LM76002: Converter not going into regulation

Peter Seibold

Part Number: LM76002
Other Parts Discussed in Thread: TPS2549-Q1, TPS2549

Hello,

We are working on an automotive USB charger based on the LM76002. The application is space constrained that's why we have used WEBENCH® Power Designer to optimize for space, please find WEBENCH output and our schematic attached.

We are using a Bourns SRP6060FA-150M 15uH coil, two 47uF ceramics output caps and the cable compensation of the attached TPS2549-Q1 USB Charging Port Controller.

We now have the first assembled prototypes in our lab but the LM76002 does not go into regulation. Please find attached the resulting outputs at startup (Vin = 12V, Vsw red, Vout blue).

The internal LDO is producing stable 3.3V at Vcc. We already tried varying the switching frequency and adding more capacitance to the output, always resulting in the same behaviour.

What could possibly be wrong with the design resulting in this behaviour?

Please let me know if you need any additional information.

Thank you and best regards,
Peter

over 5 years ago

0 Samuel Jaffe over 5 years ago

TI__Mastermind 25590 points

Peter,

Thanks for the information. The schematic should work. Which SYNC/MODE resistor did you depopulate?

Please also share the layout.

Thanks,
-Sam

0 Peter Seibold over 5 years ago in reply to Samuel Jaffe

Prodigy 50 points

Hi Sam,

thanks for your fast response.

SYNC/MODE is pulled down with R2. R1 is not populated.

Here is the top layer of the layout, its a 4 layer design, with GND and Vout in the inner layers

Thanks,

Peter

0 Samuel Jaffe over 5 years ago in reply to Peter Seibold

TI__Mastermind 25590 points

Peter,

The layout looks good enough where I don't think it's the sole reason for the issue. The part may be hitting current limit or hiccup. I recommend removing all load, adding a soft start capacitor, and testing again.

-Sam

0 Peter Seibold over 5 years ago in reply to Samuel Jaffe

Prodigy 50 points

Hi Sam,

after lengthy Investigations I finally found out, why the device was hitting current Limitation. One of the pull-up/pull-down couples at the TPS2549 was populated with both 0 ohm resistors by accident and causing a proper short.
Trying different sizes of soft-start caps eventually burned away one of the 0402 Jumper resistors and finally pointed me to the cause of this behaviour.

Thank you very much for your great support,
Peter

0 Samuel Jaffe over 5 years ago in reply to Peter Seibold

TI__Mastermind 25590 points

Peter,

Glad we solved it! Have a great day!

-Sam

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LM76002: Converter not going into regulation