Functional Description

TLH10837 – 4

FIGURE 1 Logic Symbol

When power is applied to the LM2240 with no trigger or

reset inputs activated the circuit starts with all outputs

HIGH Application of a positive going trigger pulse to the

trigger pin initiates the timing cycle The trigger input acti-

vates the time-base oscillator enables the counter section

and sets the counter outputs LOW The time-base oscillator

period of the waveform appearing at pin 14 These clock

pulses are counted by the binary counter section The tim-

ing sequence is completed when a positive going reset

pulse is applied to the Reset pin

Once triggered the circuit is immune from additional trigger

inputs until the timing cycle is completed or a reset input is

applied If both the reset and trigger are activated simulta-

neously the trigger takes precedence

Figure 2 gives the timing sequence of output waveforms at

various circuit terminals subsequent to a trigger input

When the circuit is in a reset state both the time-base and

the counter sections are disabled and all the counter out-

puts are HIGH

TLH10837 – 5

FIGURE 2 Timing Diagram of Output Waveforms

In monostable applications one or more of the counter out-

puts are connected to the reset terminal with S1 closed

(Figure 3) The circuit starts timing when a trigger is applied

and automatically resets itself to complete the timing cycle

when a programmed count is completed Each of the vari-

ous multiplies of T shown at the LM2240’s outputs in

Figure

3 represent the duration of time after a trigger pulse that

that particular output is low (it’s

not the period of the wave-

sents the duration of time that the circuit output which is the

common point of all the counter outputs which are shorted

together is low If none of the counter outputs are connect-

ed back to the reset terminal (switch S1 open) the circuit

operates in an astable or free running mode following a

trigger input

TLH10837 – 6

FIGURE 3 Basic Circuit Connection

for Timing Applications

(Monostable S1 Closed Astable S1 Open)

Important Operating Information

Ground connection is pin 9

Reset (R) (pin 10) sets all outputs HIGH

Trigger (TRIG) (pin 11) sets all outputs LOW

Time-base output (TBO) (pin 14) can be disabled by bring-

ing the RC input (pin 13) LOW via a 10 kX resistor

Normal TBO (pin 14) is a negative going pulse greater than

500 ns

may be too narrow to trigger the counter section This can be cor-

rected by connecting a 600 pF capacitor from TBO (pin 14) to

ground (pin 9)

Reset (pin 10) stops the time-base oscillator

For use with external clock minimum clock pulse amplitude

should be 30V with greater than 10 ms pulse duration

5