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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:

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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

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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