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

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

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

TLH7768 – 20

FIGURE 9 2-Terminal Time Delay Switch

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

C1 filter the pulses to give a dc output equal to

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)

TLH7768 – 21

FIGURE 10 Zero Power Dissipation

Between Timing Intervals

TLH7768 – 22

FIGURE 11 Frequency to Voltage Converter

(Tachometer) Output Independent of Supply Voltage

TLH7768 – 23

FIGURE 12 Pulse Width Detector

9