LM61440-Q1: Simulation problem with WEBENCH values

Yael Reajev

Part Number: LM61440-Q1

Tool/software:

Hello,

I used in WEBENCH this part; Input= 4V to 22V, output = 3.3V, 0.5A and got frequencies that are lower than 200KHz as defined in the datasheet.

When simulated using LM61440AFS-Q1 PSpice Transient Model:

1. Rt has no effect on the SW waveform.

2. The output is not stable under 11V input.

Is there a problem with the WEBENCH calculations for this part?

Can I please get a PSpice simulation where the Rt works properly?

Thank you.

over 1 year ago

0 Frank De Stasi over 1 year ago

TI__Guru 73515 points

Hello

I will ask our Application engineer to look at this issue.

Thanks

0 Marshall_Beck over 1 year ago in reply to Frank De Stasi

TI__Mastermind 35210 points

Hi,

This device employs frequency foldback and can occur as light loads are approached or when duty cycle beomes very lage/small (min on time, min off time).

If you would like to exhibit the switching frequency at nominal conditions, evaluate at less extreme duty cycles and/or at higher loads.

12Vin, 3.3Vout, 4A.

If you dont want frequency foldback, utilize the FPWM orderable. LM61440AFS

0 Yael Reajev over 1 year ago in reply to Marshall_Beck

Prodigy 10 points

Hi,

My simulation is based on the WEBENCH recommendations and used the LM61440AFS model and I can see that the output is not stable.

I would like to know if I can use this device or search for another one.

0 Marshall_Beck over 1 year ago in reply to Yael Reajev

TI__Mastermind 35210 points

Hi, this device is recommended for this application.

Your webench has some exoctic values and maybe the cause of your reported instability.

I started a webench design for you and the component selection demonstrates stability and is what I would recommend.

webench.ti.com/.../SDP.cgi

0 Yael Reajev over 1 year ago in reply to Marshall_Beck

Prodigy 10 points

Marshall_Beck said:
webench.ti.com/.../SDP.cgi

Hi,

Thank you for your answer.

The attached link does not work, please resend.

Thank you

0 Marshall_Beck over 1 year ago in reply to Yael Reajev

TI__Mastermind 35210 points

Hello, you are welcome. Sorry about the link not sharing correctly.

Please see if it works this time.

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=8CCCAEE43DD85C84

I inputted 12Vin, 3.3V, with 4A load current. This BOM would be sufficient for your vin range and reduced load current. By setting it at the nominal conditions along with full-load current, the passive values are more realistic.

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LM61440-Q1: Simulation problem with WEBENCH values