# Open drain voltage detector NCP301LSN45T1G

I have to use a voltage detector to detect a decrease in voltage to reset a microcontroller. I have found the NCP301LSN45T1G to do this.

But I have a question about the Reset Output current (N-Channel Sink current).

How do I have to consider this parameter? Is it used to calculate the pull up resistor?

I thank you for your answer. So If I have understand your explanation I have to use a pull up resistor of 796 ohm to obtain a 5V at the output as explained with my schematic. Is it right ?

The NCP301 has an open drain N−Channel output, so it can only drive the voltage low and typically needs a pull-up resistor.

The 'L' or 'H' suffix in the part name determines how the comparator internally is connected.

The representative block diagrams depict active low reset output NCP301 ‘L’ suffix devices.
The comparator inputs are interchanged for the active high reset output NCP301 ‘H’ suffix devices.

In case of the ‘L’ suffix devices, if $$\ V_{in} < V_{threshold} \$$ the comparator turns the mosfet ON and the Reset Output is pulled low (hence "active low").
In case of the ‘H’ suffix devices, if $$\ V_{in} < V_{threshold} \$$ the comparator turns the mosfet OFF and (when Reset Ouput is pulled up by a resistor) the output will be high (active high).

The datasheet shows the minimum current and typical current ($$\ I_{out} \$$) that is drawn by the NMOS within the device, given the conditions Vin and Vout (= Reset Output Voltage).
So, actually it's the specification of the maximum and typical $$\R_{DS(ON)}\$$ of the internal NMOS.

The case of the NCP301LSN45T1G
The NCP301LSN45T1G has its threshold voltage at 4.5 V. So, if $$\V_{in}\$$ > 4.5 V the comparator turns the mosfet off.
If the internal mosfet is not conducting, there is no current running through the NMOS. If the Reset Output pin is pulled up by a resistor to a certain supply voltage, there will neither be running a current through the pullup resistor and Reset Output voltage will be equal to that supply voltage independent of the value of the pull-up resistor.

If $$\V_{in}\$$ < 4.5 V the comparator turns on the mosfet and the Reset Output is pulled low with a sink current $$\ I_{out} \$$ as shown in the picture below marked in blue (‘L’ suffix!), not in yellow as in OP.

The datasheet shows two conditions, because with decreasing Vin, the mosfet will (eventually) be driven worse and the $$\ R_{DS(ON)}\$$ of the internal NMOS will therefore become higher.

With condition (VOUT = 0.5V, Vin = 1.5V) $$R_{DS(ON)} \leq \frac{ V_{out} }{ I_\text{out min}} = \frac{0.5 \text{ V}} {1 \text{ mA}} = 0.5 \text{ k}\Omega$$

With condition (VOUT = 0.05V, Vin = 0.70V) $$R_{DS(ON)} \leq \frac{ V_{out} }{ I_\text{out min}} = \frac{0.05 \text{ V}} {0.01 \text{ mA}} = 5 \text{ k}\Omega$$

Connecting the pull-up resistor to the same supply voltage as the NCP301LSN45T1G and using these $$\ R_{DS(ON)} \$$, the minimum value of the pull-up resistor can be calculated that is required to make the Reset Output voltage sufficiently low.
This "sufficient low" is determined by the microcontroller's maximum reset voltage $$\V_\text{reset,max}\$$ that is still recognised as "low" to reset the microcontroller.

$$R_{pull-up} > \frac { V_{supply}-V_\text{reset,max} }{ \frac{ V_\text{reset,max} }{ R_{DS(ON)} } }$$

Example If e.g. the microcontroller resets when a voltage between 0 V and 0.3 V is applied to the reset pin, then $$\ V_\text{reset,max} = 0.3 V\$$.

For the condition Vin = Vsupply = 0.70V $$R_{pull-up} > \frac { 0.7V-0.3V }{ \frac{ 0.3V }{ 5 \text{ k}\Omega } } = 6.7 \text{ k}\Omega$$

For the condition Vin = Vsupply = 1.5V $$R_{pull-up} > \frac { 1.5V-0.3V }{ \frac{ 0.3V }{ 0.5 \text{ k}\Omega } } = 2.0 \text{ k}\Omega$$

Although the $$\ R_{DS(ON)} \$$ will likely decrease at higher Vsupply, let's assume it stays 0.5kohm minimum, then for the condition Vin = Vsupply = Vthreshold = 4.5V $$R_{pull-up} > \frac { 4.5V-0.3V }{ \frac{ 0.3V }{ 0.5 \text{ k}\Omega } } = 7.0 \text{ k}\Omega$$

So, pick $$\ R_{pull-up} = 10 \text{ k}\Omega \$$, so it satisfies all above conditions.

• Comments are not for extended discussion; this conversation has been moved to chat. Any conclusions reached should be edited back into the question and/or any answer(s). Commented Jun 25, 2019 at 14:50