Why is there no problem if I do not need an external resistor Rin when using the NJU72750A with dual power supplies?

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The reason why Rin is not required when using dual power supplies is that the bias potential of the input resistor is different in single-supply and dual-supply operation.

For stable operation, the coupling capacitor connected to the input terminal must be charged to the signal midpoint.
When operating with dual power supplies, the bias point is GND. Therefore, there is no need to charge the coupling capacitor when power is turned on, and the device will immediately reach a stable state.
On the other hand, when operating with a single power supply, the bias point is the midpoint of the power supply. When power is turned on, the midpoint potential charged to REF_A/B charges the input coupling capacitor via the internal input resistor (470 kΩ), so it takes time to reach a stable state.
For this reason, we recommend connecting an external resistor to shorten the charging time of the input coupling capacitor only when operating with a single supply.

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