LM5160-Q1: Question about LM5160-Q1 Flybuck-boost test result

NOBUO FUJIHARA

Genius 9335 points

Part Number: LM5160-Q1
Other Parts Discussed in Thread: LM5160, PMP15005

Hi Champs,

My customer started evaluation HV buck-boost circuitry with LM5160 Flybuck-boost configuration.

As primary side output Vout1 -12V is targeted, but -16V is monitored as shown in attached file.

I also attached schematic and parameters for each external parts.

Could you give some advice to solve this?

Best regards,

FlyBuck-Boost test.xlsx

over 6 years ago

0 NOBUO FUJIHARA over 6 years ago

TI__Genius 9335 points

I forgot to add one additional information.
Customer measured FB node voltage, it's 2.6V.

If Vref is shifted to 2.6V, VOUT would be 12*2.6/2 = 16V.

Best regards,

0 Katelyn Wiggenhorn over 6 years ago in reply to NOBUO FUJIHARA

TI__Genius 11040 points

Hello,

Can you try to remove Rr to see if this fixes the problem? The R1 and R2 are sized correctly to set a -12V output, as described below:

Thank you,

Katelyn

0 NOBUO FUJIHARA over 6 years ago in reply to Katelyn Wiggenhorn

TI__Genius 9335 points

Hi Katelyn-san,

Thanks for your reply.

LM5160 is COT device, so Rr and Cr, Cac are required for ripple injection purpose.

I assume root cause is coming from the thing FB being regulated to 2.6V.

Can you guess the reason why reference voltage looks 0.6V higher than device specification?

Best regards,

Nobuo Fujihara

0 Katelyn Wiggenhorn over 6 years ago in reply to NOBUO FUJIHARA

TI__Genius 11040 points

Hello Fujihara-san,

My thought is that the ripple injection is interfering with the feedback voltage. However, I do not expect the feedback voltage to move 2.6V under normal operation.

You can see PMP15005 is an exmaple of an inverting flybuck design: www.ti.com/.../PMP15005

I would recommend changing Rr, Cr, and Cac to match PMP15005 to verify if the problem is in component selection.

Best Regards,
Katelyn

0 NOBUO FUJIHARA over 6 years ago in reply to Katelyn Wiggenhorn

TI__Genius 9335 points

Hello Katelyn-san,

Thanks for your advice.

There must be layout issue on customer board, will also check if it's align with our recommendation.

Best regards,

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LM5160-Q1: Question about LM5160-Q1 Flybuck-boost test result