At power-up, while the comparator supply (Vcc) is below the minimum supply voltage (< 2V), there can be transitions on the output depending on the supply voltage and input voltages applied at that point-in-time. This can cause problems with designs that require a known start-up state, such as a latching circuit or oscillator. Some existing designs can have inadvertently relied on this behavior.

Figure 6-10 Start-up for Classic die, Output set HIGH

Figure 6-11 Start-up Behavior for Classic die, Output set LOW

Starting from Vcc = 0V up until approximately 0.55V, the output is high-impedance as there is not yet enough supply voltage to bias the output driver current source (Q12) and the output transistor (Q8) base-emitter junctions. The output tracks the pull-up supply through the pull-up resistor.

When the supply reaches the range of 0.55V, the output stage now has just enough bias to become active, but the input stage still does not have enough voltage to operate. This is show as Output Awakens in Figure 6-10 and Figure 6-11.

With the output stage functioning and the input stage still inoperative (cut-off), the output behavior in this region is similar to when Section 6.7. This is shown as Out Of VCM Behavior in Figure 6-10 and Figure 6-11. The classic die output goes low, and the B version remains high-impedance (high).

When the V_CC supply reaches the range of 1V, there is just enough supply voltage to weakly bias the input stage. At this point, the output can start responding to input signals, but proper output is still not specified. This is shown as Input Stage Starts Functioning in Figure 6-10 and Figure 6-11.

Once the supply reaches 2V, the input and output stage are fully biased, but the input voltage range is essentially zero (Vcc-2V). The inputs can properly respond to input voltages around zero volts. The input voltage range now increases proportionally with the increasing supply voltage.

Figure 6-12 Start-up for B die, Output set High

Figure 6-13 Start-up for B die, Output set Low

For the B device, shown in Figure 6-12 and Figure 6-13, the behavior is similar, but the output goes high during the out of VCM range. Due to differences in the B process, the output awakens a little higher at 0.7V, and the input stage starts functioning at 1.2V.

For circuits with a slow supply start-up ramp, keep the operating threshold voltages low to make sure the voltages are within the proper operating range as quickly as possible during start-up.

Similar effects can occur during power-down, but in reverse sequence. The remedies can be similar to power-up.

One way to make sure that the comparator is operational during power-down is to add a diode in series with the comparator circuit power supply (including any reference dividers), along with a large comparator bypass capacitor for storage. This way, when the main supply drops, the diode isolates the comparator from the main supply and holds up the comparator circuit supply allowing the comparator to still function until the capacitor discharges. Since the LM339 family is an open collector output, the supply current is constant regardless of output state. Some thought is needed as to the pull-up and any reference divider voltage source sequencing during power down.