LM5160-Q1: the PWM signal is stop during start-up;

Hi Xiaobo,

Please share the schematic to understand the circuit. 

Also, it would be better if you can fill the excel tool LM5160DNTFBK-CALC Calculation tool | TI.com [select flybuck] with your operating conditions and share with us.

Regards

Arpita

we use the EVM board （LM5160DNTFBKEVM）for test；only change the value of  C13= 0.082uF capacitor with 2.2nF capacitor；

To simplify the issue, we do not discuss the user‘s own circuit schematic; TI EVM board has a same issue; Can you help test and confirm this issue? for evm board, only change the value of  C13= 0.082uF capacitor with 2.2nF capacitor；

So, you mean the current limit value when the chip starts up is different from what's listed in the datasheet（typ 2.5A）? Is this value related to the voltage of the SS pin? Is there a precise formula to calculate what this current limit value should be?

thanks Arpita；

That's the point, I don't understand；because we used the current probe to measure the peak current (pin12&pin11 output current)；it's about 0.43A， which is well lower than the limit value in datasheet （2.5A）.

yes,in my test case, the off time is 47us; so,then we use the EVM board to test with min modification. but EVM still have off time(less than 47us);

We want to know why it have this off time,because  we also used the current probe to measure the peak current for EVM board. and don't found big current value which can go up to limit of 2.5A.

Regards

• I would like to recommend using resistive load and check if start-up is smoother
• Any update on this?

• yes,in my test case, the off time is 47us; so,then we use the EVM board to test with min modification. but EVM still have off time (less than 47us)

          It is very difficult to comment without having knowledge of Cout values and EVM modifications.

• it's about 0.43A， which is well lower than the limit value in datasheet （2.5A）.

Where are you exactly measuring current? please confirm if you have checked between IC pin11/12 and T1 pin 1 like this. Also, can you please attach

waveform of iL with good resolution with A/div, BW info etc for proper understanding? Sorry I cannot see iL_peak level due to poor resolution.

1）yes, we use resistive load; for EVM test, only change value of the C13 from 0.082uF to 2.2nF;

2) yes, it's difficult to comment the customer board; you can focus on EVM board test;

3)We only want to know why  the chip have some cycle of off time  during the startup;(even the EVM also have this issue)

4) yes, the measurement condition is same with the picture above;

5)for waveforms, which you can test reproduce on an EVM board;

Hi Xiaobo,

As I have explained, the EVM behavior is not an issue and looks expected to me.

Please go through the datasheet section 7.3.7 

The length of the off time is controlled by the FB voltage and the input voltage VIN. As an example, when VFB = 0 V and VIN = 24 V, the off time is set to 10 µs. This condition occurs if the output is shorted or during the initial phase of start-up.

Also, any update on below?

• waveform of iL with good resolution with A/div, BW info etc for proper understanding? Sorry I cannot see iL_peak level due to poor resolution.
• Kindly share the results with resistive load in default EVM.

Regards

Arpita

Thanks for your patient explanation!

1）but we don't find the condition of when VFB = 0 Vand VIN = 24 V,  for both my board and TI EVM board；（The red arrow is shown in the image）；

this is why i don't understand this issue；

don't  have VFB = 0 and don't exceed the limit of 2.5A; why does EVM have about 10us off time ?

2） for waveform of iL with good resolution with A/div, BW info etc；This may be due to the website, my image became blurred after uploading.

Hi Xiaobo,

It looks like inrush current would be much higher than 0.43A for the Css=2nF. As from the shared waveforms current values cannot be read, would advise to reattach the waveforms to confirm that the current amplitude is ~0.43A.

Also, for further debugging, please share the start-up behavior with resistive load in EVM as suggested earlier.

Regards

Arpita

Hi Xiaobo,

Did you check with a resistive load start-up? Does it resolve the issue?

Regards

Xiaobo

hi Arpita,

not resolve the issue yet

Did you check with resistive load? Can you please share the waveforms with pure resistive load vs e load? Sometimes the electronic load in CC mode while start-up can cause the issue.

Regards

Arpita

hi Arpita；

we had not do any test with  electronic load ；

I mentioned it about，for EVM test, only change value of the C13 from 0.082uF to 2.2nF; no change for other parts；

it still have some cycle of off time.

Hi Xiaobo,

I have understood that you used EVM but what was connected to the EVM load terminals? Could you please confirm?

Regards

Arpita