LM3905N National Semiconductor, LM3905N Datasheet - Page 10

• Manufacturer Part Number: LM3905N
• Description: IC TIMER PRECISION 8-DIP
• Manufacturer: National Semiconductor

Specifications of LM3905N

• Voltage - Supply: *
• Current - Supply: 2.5mA
• Operating Temperature: *
• Package / Case: *
• Frequency: *
• Count: *
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LM3905N; LM3905

Figure 13 is an application where the LM122 does not use
Typical Applications
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 R
C
minus R
trigger pulses by prematurely resetting the timing capacitor
when the trigger pulse drops C
which would occur at the output due to propagation delays
during switching
5V Switching Regulator
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 5 5V minimum input
voltage at 1A output current low part count and good effi-
ciency (
regulation are less than 0 5% 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 250
must be connected between V
point tied to V
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
No 22 Wire Wound on Molybdenum Permalloy Core
t
Ground V
Raise R C more positive than V
Wire ‘‘OR’’ the output
The output pulse width is equal to the input trigger width
l
t
FIGURE 13 5V Switching Regulator with
1 Amp Output and 5 5V Minimum Input
75%) for input voltages to 10V Line and load
C
ADJ
t
ADJ
C2 insures no output pulse for short (
REF
L
(Continued)
filters the narrow spikes
ADJ
and ground with its tap
Thevinin resistance
TL H 7768–24
k
t
RC)
and
10
Grounding V
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 V
current is about 300 A
A timing cycle may also be ended by a positive pulse to a
resistor (R
The pulse amplitude must be at least equal to V
but should not exceed 5 0V When the timing capacitor dis-
charges a negative spike of up to 2 0V will occur across the
resistor so some caution must be used if the drive pulse is
used for other circuitry
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 V
nal with a divider tied to V
voltage is typically
b
typically 25 V C in the boosted mode and 50 V C un-
boosted A resistor can be inserted in series with the input
to allow overdrives up to
There is actually no limit on input voltage as long as current
is limited to
case of 5 mV to initial offset In the unboosted mode the
error drops to 0 25 mV maximum The capability of operat-
ing off a single 5V supply with internal reference should
make this comparator very useful
55 C to
ADJ
a
s
g
ADJ
125 C Offset voltage drift in the comparator is
to as near ground as possible Worst case sink
1 mA The resistor shown contributes a worst
R
FIGURE 14 Cycle Interrupt
t
will end the timing cycle just as if the timing
100) in series with the timing capacitor
g
1% over a temperature range of
g
REF
50V as shown in Figure 15
Stability of the reference
TL H 7768 – 25
ADJ
ADJ
(2 0V)
termi-