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Signetics Application Note 170 

PDF version AN170

AN97.pdf  LM122 App Note

Data Sheets
 NE555.pdf         ~130-kbytes




 LM556.pdf         ~159-kbytes

 NE558.pdf          ~75-kbytes

 ICM7555C.pdf    ~106-kbytes


General Description
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 components.

 The output of the timer is a floating transistor with built in
 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.

 The trigger input to the LM122 series has a threshold of 1.6V
 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 a
 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 µs
 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 0°C
 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 interval 
     Timing periods from microseconds to hours 
     Internal logic reversal 
     Immune to power supply ripple during the timing interval 
     Operates from 4.5V to 40V supplies 
     Input protected to ±40V 
     Floating transistor output with internal current limiting 
     Internal regulated reference 
     Timing period can be voltage controlled 
     TTL compatible input and output 



      555 Timer Circuits  Over 80 circuits. Great Resource 



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