Patent Publication Number: US-2005134323-A1

Title: Single event transient filter for comparator

Description:
FIELD OF THE INVENTION  
      The present invention relates to filters, and in particular to a single event transient filter for a comparator.  
     BACKGROUND OF THE INVENTION  
      Some comparators are susceptible to single event transients (SETs) caused by solar flares and other radiation events. This can cause disruption in the comparator output level, which can cause problems in some circuits, such as power supply monitoring circuitry, which may inadvertently cause board-level or system-level resets.  
      Many of today&#39;s commercial integrated circuit (IC) devices and multi-chip modules (MCM) cannot be utilized in deep space and earth orbiting applications because of radiation induced transient pulses or other SETs. The commercial IC devices are developed and manufactured for the computer and mass market applications and cannot withstand the effects of radiation induced single event transients from either the natural space environment caused by solar flares, galactic cosmic radiation and the Van Allen electron and proton belts or man-made radiation induced events (neutrons and gamma radiation).  
      Common methods of preventing SETs from degrading performance are to design special radiation tolerant integrated circuits. One example uses source/drain masks to locally implant the minority carrier lifetime reducer (MCLR) before the source/drain dopants are implanted. This requires control of the die mask production process and production of the die. Another method implements system level monitoring and system or subsystem level shutdown of latched circuitry. A fuse is used in a further method to limit the current through the device. This has the disadvantage of being a non-recoverable latch-up unless the fuse is reset. In a further device, circuitry, integrated into the IC package provides protection through the automatic limiting and removal of power during an SEL event, allowing the device to reset from the event and then power-up of the device.  
     SUMMARY OF THE INVENTION  
      A R-C low pass filter is used in series with a Schmitt-trigger to form a mask for single event transients (SETs) in a comparator. Transients are masked to logic devices attached to an output of the comparator. A mask time is determined in part by the time constant of the R-C filter, and in part by hysteresis trip points of the Schmitt-trigger input. A Schmitt-trigger inverter provides a stable logic level edge rate, which may have been affected by the R-C filter.  
      In a further embodiment, a reverse biased diode is positioned to bypass the filter when the comparator output is low. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a simplified circuit diagram of a single event transient filter with a comparator according to an embodiment of the invention.  
       FIG. 2  is a simplified circuit diagram of a further embodiment of the single event transient filter with a comparator according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.  
      A circuit  100  in  FIG. 1  comprises a comparator circuit  110  having an output  115  coupled to a R-C filter  120 . The comparator circuit  110  has a comparator  125  with a first input  130  and a second input  135 . Comparator  125  is a class of operational amplifier, such as one from the  139  family. (i.e. DSCC number 5962-96738—National Semiconductor&#39;s LM139A, DSCC number 5962-98613—Intersil&#39;s HS-139RH). Other types of comparators may also be used. A feedback resistor  140  is coupled between an output  145  of the comparator  125  and the first input  130 . A bias resistor  150  is coupled between the output  145  and a bias voltage.  
      The low pass filter  120  comprises a resistor  155  coupled to a capacitor  160  to ground or other current sink. An input  165  of a logic circuit  170  is coupled between the resistor  155  and capacitor  160 . The logic circuit  170  in one embodiment comprises a CMOS Schmitt-trigger inverter that provides a sharp edge output at a CMOS level.  
      Comparator circuit  110  is used in one embodiment to measure a difference in voltage between inputs  130  and  135 . The comparator  125  may be susceptible to single-event transients (SETs) caused by exposure to heavy ion environments, such as found in space, such as in satellites or other high altitude devices. These transients show up on the output  115  of the comparator as a positive or negative going voltage spike. Low pass filter  120  is placed at the output of the comparator  115  and has a time constant set by the combination of capacitor  160  and the thevenin equivalent resistance formed by resistors  140 ,  150  and  155 . The Schmitt-trigger inverter  170  is used on the output of the low pass filter to avoid slow rise and fall times inherent in R-C filters with very small rise and fall times. It also provides circuit  100  with nice clean CMOS edges.  
      In one embodiment, the R-C filter time constant is selected to accommodate a 3V transient of ˜3.4 us duration out of comparator  125 , a LM139. The time constant is large enough to mask the transient, but small enough not to mask “real” problems (i.e. power supply glitches in the event that the power supply is being monitored). In further embodiments, different transients may be of concern, and the time constant may be smaller, or much larger if desired. Values for components in the figures are for one embodiment, and are not meant to limit the scope of the invention to those values.  
       FIG. 2  shows an alternative circuit  200  that reduces delay caused in resetting the circuit. A reverse bias diode  210  is positioned across resistor  155 , and provides a bypass of the R-C filter  120  when the comparator output  115  transistions from high to low. It forces a board reset, and a high output value at  175 .  
      By using an R-C low-pass filter in series with a Schmitt-trigger inverter, transients are masked to logic devices attached to the output of the comparator. The mask time is determined in part by the time constant of the R-C filter, and in part by the hysteresis trip-points of the Schmitt-trigger  170  input  165 . The inverter portion of the Schmitt-trigger provides a stable logic level edge rate.