LM5007: The low temperature issue for LM5007

Hi,

1) At what load/load range does this occur?

2) Does this problem occur with multiple devices? Have you tried replacing the device on the board with another one and checking?

3) Can you send the waveforms of SW, VOUT, and BST-SW where you observe the issue?

Regards,

Niranjan

Hi Niranjan,

Please refer to below:

Niranjan Nair said:

1) At what load/load range does this occur?

The 12V output flows through the relay coil, and the 12V voltage is reduced to 5V to power the CPU. The load current is approximately 130mA;

Niranjan Nair said:

2) Does this problem occur with multiple devices? Have you tried replacing the device on the board with another one and checking?

The two boards currently being tested have both experienced this situation, and one of them also experiences this situation at room temperature,

but the probability is relatively small.

Niranjan Nair said:

3) Can you send the waveforms of SW, VOUT, and BST-SW where you observe the issue?

The waveform diagram is as follows: Channel 1 is the SW voltage, Channel 2 is the BST voltage, Channel 3 is the BST-SW voltage,

and Channel 4 is the voltage after L35;

Hi,

Can you send the full schematic? Some components from the output and input side have been cut-off in the original image.

Also can you confirm once more if the yellow waveform is the one at SW pin (before L35)? Because even when output is proper I do not see any switching and it just seems to be same as VOUT.

Regards,

Niranjan

Hi Niranjan,

Please refer to below schematic diagram:

Niranjan Nair said:

Also can you confirm once more if the yellow waveform is the one at SW pin (before L35)? Because even when output is proper I do not see any switching and it just seems to be same as VOUT.

Yes, the yellow waveform is the one at SW pin (before L35)

Hi,

I will review the schematic and get back to you.

Regards,

Niranjan

Hi Niranjan,

Please kindly review our schematic if there are any issue, thanks.

Hi Zhang,

The recommended value for VCC capacitor is 0.1 uF. I am not able to make out the value of bootstrap capacitor, but if it is not 0.01 uF please change it to that.

Could you make these changes and try again?

You will also need a ripple injection circuit for stable switching. although I don't think that is causing the issue at hand. You can refer 7.3.9 for the methods of implementation.

Regards,

Niranjan

Hi Niranjan,

We copy the reference design that the VCC capacitor is  0.1uF and bootstrap capacitor of the SW-BST is 0.01uF, and there are no improve for this case.

Hi Zhang,

Can you send me a waveform of SW and VOUT at room temp. where you are not seeing this issue?

What I find strange is that switching is not visible here (marked red circle) when it regulates the output voltage. From the waveform it looks like SW is following VOUT. Can you also isolate the buck from the downstream relay coils and try to run this at a higher load, say 500 mA and send the SW and VOUT waveforms?

Regards,

Niranjan

Hi Niranjan,

Sorry, customer engineer will test the spec. and feedback to us soon.