LM3905N National Semiconductor, LM3905N Datasheet - Page 9

• 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 9 the timer is used to drive a relay ‘‘on’’ R
Typical Applications
The average charging current through R
some attention must be paid to parts layout to prevent stray
leakage paths The suggested timing capacitor has a typical
self time constant of 300 hours and a guaranteed minimum
of 25 hours at
sufficient data is available on their leakage characteristics
Two Terminal Time Delay Switch
The LM122 can be used as a two terminal time delay switch
if an ‘‘on’’ voltage drop of 2V to 3V can be tolerated In
seconds after application of power ‘‘Off’’ current of the
switch is 4 mA maximum and ‘‘on’’ current can be as high
as 50 mA
Zero Power Dissipation Between Timing Intervals
In some applications it is desirable to reduce supply current
drain to zero between timing cycles In Figure 10 this is
accomplished by using an external PNP as a latch to drive
the V
Between timing periods Q1 is off and no supply current is
drawn When a trigger pulse of 5V minimum amplitude is
received the LM122 output transistor and Q1 latch for the
duration of the timing period D1 prevents the step on the
V
input is a short pulse C1 and R2 may be eliminated R
must have a minimum value of (V
Frequency to Voltage Converter
An accurate frequency to voltage converter can be made
with the LM122 by averaging output pulses with a simple
one pole filter as shown in Figure 11 Pulse width is adjusted
with R2 to provide initial calibration at 10 kHz The collector
of the output transistor is tied to V
plitude pulses equal to V
C1 filter the pulses to give a dc output equal to
(R
put If better linearity is desired R5 can be tied to the sum-
ming node of an op amp which has the filter in the feedback
path If a low output impedance is desired a unity gain buff-
er such as the LM110 can be tied to the output An analog
meter can be driven directly by placing it in series with R5 to
ground A series RC network across the meter to provide
damping will improve response at very low frequencies
Pulse Width Detector
By driving the logic terminal of the LM122 simultaneous to
the trigger input a simple accurate pulse width detector can
be made (Figure 12)
a
t
)(C
pin from coupling back into the trigger pin If the trigger
a
t
)(V
pin of the timer
FIGURE 9 2-Terminal Time Delay Switch
REF
)(f) Linearity is about 0 2% for a 0V to 1V out-
a
25 C Other capacitor types may be used if
REF
at the emitter output R4 and
CC
REF
(Continued)
) (2 5 mA)
giving constant am-
t
is about 30 nA so
TL H 7768– 20
t
C
L
t
9
(Tachometer) Output Independent of Supply Voltage
FIGURE 11 Frequency to Voltage Converter
FIGURE 10 Zero Power Dissipation
FIGURE 12 Pulse Width Detector
Between Timing Intervals
V
Pulse Out
OUT
e
0 for W R
e
W
b
1
R
C
1
1
C
1
for W R
1
TL H 7768 – 21
TL H 7768 – 22
TL H 7768 – 23
C
1