothe
output are recirculated and transferred back to the input
capacitors. This is a source of inefficiency that brings
about the motivation for pulse-skipping mode operation,
OVPGM
Figure 65 shows the optional network one can apply to
alter or tighten the OV
setpoint.
PGM
1V
REF
R
TOVPGM
LTM4641
OV
PGM
to turn off M
when the inductor current ramps down
BOT
C
R
BOVPGM
OVPGM
to 0A. This concept is also described in the industry as
“diode emulation”, because M is made to mimic the
SGND
4641 F65
BOT
behavior of a Schottky rectifier. In pulse-skipping mode
Figure 65. Optional OVPGM Network to Alter or Tighten VOVPGM
4641f
59
Linear [ Linear ]
