LM3905N National Semiconductor, LM3905N Datasheet - Page 12

• 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

Application Hints
Triggering with Negative Edge
Although the LM122 is triggered by a positive going trigger
signal a differentiator tied to a normally ‘‘high’’ trigger will
result in negative edge triggering In Figure 19 R1 serves
the dual purpose of holding the trigger pin normally high and
differentiating the input trigger pulse coupled through C1
The timing diagram included with Figure 21 shows that trig-
gering actually occurs a short time after the negative going
trigger while positive going triggers have no effect The de-
lay time between a negative trigger signal and actual starts
of timing is approximately (0 5 to 1 5) (R1
on the trigger amplitude or about 2 5 to 7 5
values shown This time will have to be increased for C
larger than 0 01 F because C
er the trigger pin is kept high and must reset itself during the
short time that the trigger pin voltage is low A conservative
value for C1 is
Chain of Timers
The LM122 can be connected as a chain of timers quite
easily with no interface required In Figure 20A and 20B two
C1
t
(Continued)
t
10
is charged to V
C
t
C1) depending
REF
FIGURE 20 Chain of Timers
s with the
whenev-
t
(a)
(b)
12
possible connections are shown In both cases the output
of the timer is low during the timing period so that the posi-
tive going signal at the end of the timing period can trigger
the next timer There is no limitation on the timing period of
one timer with respect to any other timer before or after it
because the trigger input to any timer can be high or low
when that timer ends its timing period
FIGURE 19 Timer Triggered by Negative
Edge of Input Pulse
TL H 7768 – 33
TL H 7768– 30
TL H 7768– 31
TL H 7768– 32