What is Nch open drain output mentioned as a feature of a voltage detector? How should I estimate the output voltage when the output is at 'L' state?

Voltage Detectors (Reset ICs)How to use

The word 'open drain output' refers to a structure of the output pin of a voltage detector.

In general, a voltage detector (VD) has either of the following digital output types: CMOS output which has two FETs, one on the power supply side and the other on the GND side, and Nch open drain output which has an FET on the GND only.

The former CMOS output type switches two internal FETs ON/OFF and outputs either High (= supply voltage) or Low (= GND). To use this CMOS output type, it is necessary to adjust high-side voltage to the subsequent circuit requirement; otherwise, the VD will fail to transmit signals properly. On the other hand, the latter Nch open drain output type has an FET only on the GND side, so high-side voltage output is performed by pulling up VD output voltage to a fixed-voltage source such as the power supply with a pull-up resistor.

This Nch open drain type is easy to adjust its high-side voltage to the high-side voltage of the subsequent circuit, thanks to the capability of varying high-side voltage according to the pull-up voltage. Moreover, shorting multiple Nch open drain voltage detectors can fabricate a wired-OR logic circuit.

Output voltage at low-side state is, however, a divided voltage by the ON resistance of the internal Nch FET and the pull-up resistor. So, it must be taken care of that a too small pull-up resistor increases the low-side voltage and the amount of current flowing via the pull-up resistor. On the contrary, a too large pull-up resistor can decrease high-side voltage due to OFF leak current of the Nch FET. This must be taken into consideration, too. 

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  Figure 1: Voltage Detector with Nch Open Drain Output

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  Figure 2: Nch Open Drain Connection