LM5009: LM5009 current limit status

hi, Xiaodng,

my suggestion is to check the input current when Vout have drop, it looks like it drop about 1V from 14V right, make sure the current will not increase higher .

And just curious, Vin（yellow one）  is drop at the end if this is the right input ongoing

Elena

Dear Elena,

Vin is dropping due to power is switching off.

What is the meaning of input current ? current for inductor ? make sure the current will not increase higher ?

Yes, it drop about 1V from 14V if RCL is 300K, of course, if RCL is increased, drop voltage will be increased. e.g. RCL = 1000K, Vout is about 11V.

Yes, you are right.

I am thinking if there is risk in current increase during the Vout transient. 

Tks. Hope to get your feedback soon. If you need more information, please let me know.

xiaodong,

One risk is the current limit  , it is now working close to 54ohm, 300mA , the risk is if LM5009 get to  low limit, 250mA, the relay can/can't start up normal, 

so please try 200mA current for the relay, if it works, then you can use. 

Another question is you need to consider thermal, if relay working ON a long time ,the lm5009 is working under overcurrent ,this may have risk of TSD or damage.

Elena

Hi Elena,

As mentioned in the begin of topic, our load is fixed which means LM5009 maybe goes to current limit. but we can't  change LM5009 due to delivery issue.

 we accept this current limit action in LM5009 and there is a small voltage drop for example voltage drop from 14V to 12V. We still drive the relay.

It sounds this way, 14v setup --> relay drive （54 ohm）--> maybe some LM5009 goes to current limit due to current limt threshold is different -->  Vout drops mabye Vout = Vin*ton/(ton+toff) --> current drop to 220mA --> continually drive the relay 100ms --> recover to 14V. as shown in pic in the begin of topic.

And my question is that the risk of said way?

Of course, we only drive 100ms, we will consider the dynamic thermal for LM5009.

Thanks in advance.

Xiangdong,

Yes, your described and risk you mentioned are right .LM5009 work under this condition is already close to OC ,this is the risk , but you have no other change.

Elena

Hi Elena,

Do you think LM5009 will drop from12V to very low voltage (e.g. 3V), if 220mA is still current limit?

or LM5009 will drop from 12V to 10.8V slowly to meet the current limit e.g. 200mA?

Vout 12V to 10.8V is reasonable under control of COT mode, if Vout 12V to 3V or even shutdown, this is much like a thermal concern, not a vout drop.

Got it. do you have a dynamic thermal for LM5009? for example, signal pulse, 100ms

Sorry we don't have this kind of test. We provide a  Webench simulation of TJ, but under you case it's better to do multi- test on board.