UCC256304: UCC256304 question about FBreplica voltage.

Bob Lee62837

Part Number: UCC256304
Other Parts Discussed in Thread: UCC256302

Dear supportmember,

My customer used UCC256304.

I have a question.

Q1
It is described as "When the FBreplica is less than the burst mode threshold, stop LLC switching" in datasheet 7.3.5 of UCC256304.

The burst mode threshold voltage can be confirmed with 9 pin LL / SS voltage.
But which pins can you see FBreplica voltage?

Best regard.
Bob Lee.

over 5 years ago

0 Benjamin Lough over 5 years ago

TI__Mastermind 26225 points

Hi Bob,

Thanks for your interest in UCC256304. You cannot directly measure the internal FBreplica from outside the chip. What you can do is measure the current out of the FB pin and use the equation FBreplica=Rfb*(I_internalcurrentsource-I_fbpin) to calculate the value of FBreplica. Rfb is equal to 101.5kΩ and I_internalcurrentsource is equal to 85.1uA.

Best Regards,
Ben Lough

0 Bob Lee62837 over 5 years ago in reply to Benjamin Lough

Mastermind 7230 points

Benjamin Lough,

Thank you very much for reply.

May I ask a add question?

Your answer "Rfb is equal to 101.5kΩ and I_internalcurrentsource is equal to 85.1uA".
When calculated,FBreplica = 8.637V is became.

The current circuit diagram is remove the RLLUpper.
In this way,
the VLL voltage is at its minimal value 0.7 V burst mode threshold voltage is became.

If VLL is 0.7V,
"When the FBreplica is less than the burst mode threshold, stop LLC switching"

When VLL is less than FBreplica,
Is IC never start up?

Current status、RVCC is 1s interval restart,
Vout start up is not.

Best regard.
Bob Lee.

0 Bob Lee62837 over 5 years ago in reply to Bob Lee62837

Mastermind 7230 points

Benjamin Lough,

I'm sorry.
I was wrong about question.

Current status、RVCC is 1s interval restart,
Vout start up is not.

Is there anything confirmation method?

Best regard.
Bob Lee.

0 Benjamin Lough over 5 years ago in reply to Bob Lee62837

TI__Mastermind 26225 points

Hi Bob,

If RVCC is restarting in 1s intervals then the controller is detecting a fault. Take a look at the ISNS and BW pins. The controller is likely detecting either an OCP or OVP fault.

Best Regards,

Ben Lough

0 Bob Lee62837 over 5 years ago in reply to Benjamin Lough

Mastermind 7230 points

Dear Ben Lough,

Thank you very much for reply.

I measureed.

ISNS pin was 0V.
BW pin was 0V.

HO pin was 1s interval voltage change.
LO pin was 0V.

What is there anything detecting a fault?

Do "system States and Faults State Machine" have relationship ?

Best regard.
Bob Lee.

0 Benjamin Lough over 5 years ago in reply to Bob Lee62837

TI__Mastermind 26225 points

Hi Bob,

Could you share waveforms of ISNS and VCR with me? Also could you make sure you trigger the scope capture when RVCC is high? The controller has a 1s fault timer so for the majority of time, VCR and ISNS would be near 0V. Also check VCC to make sure it is not dropping below 10.5V during startup.

Best Regards,
Ben Lough

0 Bob Lee62837 over 5 years ago in reply to Benjamin Lough

Mastermind 7230 points

Dear Benjamin Lough,

Thank you very much for reply.

I will confirm my customer measure waveform.
Please wait for a little while longer.

Best regard.
Bob Lee.

0 Bob Lee62837 over 5 years ago in reply to Bob Lee62837

Mastermind 7230 points

Dear Benjamin Lough,

I attached waveforms of ISNS and VCR.

Could you check it?

Best regard.
Bob Lee.

UCC256304_waveform_181017.pdf

0 Benjamin Lough over 5 years ago in reply to Bob Lee62837

TI__Mastermind 26225 points

Hi Bob,

Are you applying both the DC input and the AC input to the EVM?

Best Regards,
Ben Lough

0 Bob Lee62837 over 5 years ago in reply to Benjamin Lough

Mastermind 7230 points

Dear Benjamin Lough,

Thank you very much for reply.

We are using only DC input.

Because the varistor of RV 1 was destroyed,
We input 35 VDC to TP10.

Best regard.
Bob Lee.

0 Benjamin Lough over 5 years ago in reply to Bob Lee62837

TI__Mastermind 26225 points

Hi Bob,

Thanks for the info. UCC256304 has an X cap discharge function which will not function properly if a DC voltage is applied to TP10. You will need to apply an AC voltage between TP3 and TP4 in order for the X cap discharge function to work properly. This is likely causing some of the odd behavior you were seeing.

You will also want to make sure the BLK pin is seeing an appropriate voltage in order to start up.When modifying the EVM to use UCC256304, we recommend changing R4 to 44.2kΩ

Best Regards,

Ben Lough

0 Bob Lee62837 over 5 years ago in reply to Benjamin Lough

Mastermind 7230 points

Dear Benjamin Lough,

Thank you very much for reply.
I will confirmation.

By the way,
Is there a way to stop the Xcap function?

Since there is a case that the varistor is broken if there is an AC line and GND when measuring with an oscilloscope.
（AC Level and oscilloscope GND level is not same level.）

So DC input only using.

Best regard.
Bob Lee.

0 Benjamin Lough over 5 years ago in reply to Bob Lee62837

TI__Mastermind 26225 points

Hi Bob,

There isn't a way to disable X cap discharge on UCC256304. You can move to UCC256302 which does not have X cap discharge however.

Best Regards,
Ben Lough

0 Bob Lee62837 over 5 years ago in reply to Benjamin Lough

Mastermind 7230 points

Dear Benjamin Lough,

Thank you very much for advice.

I was wrong schematic constant.

Resonant capacitor C7,C8 was using wrong constant.

As I changed constants,
IC worked.

Best regard.
Bob Lee.

0 Benjamin Lough over 5 years ago in reply to Bob Lee62837

TI__Mastermind 26225 points

Hi Bob,

Thanks for the update. Let me know if there are any other problems.

Best Regards,

Ben Lough

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UCC256304: UCC256304 question about FBreplica voltage.