Technical Tips | Solving the main challenge of comparators: exceeding the input common-mode range

The term input common-mode voltage range (often abbreviated as V CM or VICR ) is widely recognized in the analog world, but it is less understood in the world of comparators. For amplifiers, V CM is defined as the average voltage applied to the two inputs. But for comparators, it means something completely different.

Normal operation of a comparator means that the two inputs cross, causing the output to change. Let's analyze the non-inverting comparator configuration shown in Figure 1, and its transient behavior is shown in Figure 2.

Figure 1: Non-inverting comparator configuration

Figure 2: Non-inverting comparator transient behavior

In Figure 2, if the voltage at the non-inverting terminal (olive green) is greater than the voltage at the inverting terminal (red), the output (green) is high. If the inverting terminal voltage is greater than the non-inverting terminal voltage, the output is low level. The comparator input VCM is the switching point where the two input voltages cross each other. The switching point voltage range that still ensures normal operation is called the comparator's input V CM .

Table 1 shows the V CM of the LM393 , which ranges from the negative supply voltage, V–, to 1.5V or 2V below the positive supply voltage, V+, depending on the temperature of the device.

Table 1: From the LM393 datasheet "Table 6.7 Electrical Characteristics"

The input stage of the comparator limits the input V CM . Let’s first look at the functional block diagram from the TL331 data sheet (Figure 3). The input stage consists of a P-channel, N-channel, P-channel (PNP) differential pair with the emitters connected to a current source. In order for the PNP transistors on the differential pair to conduct, a voltage drop (V EB ) is required from emitter to base (or base to emitter [V BE ] for an N-channel, P-channel, N-channel configuration ).

Figure 3: TL331 functional block diagram

The current source is typically a current mirror, as shown in Figure 4. For the current source to operate, the emitter-to-collector voltage of Q2 needs to exceed the minimum forward effective voltage requirement (V EC (Sat) ).

Figure 4: PNP current mirror

随着比较器输入电压的增加，可满足差分对晶体管的 V EB 导通和电流镜晶体管的 V EC (Sat) 的余量越来越小。 当这种情况发生时，V EC 开始减小，而 V EB 仍然保持稳定。当 V EC (Sat) 不超过 Q2 时，电流镜的电流产生能力显著降低，因此一些比较器的 V CM 会上升到低于电源电压的值。其他具有 NPN 晶体管输入级的器件指定的 V CM 范围为某个值至正电源电压。

The V CM value in the datasheet takes into account performance across process, voltage, and temperature (PVT). Looking back at the TL331 example in Figure 4, we can see that V BE and V CE fluctuate with temperature.

Figures 5, 6, and 7 show the performance of the LM393 in a non-inverting configuration similar to Figure 1. In this case, the supply voltage is set to 5V. A 100Hz, 1 Vpp, 3.9 V DC ramp waveform (yellow) is connected to the non-inverting terminal, while a 3.9 V DC reference signal (purple) is applied to the inverting terminal. This is an example of an application where the 3.9V switch point is outside the range of V CM ; for this supply, the compatible V CM is 3V or less.

At room temperature, the comparator operates and switches above 3.9V, as shown in Figure 5. The comparator also operates at elevated temperatures, as shown in Figure 6. As the device cools, it begins to malfunction, as shown in the output (green) in Figure 7.

Figure 5: LM393 room temperature performance outside the V CM range

Figure 6: LM393 high temperature performance outside the V CM range

Figure 7: LM393 low temperature performance outside the V CM range

When the input voltage is reduced to ensure that the switch point is within the V CM range of the device, the device operates correctly over all temperature ranges. The ramp waveform is now at a lower DC offset voltage of 2V (referenced to 2V), ensuring that the switch point is within the V CM range. Figures 8, 9, and 10 show how the LM393 operates correctly over the entire temperature range.

Figure 8: LM393 room temperature performance over V CM range

Figure 9: LM393 high temperature performance over V CM range

Figure 10: Low temperature performance of LM393 over V CM range

All of these oscilloscope captures were taken from a single unit. If you were to test another LM393 sample, the performance outside the specified V CM range might be different. At room temperature, the upper V CM limit might be different from the unit tested. This is called process variation, where performance can vary due to batch differences or inconsistent manufacturing processes.

The final component of this deviation is the voltage deviation. The device may behave differently at different supply voltages. In the setups of Figures 6, 7, and 8, if the supply voltage is increased to the upper allowed limit of 36V and the same out-of-V CM condition occurs, the performance may be different. When designing a system using a comparator, it is important to keep the input within the V CM range to avoid these issues.

When either input is out of range, the input V CM may cause the device to not operate properly. Exceeding this range can cause problems with the biasing of the comparator input stage. Keep in mind that if one or both inputs are outside the data sheet specifications, the V CM range is exceeded. Even if the system operates properly outside the V CM range, it may not operate properly at a different supply voltage, operating temperature, or if the integrated circuit is replaced . When creating a system that uses a comparator, keep each input signal within the data sheet specification range for the input V CM to maintain PVT functionality.