LM74900Q1EVM: Short circuit protection

Hi Audrius,

Thanks for reaching out to us!

Can you elaborate on the issue you are facing with EVM.

Regarding Pspice for TI, did you click on request button and then failed to proceed?

Regards,

Shiven Dhir

1: Vin = Vout 3.3V, then output was shorted

TMR - not working.

2: Vin = Vout 6.3V, then output was shorted


EWB board settings:
R16 was changed to 22.5k

"Regarding Pspice for TI, did you click on request button and then failed to proceed?"
I didn't get any link for download it.

Hi Audrius,

I will check with my team regarding short circuit issue.

Regarding Pspice for TI, you will need to request using your TI account and fill a form and will receive the link via email.

Did you complete the following procedure?

Regards,

Shiven Dhir

yes I did, but I'll try it again

Hi Audrius,

Regarding the SCP trigger, can you share the following signals in a scopeshot

• VIN, VOUT, HGATE, IOUT
• VIN, TMR, HGATE, IOUT

Also, I checked with my team, LM74900/910 does not have a model that can run in Pspice for TI yet.

Regards,

Shiven Dhir

VIN, VOUT, HGATE, IOUT

3.3V

6.3V

VIN, TMR, HGATE, IOUT

3.3V

6.3V

Hi Audrius,

Thanks for the waveforms.

Looks like when Short is applied, VIN drops below 3V and hence TMR resets. This behavior is expected due to VIN drop.

Adding very high value input capacitors can help.

Regards,

Shiven Dhir

I added 820uF to Vin, same issue

Vin 3.3V

Vin 5V with 820uF

any other suggestions?

Hi Audrius

Please increase the capacitor value further, around 2mF. Idea is to not let VIN drop during short circuit that can trigger UVLO or VS_PORR.

Also, it is better to connect UV directly to VIN or anything > 0.6V for this specific test. This will disable UVLO functionality and hence reduce our variables.

Regards,

Shiven Dhir 

Vin CAP 2200uF. UV Connected directly to VIN

Yellow Vin, GREEN - TMR, and ORANGE - CURRENT

Do you have any data to confirm that this IC works properly for 3.3V or 5V applications?

Hi Audrius,

Datasheet is data that confirms that this IC works properly. I will try to find if we have any waveforms that proves this functionality at 3.3V and 5V.

Regards,

Shiven Dhir

Thanks Shiven Dhir,
As we can see, adding an additional 2200u capacitor to Vin did not solve this problem.

Hi Audrius,

As far as I could investigate, VIN dropping is the only visible cause of this behavior. Controller needs 3V at least to function.

Regards,

Shiven Dhir

does the VS voltage must be higher then 3V or VA or both?

Hi Audrius,

VS need to be higher than 3V.

Regards,

Shiven Dhir

Shiven Dhir said:

Datasheet is data that confirms that this IC works properly. I will try to find if we have any waveforms that proves this functionality at 3.3V and 5V.

any info?

Hi Audrius,

I couldn't get the waveforms as of now, I will test the board myself and update you early next week. Meanwhile, can you help me with this info:

Here VIN is drooping although HGATE has fallen already. Which means path is cutoff and hence VIN shouldn't droop further. Why is this happening?

Can you also measure VS pin voltage, you can remove IOUT. (VIN,VS,TMR,HGATE)

Please share the waveform in this time division, it is easy to read in this.

Regards,

Shiven Dhir

VIN - Yellow
VS - Green
TMR - Orange
HGATE- Violet

PSU 3.3V, 12A

Changed PSU to another and set the same 3.3V 12A

Hi Audrius,

 We can see device is able to function at 3.3V, the TMR pin is working fine. It mostly is a setup issue. Maybe try increasing the current limit of power supply.

Regards,

Shiven Dhir 

but OCP limit is 5A,
For example if my DC/DC will provide max current 10A and I want to set OCP of LM74900 to 5A it will not work properly?

Hi Audrius,

We are working on the issue.

Appreciate the patience.

Regards,

Shiven Dhir

As I understand it will not solve my issue and  I can not use this IC  for 3.3V and 5V applications.

Hi Audrius,

At such low voltage like 3.3V, it might be hard to hold voltage on VS, adding high cap at VS can help further, but it will delay the turn-off also in some cases. For 5V, the device works fine in our setup as shown in waveforms.

Holding voltage at VS is the only challenge here.

Regards,

Shiven Dhir