TPS745-Q1: EN=hi before VIN

Hello Jason,

Thank you very much for your quick reply.

Please let me confirm if my understanding is correct.  Do you mean IN pin must sink ≧ 1mA by the time VEN > VUVLO(=1.21V) to avoid turning on the device?  If so, the device can't turn on even IN is powered correctly after EN=hi like this.

EN=hi, IN=0V

-> IN=6.0V(can sink >> 1mA)

-> EN > VUVLO

-> It doesn't turn on

What would happen if it doesn't meet IN ≧ 1mA by VEN > VUVLO like my customer's case?

Best Regards,

Yoshikazu Kawasaki

Hi Kawasaki-san, 

If VEN > VUVLO rising (min) and VIN<VUVLO , the input pin (IN) must sink 1 mA of current to avoid the device being turn on with a floating input pin.

This statement only applies to the application that has VEN > VUVLO and VIN<VUVLO. When Vin = 6V, with EN > VUVLO, the device will turn on. Sorry for the confusion. 

What would happen if it doesn't meet IN ≧ 1mA by VEN > VUVLO like my customer's case?

-Input pin cannot sink 1mA of current, the leakage from enable pin could turn on the device even when Vin is less than VUVLO. 

Does this answer your question? 

Regards, 
Jason Song

Hello Jason,

Thank you very much for your quick reply again.  Please let me ask you again because my customer may not need to change the sequence.

You said this.

> Input pin cannot sink 1mA of current, the leakage from enable pin could turn on the device even when Vin is less than VUVLO

Questions

1. Would it damage the IC?  Or would it just try to turn on if it has enough leakage current from EN pin without any damages?

2. What would happen if it doesn't have enough leakage current then?  Would it try to turn on, but it can't do that, so would it repeat that?

3. Would it work well once IN is powered externally after EN=hi?  If there is no damage and it works correctly once IN is powered, it sounds like the IC can work even EN->IN sequence.  Am I correct?  If so, my customer doesn't have to change the sequence.

Best Regards,

Yoshikazu Kawasaki

Hi Kawasaki-san, 

Please see my comments below your questions. 

Questions

1. Would it damage the IC?  Or would it just try to turn on if it has enough leakage current from EN pin without any damages?

For this scenario we discussed, the leakage from EN to Vin is less than 1mA, and as long as all the required absolute max have been met, this small amount of leakage alone will not damage the IC. 

2. What would happen if it doesn't have enough leakage current then?  Would it try to turn on, but it can't do that, so would it repeat that?

If the Vin cannot take the current during the floating condition, the leakage can build up the voltage on Vin that may turn on the output slowly. For the case that the output may need higher current to charge an output cap or pulling a load voltage high, the LDO's output may drag the Vin below UVLO again and this process may repeat. 

3. Would it work well once IN is powered externally after EN=hi?  If there is no damage and it works correctly once IN is powered, it sounds like the IC can work even EN->IN sequence.  Am I correct?  If so, my customer doesn't have to change the sequence.

Yes, the device is okay for EN>IN, and the only case that may have an issue is the one we discussed here as the following:

If VEN > VUVLO rising (min), the input pin (IN) must sink 1 mA of current to avoid the device being turn on with a floating input pin or IN<VUVLO.

Please click the "Resolved" link at the bottom of this post if I have answered your question.

Regards, 

Jason Song