The LM122 series are precision timers that
offer great versatility with high accuracy.
They operate with unregulated supplies from 4.5V to 40V while maintaining
constant timing periods from microseconds to hours. Internal logic and
regulator circuits complement the basic timing function enabling the LM122
series to operate in many different applications with a minimum of external
The output of the timer is a floating transistor with built in
The trigger input to the LM122 series has a threshold of 1.6V
current limiting. It can drive either ground referred or supply
referred loads up to 40V and 50 mA. The floating nature of
this output makes it ideal for interfacing, lamp or relay
driving, and signal conditioning where an open collector or
emitter is required. A "logic reverse" circuit can be
programmed by the user to make the output transistor either
"on" or "off" during the timing period.
independent of supply voltage, but it is fully protected against
inputs as high as ±40V-even when using a 5V supply. The
circuitry reacts only to the rising edge of the trigger signal,
and is immune to any trigger voltage during the timing periods.
An internal 3.15V regulator is included in the timer to reject
supply voltage changes and to provide the user with a
convenient reference for applications other than a basic timer.
External loads up to 5 mA can be driven by the regulator. An
internal 2V divider between the reference and ground sets the
timing period to 1 RC. The timing period can be voltage
controlled by driving this divider with an external source
through the VADJ pin. Timing ratios of 50:1 can be easily
The comparator used in the LM122 utilizes high gain PNP input
transistors to achieve 300 pA typical input bias current over
common mode range of 0V to 3V. A boost terminal allows the
user to increase comparator operating current for timing
periods less than 1 ms. This lets the timer operate over a 3
to multi-hour timing range with excellent repeatability.
The LM122 operates over a temperature range of -55°C to
+125°C. An electrically identical LM322 is specified from
to +70°C. The LM3905 is identical to the LM122 series except
that the boost and VADJ pin options are not available, limiting
minimum timing period to 1 ms.
Immune to changes in trigger voltage during
Timing periods from microseconds to hours
Internal logic reversal
Immune to power supply ripple during the timing
Operates from 4.5V to 40V supplies
Input protected to ±40V
Floating transistor output with internal current
Internal regulated reference
Timing period can be voltage controlled
TTL compatible input and output