9.2.1.1 Input and Output Topology

All input / output pins are protected against excessive voltages by ESD diodes connected to V⁺ and V^– rails (see Figure 54). These diodes start conducting when the input / output pin voltage approaches 1 V_be beyond V⁺ or V^–  to protect against over voltage. These diodes are normally reverse biased. Further protection of the inputs is provided by the two resistors (R in Figure 54), in conjunction with the string of anti-parallel diodes connected between both bases of the input stage. The combination of these resistors and diodes reduces excessive differential input voltages approaching 2 V_be. This occurs most commonly when the device is used as a comparator (or with little or no feedback) and the device inputs no longer follow each other. In such a case, the diodes may conduct. As a consequence, input current increases and the differential input voltage is clamped. It is important to make sure that the subsequent current flow through the device input pins does not violate the Absolute Maximum Ratings of the device. To limit the current through this protection circuit, extra series resistors can be placed. Together with the built-in series resistors of several hundred ohms, these external resistors can limit the input current to a safe number (that is, less than 10 mA). Be aware that these input series resistors may impact the switching speed of the device and could slow down the device.