LTM4641
APPLICATIONS INFORMATION—LOAD PROTECTION FEATURES
protection during severe load current step-down events,
but is not very effective at protecting loads from genuine
fault conditions such as a short circuited high side power
switching MOSFET. Furthermore, such schemes tend to
be implemented with the overvoltage detector’s threshold
dependent on the same bandgap voltage that the output is
being regulated to. Applications needing superior output
overvoltage and load protection require the performance
achievedwiththeoutputcrowbarMOSFET,MCBandpower
interruptswitch,MSP,andLTM4641’suseofanindependent
on the CROWBAR output; internal circuitry interfacing to
CROWBAR presents itself as a ~10kΩ load (see Figure 62
in Appendix A). The use of the PN diode and 10nF capaci-
tor creates a way for the CROWBAR output to stay logic
high, even if the duration of OOV is very brief, and assures
the glitch immunity of the latchoff detection circuitry is
overcome. The 10kΩ load and 10nF capacitor provide an
upper bound for the duration of time MCB might be on
after CROWBAR activates: 400μs, or four time constants.
Parasitic capacitance on the gate of MCB may increase
this time, slightly.
reference voltage(1V ) to generate an OOV threshold.
REF
Observe that when HYST is low, the noninverting input
to the fast OOV comparator (see Appendix A) is clamped
by a Schottky diode. (When RUN is low, the noninverting
input to the fast OOV comparator is clamped by two series
Schottky diodes.) This differs from when switching ac-
tion is engaged, where the noninverting input to the fast
Power-Interrupt ꢁOSFET (ꢁSP), CROWBAR Pin and
Output CROWBAR ꢁOSFET (ꢁCB)
Within 500ns (maximum) of the control-loop-referred
feedback signal, V , exceeding the voltage on OV
FB
PGM
(plus-or-minus OVP ), an OOV event is detected, and
ERR
OOV comparator is normally the V signal. Therefore, be
the CROWBAR output swings high enough to turn on an
FB
aware that the CROWBAR output is nominally inhibited
when switching action is inhibited.
optional crowbaring device (MCB) residing on V . No
OUT
more than 2.6µs after OOV detection, V is discharged
ING
and an optional power interrupt switch, MSP, disconnects
the LTM4641’s power stage from the input source supply.
Restarting regulation after a latchoff event has occurred
is explained in detail in the Start-Up/Shutdown section.
When MCB and MSP are used in conjunction as shown in
the Figure 46 circuit, the LTM4641 is able to provide best-
in-classoutputovervoltageprotectionagainstarguablythe
mostdespisedfailuremodehighstep-downbuckconvert-
ers can theoretically suffer: an electrical short between
the input source to the output, via the switching node.
Turning on MCB upon detection of OOV helps discharge
the output capacitors and prevent any further positive ex-
cursion of output voltage by transforming residual energy
in LTM4641’s power stage into heat; meanwhile, turning
off MSP removes a path for current flow between the in-
put power source and the output—preventing hazardous
(input) voltage from reaching the precious load.
MCBshouldbeplacedclosetothemajorityoftheload(s)’s
bulkandMLCClocalbypasscapacitors.CROWBARshould
be connected to the gate of MCB with a generous signal
tracewidth(20mils,or0.5mm),tosupportdrivingthepeak
currentneededtoturnonMCBuponOOVdetection. Atthe
instant that MCB turns on, it typically draws hundreds of
amps from the output capacitors which are mainly located
near the load. When MCB turns off, the B-field that may
have been built up in the parasitic inductance in the cop-
per plane between the output capacitors and MCB cannot
vanish instantaneously, and the collapsing of that B-field
caninduceanegativevoltageacrosstheoutputcapacitors
and load. Closer proximity of MCB to the majority of the
output capacitors minimizes this parasitic inductance and
hencetheresultingmagnitudeofthenegativevoltagespike.
It should be noted that when an OOV event is detected,
CROWBAR is not held high (equivalently, MCB is not left
turned on) indefinitely. The act of pulling CROWBAR high
(above 1.5V nominal), whether due to internal or external
circuitry,invokesalatchoffresponseandstrongdischarge
MCB must be selected according to the following criteria:
•ꢀ MCBꢀmustꢀbeꢀaꢀlogic-levelꢀN-channelꢀMOSFET
ofV ;HYSTislatchedlowandswitchingactionisinhibited
ING
•ꢀ Theꢀdrain-to-sourceꢀratingꢀofꢀMCBꢀmustꢀbeꢀgreaterꢀthanꢀ
after CROWBAR overcomes the glitch immunity require-
ment (see Figure 12). The fast OOV comparator’s output
is fed through a blocking PN diode into a 10nF capacitor
the maximum output voltage, V
OUT(PEAK,OOV_DETECTED)
4641f
33
Linear [ Linear ]
