LMR33630: About LMR33630 DATASHEET IZC

Hi Young

We do bench test to verify the IZC, and this is the bench test result of the iL when the LS turns off, the value slightly higher or lower than 0 is all right.

Thanks and best regards.

Hello   Gui   He：

I want to ask,  is the design value basis a negative current? Because the zero current of all products I see is basically positive current, in order to improve the light-load efficiency, why this product is made into negative current? Is there any other design consideration？

thanks very much！

Hello   Gui   He：

I want to ask,  is the design value basis a negative current? Because the zero current of all products I see is basically positive current, in order to improve the light-load efficiency, why this product is made into negative current? Is there any other design consideration？

thanks very much！

Hello   Gui   He：

I want to ask,  is the design value basis a negative current? Because the zero current of all products I see is basically positive current, in order to improve the light-load efficiency, why this product is made into negative current? Is there any other design consideration？

thanks very much！

Hello   Gui   He：

I want to ask,  is the design value basis a negative current? Because the zero current of all products I see is basically positive current, in order to improve the light-load efficiency, why this product is made into negative current? Is there any other design consideration？

thanks very much！

HI Young

The ideal design value is 0. But it's normal to have some deviation, slightly positive or negative values both are ok.

Thanks and best regards.

Hello  Gui  He：

I understand what you mean, because it can't be made absolutely zero current shutdown. Therefore, the reference at the internal comparator is made to switch off at the inductor current slightly positive current, so as to prevent the output from reversing the input when the negative current is switched off, which reduces the efficiency. My question is, all the ones I have seen are turned off with positive current. Why is this one the only one that is turned off with negative current? Is it the internal reference current that is made to turn off with slightly negative current? Or are there other design considerations?

Hello  Gui：

All of the other products bench tested had positive zero crossing points, so there are questions about this  product！

thanks  very  much

Hi YOung

Let me check and get back to you later.

Thanks

Hi Gui

Too troublesome you. thanks!

Hello  Gui：

Could you help me look at this problem? I was afraid you'd forget!

thanks 

Hi Young

Sorry for late. I am checking with our expert and will get back to you soon.

Thanks!

Hi Young,

The design target for zero cross current limit is 0mA, but the typical value may changed with inside circuit device variation, so the final spec. maybe positive or negative. LM53602 is -20mA,  LMR23615 is -40mA,  LMR38020 is +10mA. there was no special design purpose there.

B R

Andy

Hi Andy，

Thanks for your reply Andy, we all know that the ideal value of design is 0mA, but we can't do it! Due to the offset of the comparator and the circuit delay, the 0mA shutdown delay in bench test was caused! So guess the other way around: Is your design value, is the inductor current turned off before it reaches 0mA, or is the 0 current detection point greater than 0mA?

thanks！

Hi Young

Andy will get back to you on Monday. 

Thanks

Hi Young,

The detect circuit always intends to turn off LS switch at target of positive few 10mA to 0mA ( or circuit consider it as a small positive current), no matter the actual value may varies to larger positive value or even the negative value which is caused by drift, offset or tolerance of circuit itself.

B R

Andy  

Hi  Andy：

Some chips made of negative current,  Are you going to evaluate whether it's possible to make light load less efficient？

Thanks！

Hi Young,

the small negative current at LS turn-off has below benefits:

1, no power loss caused by LS body diode reverse recovery

2. the negative current also help HS FET to turn on softly.

So light load eff may be improved than positive cross zero current. 

B R

Andy