LTM4641
APPLICATIONS INFORMATION—LOAD PROTECTION FEATURES
•ꢀ The drain-to-source breakdown voltage of MSP must
be greater than the maximum input source voltage.
Consult the MOSFET vendor’s data sheet and consider
temperature effects.
•ꢀ WhenꢀCROWBARꢀgoesꢀlogicꢀhigh,ꢀtheꢀpeakꢀdrainꢀcur-
rent in MCB will be given by V
/
OUT(PEAK,OOV_DETECTED)
. The peak drain current, and its duration, must
R
DS(ON)
not exceed the maximum safe operating area of the
MOSFET; consult the MOSFET vendor’s data sheet. An
upper bound for MCB’s on-time is 400μs. However, this
worst-caseconductiontimecanonlyhappeniftheoutput
•ꢀ Inꢀorderꢀtoꢀsupportꢀveryꢀfastꢀturn-onꢀofꢀoutputꢀvolt-
age (e.g., sub 1ms ramp up), MSP should be turned
on quickly to bring up V quickly. Therefore, a gate
INH
capacitance on V
is extraordinarily large. The length
OUT
input capacitance (C ) below 4.7nF is preferred (less
ISS
of time that MCB can possibly conduct ultrahigh drain
currentꢀisꢀalsoꢀboundedꢀbyꢀ4ꢀ•ꢀR ꢀ•ꢀC
is better).
.
OUT(TOTAL)
DS(ON)
•ꢀ MSPꢀ mustꢀ beꢀ ableꢀ toꢀ conductꢀ theꢀ maximumꢀ inputꢀ
current to the LTM4641’s power stage without getting
too hot. Choose a suitable MOSFET package size and
In a majority of applications, output capacitance is low
enough that MCB does not conduct ultrahigh drain
current for longer than a few microseconds, as seen
on the front page.
R
that results in reasonable MOSFET junction
DS(ON)
temperature rise. Be mindful that I
during low line operation.
is highest
Q(VINH)
•ꢀ MCB’sꢀ junctionꢀ temperatureꢀ mustꢀ notꢀ exceedꢀ itsꢀ
specified maximum at any time. Consult the MOSFET
vendor’s data sheet for device thermal characteristics
for “single shot” thermal transients or “single pulse”
Blowingaseries-passinputfusewithacrowbaringSCRcan
be an effective overvoltage protection scheme for higher
output voltages, e.g., 5V, but a crowbaring MOSFET on the
output of the converter is more effective at clamping the
output voltage. For the same current, the power MOSFET
will have much less voltage drop than the PN-junction
voltage drop of an SCR. SCR-based circuits involving the
LTM4641 are not presented here. Evaluation of induced or
simulated overvoltage events on a demo board (such as
DC1543) is recommended to ensure the end result meets
the user’s expectations.
power-handling capability. The peak power sustained
2
by MCB is V
/R
.
OUT(PEAK,OOV_DETECTED)
DS(ON)
If MCB is used and it is expected that LATCH will be
toggled high (to unlatch the LTM4641) or held logic high
continuously (for automatic LTM4641 restart after fault-
off), recognize that peak power sustained by MCB during
CROWBAR activity may not be single pulse anymore.
Therefore, to prevent MCB thermal overstress in such
applications, it is recommended to use C
to set a rea-
TMR
Fast Output Overvoltage Coꢃparator Threshold
sonable cool-down period for the MOSFET. Additionally,
one may opt to implement a circuit that shuts down the
LTM4641 when MCB temperature is detected to be too
high: a minor modification to Figure 47, RT1 would be
located as close in proximity to MCB as possible (instead
of MSP), and R1, R2, and R3 would be experimentally
determined. Consult the MOSFET vendor’s data sheet for
maximum rated junction temperature and device thermal
characteristics for repeated pulsed-power transients.
O
V
is nominally biased by internal circuitry to
PGM
666mV, according to a 499kΩ and 1MΩ resistor-divider
network internal to the LTM4641 driven from the 1V
.
REF
This pin connects directly to the inverting input of the
fast OOV comparator—setting the trip threshold that
the control-loop-referred feedback voltage, V , would
FB
have to exceed to result in CROWBAR becoming logic
high. Recall that the control-loop pulse frequency modu-
lates M
such that V is driven to the lesser of the
TOP
FB
When using MSP, connect V to V
and to the gate of
INGP
ING
TRACK/SSpinorthebandgapreferencevoltageof600mV.
When TRACK/SS (and hence, the output voltage) has
MSP.SeetheInputCapacitorssection(earlier)forinforma-
tion on the input bypassing technique when MSP is used.
been fully ramped up, the 666mV on OV
represents
PGM
an OOV setting 11% above nominal output voltage. To
increase the OOV threshold, a resistor can be connected
MSP must be selected according to the following criteria:
•ꢀ MSPꢀcanꢀbeꢀeitherꢀaꢀstandardꢀlogicꢀorꢀaꢀlogic-levelꢀ
externally from 1V
to OV
; to decrease the OOV
REF
PGM
N-channel MOSFET.
threshold, a resistor can be connected externally from
4641f
34
Linear [ Linear ]
