LM1117, LP38852 - operation starts under pre-bias condition

Please define your 'pre-bias condition'.

Donald San,

After the pre-bias condition occurs, what does the device behave?
We concern the behavior after the incident.

In addition, the datasheet does't show correspondence of pre-bias start up,
so, we understand that any pre-bias are not recommended.
Our opinion is correct or uncorrect?

Kind regards,

Hirotaka Matsumoto

As a general rule, a pre-biased output is not recommended for LP38852.

Please refer to the ‘REVERSE VOLTAGE’ section in the datasheet.

There are some specific conditions that should be avoided, as well as some limited conditions that might be acceptable.

Without knowing any details of the pre-biasing condition, I am unable to make any comment.

Donald San,

Thank you for your reply.

We got that a pre-biased output is not recommended for LP38852.
So, how about LM1117?
A pre-biased output is not recommended for LM1117 the same as LP38852, right?

kind regards,

Hirotaka Matsumoto

Donald San,

If LDO doesn't have the fuction of  pre-bias start-up as like LM1117, LP38852,
do you have any ideas of circuit protection which they start from pre-bias condition with external parts? 
If you have any ideas for them, please inform us.

Kind regards,

Hirotaka Matsumoto

LM1117 vs LP38852

Two totally different designs, two totally different fabrication processes.

I will leave any response regarding the LM1117 pre-bias start-up to someone more knowledgeable with the device characteristics and behavior.

Hi Tim San,

Thank you for your good update!

Kind regards,

Hirotaka Matsumoto

Hi Tim-san,

I'm working with Matsumoto at disty in Japan. Please let me ask further about pre-bias start-up at LM1117-N and LP38852.

I'd like to know that there is a possibility of start-up issue (not damage) under pre-bias condition. Now I'm assuming that the surge current from output to input through the device is within the spec described in datasheet as below. 

I believe that it should be start-up without any failure.

Best Regards,

Sonoki

Hi Sonoki,

Although the LM1117-N contains an internal diode that can withstand the current described in the datasheet, it is not recommended to put this diode under repeated stress as this can reduce the life of the device.

It is meant to protect from infrequent shorts of Vin to GND. 

if the system will frequently start up in a pre biased/reverse voltage condition with excessive reverse currents, then you may damage the part over time.

Tim

Hi Tim-san,

Thank you for your comments.

I understand that the regulator can start up without any failure under pre-bias condition by adding protection like reverse current blocking diode on Vin line to prevent reverse current.

Is my understanding correct?

Best Regards,

Sonoki

Hi Sonoki,

You do not want to "block" reverse current, but instead, you want to divert current from going through the internal reverse protection diode of the LM1117-N to avoid damage.

I would not say there will be no failure, but adding a protection diode like one shown on the datasheet will definitely increase the reliability of the part from any reverse currents. The key here is that the external diode must be able to handle the amount of reverse current, as well as divert most of the current from passing through the internal diode.

Tim

Sonoki,

To add a bit of information, yes to some extent, you may use a reverse current blocking diode at Vin to protect from reverse current. Just note that during normal operation, you will always dissipate power on the diode.

This method will not provide protection in all instances, though, and is meant to provide reverse battery protection. For example, if the Vin pin of LM1117 is directly shorted to gnd with a large amount of energy stored in Cout, this diode will not prevent reverse current through the LM1117. In this case, an external diode as shown in the datasheet would be needed if a large amount of reverse current is expected.

Tim