In this application the logic terminal is normally held high by

R3 When a trigger pulse is received Q1 is turned on driv-

ing the logic terminal to ground The result of triggering the

timer and reversing the logic at the same time is that the

output does not change from its initial low condition The

only time the output will change states is when the trigger

trigger pulses by prematurely resetting the timing capacitor

which would occur at the output due to propagation delays

during switching

5V Switching Regulator

Figure 13 is an application where the LM122 does not use

its timing function A switching regulator is made using the

internal reference and comparator to drive a PNP transistor

switch Features of this circuit include a 55V minimum input

voltage at 1A output current low part count and good effi-

regulation are less than 05% and output ripple at the

switching frequency is only 30 mV Q1 is an inexpensive

plastic device which does not need a heatsink for ambient

temperature up to 50 C D1 should be a fast switching di-

ode Output voltage can be adjusted between 1V and 30V

by choosing proper values for R2 R3 R4 and R5 For out-

puts less than 2V a divider with 250X Thevinin resistance

No 22 Wire Wound on Molybdenum Permalloy Core

TLH7768 – 24

FIGURE 13 5V Switching Regulator with

1 Amp Output and 55V Minimum Input

Application Hints

Aborting a Timing Cycle

The LM122 does not have an input specifically allocated to

a stop-timing function If such a function is desired it may be

accomplished several ways

capacitor had reached its normal discharge point A new

timing cycle can be started by the trigger terminal as soon

as the ground is released A switching transistor is best for

current is about 300 mA

A timing cycle may also be ended by a positive pulse to a

but should not exceed 50V When the timing capacitor dis-

charges a negative spike of up to 20V will occur across the

resistor so some caution must be used if the drive pulse is

used for other circuitry

TLH7768 – 25

FIGURE 14 Cycle Interrupt

The output of the timer can be wire ORed with a discrete

transistor or an open collector logic gate output This allows

overriding of the timer output but does not cause the timer

to be reset until its normal cycle time has elapsed

Using the LM122 as a Comparator

A built-in reference and zero volt common mode limit make

the LM122 very useful as a comparator Threshold may be

b

typically 25 mV C in the boosted mode and 50 mV C un-

boosted A resistor can be inserted in series with the input

Figure 15

There is actually no limit on input voltage as long as current

case of 5 mV to initial offset In the unboosted mode the

error drops to 025 mV maximum The capability of operat-

ing off a single 5V supply with internal reference should

make this comparator very useful

10