TPS65400: VREF_COMMAND setting issue

NObody

Part Number: TPS65400

Hi, All：

We got a problem about VREF_COMMAND(D8h) setting.

Customer remove low side feedback resister and measure output voltage, customer think Vout = Vref in this case.

When they set D8h = 0x00h, they got Vout = 0.65V, this isn't match with spec. they set D8h = 0x7Fh, they can got correct value ---- 1.87V.

Any possible cause make this issue?

Customer's schematic as below.

over 7 years ago

0 Sam(MPAA) Zhang over 7 years ago

TI__Mastermind 19080 points

Hi,

I'm looking into this question. May need some time to do the test.
I've one question, when 0.65V measured, is there any loading present?

Regards,
Sam

0 Sam(MPAA) Zhang over 7 years ago

TI__Mastermind 19080 points

Hi,

The reason is the loop is not stable any more when the Vout is set to be 0.65V.

You can probe the SW waveform when Vref=0.65V, you will find the abnormal waveform. Attached is my test result in your condition.

Current compensation is designed for higher Vout. If Vout in your design is 0.65V, compensation network needs to be re-designed.

Different Vref.xlsx

Regards,

Sam

0 NObody over 7 years ago in reply to Sam(MPAA) Zhang

Expert 1100 points

Hi, Sam：

Is it helpful if I adjust DFh? or other register can improve it.

My calculate as below could you help to double check(Rc, Cc, Croll)??

0 NObody over 7 years ago in reply to NObody

Expert 1100 points

Hi, Sam：

If Fsw = 2MHz, Vin = 12V, Vout =0.6V

Is this OK? I think on time will less than minimum value, Am I right?

0 Sam(MPAA) Zhang over 7 years ago in reply to NObody

TI__Mastermind 19080 points

Hi,

It can't help the on stability issue by adjusting the transition rate (DFh).

To avoid the minimum on time limit, ton_min MAX value (150ns in datasheet) should be used for the calculation.
This means, if Fsw=2MHz, Vin=12V, minimum Vout should be 3.6V.
So neither 0.6V nor 3.3V Vout would work well...

You may have to decrease the Fsw or VIN. Please post the new value, and then I can help to check the compensation design. Thanks.

Regards,
Sam

0 NObody over 7 years ago in reply to Sam(MPAA) Zhang

Expert 1100 points

Hi, Sam：

Copy it.

One more question, if customer want to change Vout by VREF_COMMAND setting (D8h).

Any side effect? (ex: compensation or others....)

0 Sam(MPAA) Zhang over 7 years ago in reply to NObody

TI__Mastermind 19080 points

Hi,

It should be fine as long as

1. Compensation network can work well with the new Vout.

2. Minimum on time(max value in EC table, 150ns) limit is not touched.

Regards,

Sam

0 NObody over 7 years ago in reply to Sam(MPAA) Zhang

Expert 1100 points

Hi, Sam：

Thanks for your reply soon.

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TPS65400: VREF_COMMAND setting issue