Timer Protected

Against Damage

for up to 50V

TLH7768 – 26

FIGURE 15 Comparator with 0V to 3V Threshold

Eliminating Timing Cycle Upon Initial

Application of Power

The LM122 will normally start a timing cycle (with no trigger

undesirable it can be defeated by tying the timing capacitor

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 RC pin and the positive end to

when the capacitor is discharged

TLH7768 – 27

FIGURE 16 Eliminating Initial Timing Cycle

Using Dual Supplies

The LM122 can be operated off dual supplies as shown in

Figure 17 The only limitation is that the emitter terminal

cannot be tied to ground it must either drive a load referred

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

‘‘high’’ is 25V R2 may be calculated from the divider equa-

tion with R1 to give these levels

TLH7768 – 28

Select for Proper Level Shift

Emitter Terminal or Emitter Load must be Tied to GND Pin of Timer

FIGURE 17 Operating Off Dual Supplies

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

TLH7768 – 29

FIGURE 18 Temperature Compensated

Linear Charging Sweep

Q1 converts the current through R1 to a current source in-

added to make the current through R1 independent of sup-

ply variations and temperature changes (D2 is a low TC

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 0015% C with a 15V supply

and 01% C with a 50V supply

11