2pin_ON/OFF input volatage range of LM2991

The On/Off threshold has high sensitivity to device junction temperature. See Figure 14 in the datasheet.

If On/Off pin control voltage is limited to between 1.0V (on)  and 2.0V (off) then as the device heats during normal operation, the threshold voltage drops and the 1.0V (on) may not keep the LM2991 on.

For the On/Off thresholds, less than 0.8V for ‘Ensured On’ and more than 2.4V for ‘Ensured Off’, are required to ensure proper operation across the entire operating range (junction temperature and input voltage).

It is quite possible that you may be able to operate successfully inside the thresholds if your operating conditions are modest, but you will need to do so at your own risk.

Hi,Mr.Jones.

Thank you for quick reply.

You are very helpful and I'm grateful for your thoughtfulness.

Best Regards!!

Hi,Mr.Jones.

I'm sorry for my wrong question.I missed questions I want to ask you.

How much is minimum voltage to turn on? Input Voltage(-26V)?

How much is maximum voltage to turn off?I recognize it is 2.4V.

Best Regards.

From the Electrical Characteristics table, the On/Off pin control voltage must 0.8V, or less, to ensure ON, and 2.4V, or greater, to ensure OFF.

The actual On/Off threshold falls somewhere in between 0.8V and 2.4V. If you want to work inside this region it is your responsibility to do the characterization for your application. TI will not ensure device operation that does not conform to the conditions described in the Electrical Characteristics table.

Figure 14 in the datasheet would provide some hint, but not any threshold value that would be ensured to always work.

The Typical threshold values would indicate that there is 100mV of hysteresis.

The variance of the On/Off threshold across the operating input voltage range is not very significant, a few tens of milli-volts across the full input voltage range.

Junction temperature will possibly dominate the threshold voltage (approx -5mV/C).

Additionally, normal process variations at the transistor level may cause a variance of a few tens of milli-volts between wafer lots.

Knowing (from Figure 14) that the 0.8V (ON) limit applies at 125C, and the 2.4V (OFF) limit applies at -50C, you could work backwards with the approx. -5mV/C to estimate most likely min/max limits at 25C, if that is your question.

   Off @ 25C = (2.4V – ((25C –(-50C)) x -5mV/C)) = (2.4V - 0.375V) = 2.025V

   On @ 25C = (0.8V – ((25C – 125C) x -5mV/C)) = (0.8V – (-0.500V)) = 1.3V

But, when the internal junction temperature moves higher than 25C, due to normal power dissipation, the calculated ON threshold limit will no longer be valid.

Thank you for specific reply about operation of 2pin.

I apologize if I give you wrong question.

I want to know absolute maximum ratings of 2pin.

Page2 on LM2991 datasheet is written down that absolute maximum ratings of Input Voltage is -26V to +0.3V.I think this value relate 3pin_Input and not 2pin_ON/OFF.

I'm sorry for asking questions many times,but I hope you will take good care if this.

Best Regards.

Hi,Jones

Thank you for quick reply.

I could get an information I had needed!

If there is another opportunity,please allow me to ask you again.

Best regard

Taichi Noda