7.2.2.1 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 above or below 25°C the VICR can range from 0V to VCC – 1.5V. This limits

the input voltage range to as high as VCC – 1.5V and as low as 0V. Operation outside of this range can yield

incorrect comparisons.

Below 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, please see the Both Inputs Above Input Range

Behavior section of the LM339 Family Application Note (SNOAA35).

7.2.2.2 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 7-2 and Figure 7-3 show

positive and negative response times with respect to overdrive voltage.

7.2.2.3 Output and Drive Current

Output current is determined by the load/pull-up resistance and logic/pull-up voltage. The output current

produces a output low voltage (VOL) from the comparator. In which VOL is proportional to the output current.

Use Output Low Voltage vs Output Current (IOL) to determine VOL based on the output current.

The output current can also effect the transient response. More is explained in the next section.

7.2.2.4 Response Time

Response time is a function of input over drive. See Section 7.2.3 for typical response times. The rise and

fall 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 determined by taking the slope of Output Low Voltage vs Output Current (IOL) in its linear

region at the desired temperature, or by dividing the VOL by Iout

TL331, TL331B, TL391B

SLVS238L – AUGUST 1999 – REVISED NOVEMBER 2024

www.ti.com

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