TPS40055: Question about abnormal waveform.

David Kim2

Genius 3545 points

Part Number: TPS40055

Below is the waveform at the start and the next waveform is the middle magnification.

Why is Vds over?

Let me know if there is a way to improve it.

-condition-

input: 15~20V

output: 5V

-sch-

Thanks

over 4 years ago

0 Timothy Hegarty over 4 years ago

TI__Guru* 79017 points

Hi David,

The duty cycle is not consistent with a 15-20V input and 5V output (where D = Vout/Vin for a buck regulator). Please check your design - there is a calculator tool available at https://www.ti.com/product/TPS40055#design-development##design-tools-simulation

Regards,

Tim

0 David Kim2 over 4 years ago in reply to Timothy Hegarty

Genius 3545 points

hi
I have an additional question.
1. If it's a duty problem, where should I fix it?
For example, modify a part value or change an L value.
2. According to the attachment, the abnormal waveform has a maximum voltage is 2V of Vgs.
What's wrong?
3. If you look at the waveform in the attached file, there is a difference between normal and abnormal. Why did you do that?6354.waveform.pdf

0 Timothy Hegarty over 4 years ago in reply to David Kim2

TI__Guru* 79017 points

Hi David,

Looks like the high-side gate drive is not going high as expected. Please send the power stage schematic including FET part numbers. Check the boot cap value and if BP10 is stable at its nominal 9.6V.

Regards,

Tim

0 David Kim2 over 4 years ago in reply to Timothy Hegarty

Genius 3545 points

Hi
I answer as follow.

Boot cap : 100nF
FET : FDMS8333L
L201 : 3.3uH
Bp10=> 9.4V

3187.mosfet sch.pdf

Thanks

0 Timothy Hegarty over 4 years ago in reply to David Kim2

TI__Guru* 79017 points

Hi David,

This is -5V output, so it's configured as an inverting buck-boost (IBB). Please check the stability via simulation or calculation. IBB compensation is somewhat more difficult with voltage-mode control, and a high Cout with ESR helps.

Here is an app note on IBB: https://www.ti.com/lit/an/snva856a/snva856a.pdf

Also, the MOSFETs are quite low Rdson and high Qg (2,4mOhm and 46nC) for a 3A output. Consider using higher Rdson for better efficiency, especially if the Fsw is high (500kHz is indicated in the plot above)

Regards,

Tim

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TPS40055: Question about abnormal waveform.