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LM122H データシート(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM122H
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)

LM122H データシート(HTML) 10 Page - National Semiconductor (TI)

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Typical Applications (Continued)
In this application the logic terminal is normally held high by
R3 When a trigger pulse is received Q1 is turned on driv-
ing the logic terminal to ground The result of triggering the
timer and reversing the logic at the same time is that the
output does not change from its initial low condition The
only time the output will change states is when the trigger
input stays high longer than one time period set by Rt and
Ct The output pulse width is equal to the input trigger width
minus Rt  Ct C2 insures no output pulse for short (kRC)
trigger pulses by prematurely resetting the timing capacitor
when the trigger pulse drops CL filters the narrow spikes
which would occur at the output due to propagation delays
during switching
5V Switching Regulator
Figure 13 is an application where the LM122 does not use
its timing function A switching regulator is made using the
internal reference and comparator to drive a PNP transistor
switch Features of this circuit include a 55V minimum input
voltage at 1A output current low part count and good effi-
ciency (l 75%) for input voltages to 10V Line and load
regulation are less than 05% and output ripple at the
switching frequency is only 30 mV Q1 is an inexpensive
plastic device which does not need a heatsink for ambient
temperature up to 50 C D1 should be a fast switching di-
ode Output voltage can be adjusted between 1V and 30V
by choosing proper values for R2 R3 R4 and R5 For out-
puts less than 2V a divider with 250X Thevinin resistance
must be connected between VREF and ground with its tap
point tied to VADJ
No 22 Wire Wound on Molybdenum Permalloy Core
TLH7768 – 24
FIGURE 13 5V Switching Regulator with
1 Amp Output and 55V Minimum Input
Application Hints
Aborting a Timing Cycle
The LM122 does not have an input specifically allocated to
a stop-timing function If such a function is desired it may be
accomplished several ways
 Ground VADJ
 Raise RC more positive than VADJ
 Wire ‘‘OR’’ the output
Grounding VADJ will end the timing cycle just as if the timing
capacitor had reached its normal discharge point A new
timing cycle can be started by the trigger terminal as soon
as the ground is released A switching transistor is best for
driving VADJ to as near ground as possible Worst case sink
current is about 300 mA
A timing cycle may also be ended by a positive pulse to a
resistor (R s Rt 100) in series with the timing capacitor
The pulse amplitude must be at least equal to VADJ (20V)
but should not exceed 50V When the timing capacitor dis-
charges a negative spike of up to 20V will occur across the
resistor so some caution must be used if the drive pulse is
used for other circuitry
TLH7768 – 25
FIGURE 14 Cycle Interrupt
The output of the timer can be wire ORed with a discrete
transistor or an open collector logic gate output This allows
overriding of the timer output but does not cause the timer
to be reset until its normal cycle time has elapsed
Using the LM122 as a Comparator
A built-in reference and zero volt common mode limit make
the LM122 very useful as a comparator Threshold may be
adjusted from zero to three volts by driving the VADJ termi-
nal with a divider tied to VREF Stability of the reference
voltage is typically g1% over a temperature range of
b
55 Cto a125 C Offset voltage drift in the comparator is
typically 25 mV C in the boosted mode and 50 mV C un-
boosted A resistor can be inserted in series with the input
to allow overdrives up to g50V as shown in
Figure 15
There is actually no limit on input voltage as long as current
is limited to g1 mA The resistor shown contributes a worst
case of 5 mV to initial offset In the unboosted mode the
error drops to 025 mV maximum The capability of operat-
ing off a single 5V supply with internal reference should
make this comparator very useful
10


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