【Introduction】When the negative input Voltage in the comparator is large and exceeds the input common mode voltage range, incorrect output behavior will occur. If negative input voltages cannot be avoided, it is important to protect the input pins of the comparators and prevent phase reversal.
In this article, I will explore the causes and effects of negative input voltages in comparators, the phase reversal behavior, and how to protect the inputs from negative voltages.
Causes and Effects of Negative Inputs
Negative voltages can be caused by many unexpected and unavoidable causes, including switching noise in DC/DC converters, AC-coupled inputs that produce bipolar voltages at the input, and ringing from bipolar output inductive sources. In systems such as automotive and industrial applications, ground offset or ground drift (where there are two different reference points at different potentials) can also produce large negative inputs; when a separate ground plane exists that separates analog circuits from high current This happens when the switch node is isolated.
Manufacturers make comparators without specialized ESD structures, instead using a junction-isolated die process to protect against large negative input voltages, where PN junctions start under each node connected to a common die substrate. acting as a diode. It is important to keep the connection to the common die substrate at the lowest negative potential of the GND pin. If the input pin is at a lower negative voltage than the GND pin (and thus the substrate), excessive reverse current can cause parasitic elements to appear and turn on internal parasitic NPN transistors that draw current from other internal nodes and channels, This causes phase reversal.
phase reversal
In a phase-reversed condition, the polarity of the output is incorrectly reversed. Figure 1 shows a comparator in a non-inverting configuration where the reference at IN is tied to 0V at GND. When the input voltage falls below 0V, the output voltage goes low as expected. However, when the input voltage reaches about –570mV, the output voltage reverses and goes high.
Figure 1: Phase inversion in a comparator
Figure 2 shows the comparator input current and supply current when the input is below 0V. As the input voltage increases negatively, the reverse current also increases significantly. Since the extra PN junction is now open and conducting, the supply current will also increase.
Figure 2: Input current and supply current due to negative input
How to protect the input from negative voltage
To protect the input of the comparator and prevent phase reversal, you must first analyze the minimum input voltage and maximum input current of the device. Figure 3 shows the absolute maximum ratings table for the LM2903B, which specifies a minimum input voltage of –0.3V and a maximum input current of 50mA. These specifications make it impossible to define operation with negative input voltages in excess of 0.3V, as this does not meet the input voltage specifications and does not ensure proper operation. The maximum current the comparator can handle is 50mA: any current greater than this can damage the device. While limiting the current below 50mA will not damage the device, phase reversal can still occur, which means you must limit the current to a value well below 50mA for the comparator to function properly.
Figure 3: Absolute Maximum Ratings Table for the LM2903B
It is not recommended to operate these devices outside the absolute maximum limits. If this cannot be avoided, use a series current limiting resistor and an external Schottky diode, placed between the input pin and GND, this helps limit the voltage and current to safe levels so that the body diode does not Conduction will begin as shown in Figure 4.
Figure 4: Current-Limiting Resistor with Schottky Diode at the Input of the Comparator Circuit
A general rule of thumb is to choose a resistor with a ratio of 1kΩ per volt to the maximum negative voltage in order to limit the current to 1mA or less. For example, if the maximum negative input voltage is –2V, the resistor must be at least 2kΩ. Following this rule will ensure that the input current is well within the absolute maximum specification, preventing damage to the comparator.
The forward voltage of a Schottky diode is lower than that of a body diode; the body diode conduction voltage is about 0.4V, while the Schottky diode is about 0.2V. This external diode (together with the characteristic of keeping the current at 1mA or less) will help to ensure that the negative input voltage signal is clamped below 0.3V, thus avoiding phase reversal.
Another approach is to use a comparator that is better at preventing phase reversal, such as the TLV1701 high-voltage micropower comparator. Figure 5 shows its absolute maximum ratings. The minimum input voltage is 0.5V lower than VS– and the maximum input current is 10mA.
Figure 5: Absolute Maximum Ratings Table for TLV1701
Figure 6 shows the TLV1701 in a non-inverting configuration with its inverting input tied to 0V at GND. When the input voltage is below 0V, the output voltage will not invert even if the input voltage exceeds the –0.5V limit. As the input increases negatively, the input current will begin to increase exponentially and exceed the maximum limit of 10mA, which can damage the comparator. Therefore, a 10kΩ resistor in series with the input helps reduce the current to a safe level.
Figure 6: TLV1701 output voltage with negative input
Epilogue
When designing with comparators, it is important to consider the effects of negative inputs and how they can cause phase reversal of the output. In many systems, such as automotive and industrial applications, the large negative voltages described above are unavoidable. Operating beyond the recommended limits is not safe, so it is critical to take steps to prevent comparator damage and phase reversal. Using the methods outlined in this article can help improve circuit performance and minimize unwanted output behavior that can cause problems with downstream devices in the system.
The Links: SKT552/16E SKB52/08
