TPS54620: TPS54620 hard short protection behavior

Randy Chen

Part Number: TPS54620

Hi,

My customer had hard short test by using Electrical loading and captured the attached waveform. The waveform shows Vout starts drop ta the very begining even the Iout still under 6A. Why the behavor Vout drops before Iout reach 6A? Another test, customer used the constant current mode to increase the Iout loading and Vout started drop at Iout 9~10A.

Best regards,

Randy Chen

over 7 years ago

0 JohnTucker over 7 years ago

TI__Guru**** 191445 points

Randy,

It is like a load transient. When the load is changed abruptly, the excess current will come from charge stored in the output capacitors, causing Vout to drop.

0 Randy Chen over 7 years ago in reply to JohnTucker

TI__Intellectual 2240 points

Hi John,

Another question about OC.

The picture is OC test by E-loading.

As picture why the high side MOSFET not turn off when Iout reach to 9.5A? This is not meet spec description.

Best regards,

Randy Chen

0 JohnTucker over 7 years ago in reply to Randy Chen

TI__Guru**** 191445 points

Randy,

I can't really tell anything from your waveform as the time scale is too long. TPS54620 uses pulse by pulse current limit. The high side FET will turn on for every cycle. When the current is above the current limit, it will turn off earlier than required for voltage regulation. It will always turn on at least for the minimum on time, 94 nsec typical, 135 nsec maximum.

0 LI Gandalf over 7 years ago in reply to JohnTucker

Prodigy 60 points

I had zoom in the ocp states

Does it meet the datasheet behavior ?

0 LI Gandalf over 7 years ago in reply to LI Gandalf

Prodigy 60 points

By the way
This voltage output is 3.3V and input is 12V

0 JohnTucker over 7 years ago in reply to LI Gandalf

TI__Guru**** 191445 points

Randy,
Something is not right with your waveform. Looking at your "high side switch", I assume that is the voltage at the PH node. It is offset above ground during the off time and the voltage is increasing during the on time. When the TPS54620 is sourcing current, the switch current should increase during the on time, causing the PH node voltage to decrease below your 12 V input. During the off time the positive current should cause the PH node voltage to be below ground, not above ground. See figure 44 in the datasheet for a representative waveform of PH at 6 A load. Can you recheck your set up?

0 LI Gandalf over 7 years ago in reply to JohnTucker

Prodigy 60 points

Sorry, I don't clear what your assumption...the picture are our setting and layout for your reference

and we want to know why the PG and Vout not pull low when current limit, thanks!!

0 LI Gandalf over 7 years ago in reply to LI Gandalf

Prodigy 60 points

Those picture are setting of E-load

0 JohnTucker over 7 years ago in reply to LI Gandalf

TI__Guru**** 191445 points

Li and Randy,
All I can say for certain is that the waveform you show "looks" like the circuit is sinking current, not sourcing current. The PH voltage is higher than Vin and increasing during the ON time and higher than ground during the OFF time. That is the characteristic of sinking current, not sourcing current. For sourcing current, PH voltage should be less than Vin and decreasing during the ON time and below ground and increasing during the OFF time. So I suspect something is wrong with your set up.
If I have an EVM available, I will take some waveforms later today.

0 JohnTucker over 7 years ago in reply to JohnTucker

TI__Guru**** 191445 points

TPS54620 Current Limit.pptxLi and Randy,

I went to the lab and took some measurements of TPS54620 at 6 A (normal output current) and in over current conditions of 10 A and 11 A load. I have annotated the wave forms to show you what type of wave forms you should expect when measuring current limit correctly. Hopefully you can resolve your set up issue.