(Refer to the simplified schematic in Figure 6-1).

If the input-to-output differential voltage were allowed to exceed the Absolute Maximum Rating of 5 V, an internal diode clamp would turn on and divert the current around the compound emitter followers of Q1/Q3 (D1 – D11 for positive input), or around Q2/Q4 (D2 – D12 for negative inputs). Without this clamp, the input transistors Q1 – Q4 would zener, thereby damaging the buffer.

To limit the current through this clamp, a series resistor should be added to the buffer input (see R₁ in Figure 6-2). Although the allowed current in the clamp can be as high as 5 mA, which would suggest a 2 kΩ resistor from a 15 V source, it is recommended that the current be limited to about 1 mA, hence the 10 kΩ shown.

The reason for this larger resistor is explained in the following: One way that the input or output voltage differential can exceed the Absolute Maximum value is under a short circuit condition to ground while driving the input with up to ±15 V. However, in the LMH6321 the maximum output current is set by the programmable Current Limit pin (C_L). The value set by this pin is specified to be accurate to 5 mA ±5%. If the input/output differential exceeds 5 V while the output is trying to supply the maximum set current to a shorted condition or to a very low resistance load, a portion of that current will flow through the clamp diodes, thus creating an error in the total load current. If the input resistor is too low, the error current can exceed the 5 mA ±5% budget.