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LM122 データシート(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM122
部品情報  Precision Timers
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM122 データシート(HTML) 6 Page - National Semiconductor (TI)

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Pin Function Description (Continued)
trollers Typical temperature drift of the reference is less
than 001% C
The trigger terminal is used to start a timing cycle (see
functional diagram) Initially Q1 is saturated Ct is dis-
charged and the latching buffer output (V1) is latched high
A trigger pulse unlatches the buffer V1 goes low and turns
Q1 off The timing capacitor Ct connected from RC to GND
will begin to charge When the voltage at the RC terminal
reaches the 20V threshold of the comparator the compara-
tor toggles latching the buffer output (V1) in the high state
This turns on Q1 discharges the capacitor Ct and the cycle
is ready to begin again
If the trigger is held high as the timing period ends the
comparator will toggle and V1 will go high exactly as before
However V1 will not be latched and the capacitor will not
discharge until the trigger again goes low When the trigger
goes low V1 remains high but is now latched
Trigger
threshold is typically 16V at 25 C and has a tem-
perature dependence of b50 mV C Current drawn from
the trigger source is typically 20 mA at threshold rising to
600 mA at 30V then leveling off due to FET action of the
series resistor R5 For negative input trigger voltages the
only current drawn is leakage in the nA region The trigger
can be driven from supplies as high as g40V even when
device supply voltage is only 5V
The RC pin is tied to the non-inverting side of the compara-
tor and to the collector of Q1 Timing ends when the voltage
on this pin reaches 20V (1 RC time constant referenced to
the 315V regulator) Q1 turns on only if the trigger voltage
has dropped below threshold In comparator or regulator
applications of the timer the trigger is held permanently
high and the RC pin acts just like the input to an ordinary
comparator The maximum voltages which can be applied to
this pin are a55V and b07V Current from the RC pin is
typically 300 pA when the voltage is negative with respect to
the VADJ terminal For higher voltages the current drops to
leakage levels In the boosted mode input current is typical-
ly 30 nA Gain of the comparator is very high 200000 or
more depending on the state of the logic reverse pin and
the connection of the output transistor
The ground pin of the LM122 need not necessarily be tied
to system ground It can be connected to any positive or
negative voltage as long as the supply is negative with re-
spect to the Va terminal Level shifting may be necessary
for the input trigger if the trigger voltage is referred to sys-
tem ground This can be done by capacitive coupling or by
actual resistive or active level shifting One point must be
kept in mind the emitter output must not be held above the
ground
terminal with a low source impedance This could
occur for instance if the emitter were grounded when the
ground
pin of the LM122 was tied to a negative supply
The terminal labled VADJ is tied to one side of the compara-
tor and to a voltage divider between VREF and ground The
divider voltage is set at 632% of VREF with respect to
groundexactly one RC time constant The impedance of
the divider is increased to about 30k with a series resistor to
present a minimum load on external signals tied to VADJ
This resistor is a pinched type with a typical variation in
nominal value of b50% a100% and a TC of 07% C For
this reason external signals (typically a pot between VREF
and ground) connected to VADJ should have a source re-
sistance as low as possible For small changes in VADJ up
to several kX is all right but for large variations 250X or
less should be maintained This can be accomplished with a
1k pot since the maximum impedance from the wiper is
250X If a voltage is forced on VADJ from a hard source
voltage should be limited to b05 and a50V or current
limited to g10 mA This includes capacitively coupled sig-
nals because even small values of capacitors contain
enough energy to degrade the input stage if the capacitor is
driven with a large fast slewing signal The VADJ pin may be
used to abort the timing cycle Grounding this pin during the
timing period causes the timer to react just as if the capaci-
tor voltage had reached its normal RC trigger point the
capacitor discharges and the output charges state An ex-
ception to this occurs if the trigger pin is held high when the
VADJ pin is grounded In this case the output changes
state but the capacitor does not discharge
If the trigger drops while VADJ is being held low discharge
will occur immediately and the cycle will be over If the trig-
ger is still high when VADJ is released the output may or
may not change state depending on the voltage across the
timing capacitor For voltages below 20V across the timing
capacitor the output will change state immediately then
once more as the voltage rises past 20V For voltages
above 20V no change will occur in the output This pin is
not available on the LM2905LM3905
In noisy environments or in comparator-type applications a
bypass capacitor on the VADJ terminal may be needed to
eliminate spurious outputs because it is high impedance
point The size of the cap will depend on the frequency and
energy content of the noise A 01 mF will generally suffice
for spike suppression but several mF may be used if the
timer is subjected to high level 60 Hz EMI
The emitter and the collector outputs of the timer can be
treated just as if they were an ordinary transistor with 40V
minimum collector-emitter breakdown voltage Normally the
emitter
is tied to the ground pin and the signal is taken
from the collector orthe collector is tied to Va and the
signal is taken from the emitter Variations on these basic
connections are possible The collector can be tied to any
positive voltage up to 40V when the signal is taken from the
emitter
However the emitter will not be pulled higher than
the supply voltage on the Va pin Connecting the collector
to a voltage less than the Va voltage is allowed The emit-
ter
should not be connected to a low impedance load other
than that to which the ground pin is tied The transistor has
built-in current limiting with a typical knee current of 120 mA
Temporary short circuits are allowed even with collector-
emitter
voltages up to 40V The power x time product how-
ever must not exceed 15 watt-seconds for power levels
above the maximum rating of the package A short to 30V
6


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