TPS563252: Buck + Parallel LDO - VOUT not regulating as expected

Austin Allen

Intellectual 1970 points

Part Number: TPS563252

Tool/software:

Hi all,

I'm working on a design that involves having an LDO in parallel with a Buck converter for increased light load efficiency

I've been following this app note: https://www.ti.com/lit/ug/tiduaa4/tiduaa4.pdf

I built the circuit in PSPICE using TPS563252 and TPS70918.

My goal is to have the output voltage be 1.8V, but based on the App Note saying I need min Buck Voltage > max LDO voltage for this to work, I have Buck voltage set closer to 1.9V.

The issue I'm seeing is that, first of all, when I enable the Buck Converter, it does not start switching. Instead, Vout stays at 1.8V which is coming from LDO.

The second issue is, when I increase the load (around 6ms in), the Buck converter does start switching but Vout still stays at 1.8V and about a ms later the output collapses altogether.

Any idea what I'm doing wrong here?

Thank you!!

TPS563252_TPS709 Switchover Circuit.pdf Transient 1 - Crossover.pdf Switchover Circuit.zip

1 month ago

0 Lucia Gao 1 month ago

TI__Genius 17380 points

Hi Austin,

Thanks for reaching out. Pls kindly let me check and get back to you during these days.

BRs

Lucia

0 Lucia Gao 1 month ago in reply to Lucia Gao

TI__Genius 17380 points

Hi Austin,

Did you see this issue with only Pspice simulation or with your board?

Thanks!

BRs

Lucia

0 Austin Allen 1 month ago in reply to Lucia Gao

TI__Intellectual 1970 points

Just on PSPICE so far, I am ordering boards as well

0 Lucia Gao 1 month ago in reply to Austin Allen

TI__Genius 17380 points

Hi Austin,

Thanks for your feedback. I will look into the details.

BRs

Lucia

0 Lucia Gao 1 month ago in reply to Lucia Gao

TI__Genius 17380 points

Hi Austin,

Just want to understand more background that why customer need to parallel LDO and DCDC. Is this mainly to increase light load efficiency or they also have a must requirement for low ripple and low noise?

(1) What is the range of the load current? Looks like it's from 200mA to 3A from the simulation? And what is the requirement for the efficiency from 200mA to 3A?

(2) What is the requirement of output ripple and noise? When the load is light and with eco mode part, the output ripple will be larger than when it's in CCM mode. I am not sure whether it will be the reason that customer choose to parallel LDO with Buck converter.

Thanks and looking forward to your reply!

BRs

Lucia

0 Austin Allen 1 month ago in reply to Lucia Gao

TI__Intellectual 1970 points

Hi Lucia,

I started an email thread internally with more details

Thanks

Austin

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TPS563252: Buck + Parallel LDO - VOUT not regulating as expected