TPS23754 startup problem

Alparslan Isikli

Other Parts Discussed in Thread: TPS23754, TPS23754EVM-420

Hi;

I designed a PoE according to reference http://www.ti.com/lit/ug/slvu301/slvu301.pdf. However, generally design could not start easily. First of all, you hear "zzzz" sound from POE300F-50L and you get no output. Then, If you touch the DT and BLNK pin, design works fine. If the design works fine with no load, board will not start to work with load (1A) in general.

I tried this at 15-20 different PCB. I think resistance of the DT and BLNK pin does not choosen correctly.

How can I solve this problem?

Thank you,

over 7 years ago

0 Darwin Fernandez over 7 years ago

TI__Mastermind 40100 points

Hi Alparslan,

The TPS23754EVM is a well known working solution. How close are these components to the IC on your board? How closely was the layout followed on your board as in the EVM? You may want to order and EVM to compare nodal impedance to make sure component selection and overall PCB connections are similar.

Regards,
Darwin

0 Alparslan Isikli over 7 years ago in reply to Darwin Fernandez

Prodigy 95 points

Hi Darwin,

Is it correct to put DT and BLNK resistors closely at bottom layer?

0 Darwin Fernandez over 7 years ago in reply to Alparslan Isikli

TI__Mastermind 40100 points

Hi Alparslan,

It should be okay to be on the bottom layer as long as the routing from pin to via to resistor to IC GND is short. Is there a way to modify your board to put a small SMT resistor on the ICs pin to GND to verify?

0 Alparslan Isikli over 7 years ago in reply to Darwin Fernandez

Prodigy 95 points

Hi Darwin,

I will try it as soon as possible. I will share the result.

Thank you,

0 Alparslan Isikli over 7 years ago in reply to Alparslan Isikli

Prodigy 95 points

Hi Darwin;

I tested system with 402 resistors connected over the pins of the TPS23754; result is the same. I think I should redesign the PCB with wider routes.

Do you suggest anything other than that?

Thank you,

0 Darwin Fernandez over 7 years ago in reply to Alparslan Isikli

TI__Mastermind 40100 points

Hi Alsparslan,

That might help. But I also recommend similar routing techniques as the EVM. Thanks!

Regards,
Darwin

0 Darwin Fernandez over 7 years ago in reply to Darwin Fernandez

TI__Mastermind 40100 points

Can you also attach a schematic of your design that I can reference? Thanks!

0 Alparslan Isikli over 7 years ago in reply to Darwin Fernandez

Prodigy 95 points

Hi Darwin,

I added the schematics. I took the same result with the new PCB. What do you suggest? I tried everything but this situation does not going well.

Thank you,

0 Darwin Fernandez over 7 years ago in reply to Alparslan Isikli

TI__Mastermind 40100 points

Hi Alparslan,

Thanks for sending the schematic. I see a lot of 0402 on components that would heat up. For example, R5, R23, R20, R6 and
R25 should be similar to the sizes used in the EVM. Also C24 should be removed.

0 Alparslan Isikli over 7 years ago in reply to Darwin Fernandez

Prodigy 95 points

Hi Darwin,

I think about your suggestion. However, EVM's power ratings are generally 1/16Watt even if they are bigger footprint sizes. I choose 1/10Watt resistors in 0402 SMD packages. So, 0402 packages will not be a problem. I removed also C24 and this did not work.

I fix the problem. I removed the capacitors (c18 and C19 from http://www.ti.com/lit/ug/slvu301/slvu301.pdf) from the design. Now it is working properly at every condition. The main transformer can not drive high capacitive load. I think TI may also remove that capacitors from the TPS23754EVM-420. Because it is not working properly in this way.

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TPS23754 startup problem