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LM293: what's the meaning of overdrive voltage?

Betty Guo
Expert 7300 points
Part Number: LM293

Hi team,

in the datasheet, there have a overdrive voltage minimum requirement, I want to know what's the overdrive voltage.

the datasheet show that:

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.


but I never see this parameter before in the amplifier or comparators, could you give more comments about this one? what's the difference between overdrive voltage and offset voltage?

thank you

  • 0
    TI__Mastermind 49906 points

    Hello Betty,

    Overdrive is usually a condition of propagation delay, not a specification itself. Overdrive is usually seen in the conditions column for propagation delay. Look in the Propagation Delay specification section of the datasheet and it should state what the "overdrive" voltage was for the Prop Delay specification. Typically it is 5mV, 20mV, 50mV up to 1V.

    Comparators react faster with larger overdrive, so to properly compare comparators, the prop delay must be compared at similar overdrive conditions.

    Overdrive is the amount of votlage over the set threshold voltage. Since the threshold is also affected by the comparator voltage offset, the more "accurate" definition includes offset voltage (as you have found above).

    So if the reference voltage is +2.5V, and the input signal goes from +1V to +2.6V, the overdrive voltage is +100mV, and NOT +1.6V.

    Offset voltage is the natural random miss-match between the inputs. The definition is the same as an Op Amp. It is seen as a small voltage source in series with the positive input. The offset is different for every device and can be positive or negative. Offset voltage for comparators generally ranges from ±100uV to ±20mV (usually a few mV).

    See the TI Precision Labs section on Offset Voltage for Op Amps:

     TI Precision Labs - Op Amps: Vos and Ib - Lab

     Ideally, if you apply 1.0000V to the negative input, the output should go high when the positive input crosses 1.000001V.

    But if the comparator has a -10mV offset voltage, the output will go high at +1.010000V. Remember that the -10mV offset is internally in series with the positive input, and "bucks" the input signal by -10mV, so the result is the comparator appears to trigger at a higher voltage.

    So because of the offset, the "actual" threshold is now 1.010V. If you wanted to test a 5mV overdrive spec, then you would set the negative input to 1.0000V, and the positive input to 1.0050V. The output would not trip because you have not reached the "actual" 1.01V threshold due to the offset voltage.

    Conversely, if the offset was +10mV, the "actual" threshold would be 990mV, causing it to trip "earlier" than expected.

    This is why the offset voltage must be taken into account when measuring the propagation delay. Either the output will not trip, or it will trip too early when measuring prop delay using small 20mV overdrive numbers.

    See Part 4 of the TIPL Comparator presentation:

    TI Precision Labs - Op Amps: Comparator Applications 4