TPS737: TPS73701 loop test is fail

user5074120

Part Number: TPS737

TPS73701 Schematic diagram is below.Test Phase and Gain Margin by removed C241,the Phase value is less than 30° when loading is 0.25A or 0A;the phase is greater than 30° when loadind is 0.5A.How can I optimize the design to ensure the phase value is greater than 30°.Please give me some suggestion,thanks!

The Schematic diagram:

The Bode diagram under 0A loading:

The Bode diagram under 0.25A loading:

over 7 years ago

0 user5074120 over 7 years ago

Intellectual 270 points

The input voltage is 5V,and the Vout is 1.8V.The max loading is 0.5A.

0 Ryan Eslinger over 7 years ago in reply to user5074120

TI__Guru 55940 points

Hi ,

As you have discovered, the phase margin depends of the LDO depends on the conditions to which the LDO is exposed. From an application standpoint the only things we can do to impact the phase margin is to adjust the feedforward capacitance (C241 in your application) and the output capacitance.

The following application report discusses how a feedforward capacitor impacts stability.

www.ti.com/.../sbva042.pdf

The following application report discuses how output capacitors impact stability.

www.ti.com/.../snva167a.pdf

It can be difficult to measure the phase margin of a modern LDO as there are often internal feedforward capacitors that make it impossible to truly break the feedback loop. The following Application Report discusses how to equate a transient response to Phase Margin.

www.ti.com/.../slva381b.pdf

Very Respectfully,
Ryan

0 user5074120 over 7 years ago in reply to Ryan Eslinger

Intellectual 270 points

Thanks very much.

0 user5074120 over 7 years ago in reply to Ryan Eslinger

Intellectual 270 points

HI Ryan，
Excuse me，is there internal feedforward capacitors in TPS73701？

0 Ryan Eslinger over 7 years ago in reply to user5074120

TI__Guru 55940 points

Hi ,

I apologize for the confusion. TPS73701 does not have an internal feedforward capacitor.

My comments were meant to be general to linear regulators and not specific to TPS73701 as the methods to improve phase margin is the same for all LDOs. I also wanted to inform you that directly measuring phase margin will not always be possible on every LDO and wanted to provide the recommended method for checking stability in an application.

Very Respectfully,
Ryan

0 user5074120 over 7 years ago in reply to Ryan Eslinger

Intellectual 270 points

HI Ryan，

Thank you for your response.

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TPS737: TPS73701 loop test is fail