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LM193-MIL
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SLIS183 –JUNE 2017
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8.2.2 Detailed Design Procedure
When using the device in a general comparator application, determine the following:
• Input Voltage Range
• Minimum Overdrive Voltage
• Output and Drive Current
• Response Time
8.2.2.1 Input Voltage Range
When choosing the input voltage range, the input common mode voltage range (VICR) must be taken in to
account. If temperature operation is below 25°C the VICR can range from 0 V to VCC– 2.0 V. This limits the input
voltage range to as high as VCC– 2.0 V and as low as 0 V. Operation outside of this range can yield incorrect
comparisons.
The following is a list of input voltage situation and their outcomes:
1. When both IN- and IN+ are both within the common-mode range:
(a) If IN- is higher than IN+ and the offset voltage, the output is low and the output transistor is sinking
current
(b) If IN- is lower than IN+ and the offset voltage, the output is high impedance and the output transistor is
not conducting
2. When IN- is higher than common-mode and IN+ is within common-mode, the output is low and the output
transistor is sinking current
3. When IN+ is higher than common-mode and IN- is within common-mode, the output is high impedance and
the output transistor is not conducting
4. When IN- and IN+ are both higher than common-mode, the output is low and the output transistor is sinking
current
8.2.2.2 Minimum Overdrive Voltage
Overdrive Voltage is the differential voltage produced between the positive and negative inputs of the comparator
over the offset voltage (VIO). To make an accurate comparison the Overdrive Voltage (VOD) should be higher
than the input offset voltage (VIO). Overdrive voltage can also determine the response time of the comparator,
with the response time decreasing with increasing overdrive. Figure 8 and Figure 9 show positive and negative
response times with respect to overdrive voltage.
8.2.2.3 Output and Drive Current
Output current is determined by the load/pull-up resistance and logic/pullup voltage. The output current will
produce a output low voltage (VOL) from the comparator. In which VOL is proportional to the output current. Use
Typical Characteristics to determine VOL based on the output current.
The output current can also effect the transient response. See Response Time for more information.
8.2.2.4 Response Time
Response time is a function of input over drive. See Application Curves for typical response times. The rise and
falls times can be determined by the load capacitance (CL), load/pullup resistance (RPULLUP) and equivalent
collector-emitter resistance (RCE).
• The rise time (τR) is approximately τR~ RPULLUP × CL
• The fall time (τF) is approximately τF~ RCE × CL
– RCE can be determine by taking the slope of Typical Characteristics in its linear region at the desired
temperature, or by dividing the VOL by Iout