LM3905N National Semiconductor, LM3905N Datasheet - Page 11

• 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 17 The only limitation is that the emitter terminal
Application Hints
Eliminating Timing Cycle Upon Initial
Application of Power
The LM122 will normally start a timing cycle (with no trigger
input) when V
undesirable it can be defeated by tying the timing capacitor
to V
nection does not affect operation of the timer in any other
way If an electrolytic timing capacitor is used be sure the
negative end is tied to the R C pin and the positive end to
V
timing capacitor to limit the surge current load on V
when the capacitor is discharged
Using Dual Supplies
The LM122 can be operated off dual supplies as shown in
cannot be tied to ground it must either drive a load referred
to V
tive coupling is shown for the trigger input (to allow 5V trig-
gering) a resistor can be substituted for C1 R2 must be
chosen to give proper level shifting between the trigger sig-
nal and the trigger pin of the timer Worst case ‘‘lo’’ on the
trigger pin (with respect to V
Timer Protected
Against Damage
for up to 50V
REF
FIGURE 15 Comparator with 0V to 3V Threshold
REF
b
A 1 0 k
or be actually tied to V
FIGURE 16 Eliminating Initial Timing Cycle
instead of ground as shown in Figure 16 This con-
a
resistor should be included in series with the
is first turned on If this characteristic is
b
(Continued)
b
as shown Although capaci-
) is 0 8V and worst case
TL H 7768– 26
TL H 7768– 27
REF
11
‘‘high’’ is 2 5V R2 may be calculated from the divider equa-
tion with R1 to give these levels
Linearizing the Charging Sweep
In some applications (such as a linear pulse width modula-
tor) it may be desirable to have the timing capacitor charge
from a constant current source A simple way to accomplish
this is shown in Figure 18
Q1 converts the current through R1 to a current source in-
dependent of the voltage across C
added to make the current through R1 independent of sup-
ply variations and temperature changes (D2 is a low TC
type) D2 and R3 can be omitted if the V
and D1 and R2 can be omitted also if temperature stability is
not critical With D1 D2 R2 and R3 omitted the current
through R1 will change about 0 015% C with a 15V supply
and 0 1% C with a 5 0V supply
Select for Proper Level Shift
Emitter Terminal or Emitter Load must be Tied to GND Pin of Timer
FIGURE 17 Operating Off Dual Supplies
FIGURE 18 Temperature Compensated
Linear Charging Sweep
t
R2 R3 D1 and D2 are
a
supply is stable
TL H 7768 – 28
TL H 7768 – 29