10
LT1249
APPLICATIONS INFORMATION
WUU
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voltages above 8V and the 12V limit is only for surge
conditions.
In normal operation, the voltage into M
OUT
does not
exceed 1.1V, but under surge conditions, the voltage
could temporarily go higher. To date, no field failures due
to surges have been reported for normal LT1249 configu-
rations, but if the possibility exists for extremely large
current surges, please read the following discussion.
Offline switching power supplies can create large current
surges because of the high value storage capacitor used.
The surge can be the result of closing the line switch near
the peak of the AC line voltage, or because of a large
transient in the line itself. These surges are well known in
the power supply business, and are normally controlled
with a negative temperature coefficient thermistor in
series with the rectifier bridge. When power is switched
on, the thermistor is cold (high resistance) and surges are
limited. Current flow in the thermistor causes it to heat and
resistance drops to the point where overall efficiency loss
in the resistor is acceptable.
This basic protection mechanism can be partially defeated
if the power supply is switched off for a few seconds, then
turned back on. The thermistor has not had time to cool
significantly and if the subsequent turn-on catches the AC
line near its peak, the resulting surge is much higher than
normal. Even if this surge current generates a voltage
greater than 6V (but less than 12V) across the sense
resistor, the standard LT1249 application will not be
affected because the chip is not yet powered. Problems are
only created if the V
CC
pin is powered from some external
housekeeping supply that remains powered when bridge
power is switched off.
A huge line voltage surge,
beyond the normal worst-case
limits
, can also create a large current surge. The peak of
the line voltage must significantly exceed the storage
capacitor voltage (typically 380V) for this to occur, so peak
line voltage would probably have to exceed 450V. Such
excessive surges might occur if a very large mains load
was suddenly removed, with a resulting line “kickback”. If
the surge results in voltage at the M
OUT
pin greater than
6V, it must also last more than 30µs (three switch cycles)
to cause FET problems.
External Clamp
The external clamp shown in Figure 6 will protect the
LT1249 M
OUT
pin against extremely large line current
surges (see above). Protection is provided for all V
CC
power methods. The 100Ω resistor and three diodes limit
the peak negative voltage into M
OUT
to less than 3V.
Current sense gain is attenuated by only 100Ω/4000Ω =
2.5%. Three diodes are used because the peak negative
voltage into M
OUT
in normal operation could go as high as
–1.1V and the diodes should not conduct more than a few
microamps under this condition.
Figure 6. Protecting M
OUT
from Extremely High Current Surges
THERMISTOR
100Ω
R
S
SURGE PATH
STORAGE
CAPACITOR
M
OUT
LT1249
+
–
BRIDGE
+
