LM5171-Q1: Undocumented Hysteretic Mode at Light Load

Dan Quixoté

Part Number: LM5171-Q1
Other Parts Discussed in Thread: LM2611, LM5171

Tool/software:

At light load, LM5171-Q1 goes into a hysteretic mode. Entry and exit between that mode and regulation seems to itself have a hysteresis threshold of load current or power. Can documentation about this be provided? We need to be able to prevent it from entering this mode, otherwise the IC cannot effectively respond to transient loads quickly enough. See below -purple is boost voltage, yellow is current. The boosted voltage collapses to input voltage for the first 2 milliseconds until some kind of internal state machine decides to exit hysteretic mode.

5 months ago

0 Dan Quixoté 5 months ago

Prodigy 70 points

0 Feng Ji 5 months ago in reply to Dan Quixoté

TI__Mastermind 22540 points

Hi Dan,

Are these waveforms from the EVM?

I can observe similar behavior on the EVM, but it happens only at very light load and the amplitude is much less.

Best Regards,

Feng

0 Dan Quixoté 5 months ago in reply to Feng Ji

Prodigy 70 points

Hi Feng Ji - It's on target hardware that we have built. We need to understand the light-load hysteretic mode to at least the degree as TI described about LM2611 here LM2611: Condition for Hysteretic mode - Power management forum - Power management - TI E2E support forums. The TI IC design teams are in the habit of implementing pulse-skipping, hysteretic modes for light loads - which are great for most applications - but need to be very well understood for supporting high-reliability automotive applications.

Thanks!

Dan

0 Feng Ji 5 months ago in reply to Dan Quixoté

TI__Mastermind 22540 points

Hi Dan,

There is no hysteric mode in LM5171. This is caused by delay, you may observe ISET and COMP pin waveform to understand the behavior.

You may add some dummy load or increase the voltage loop cross over frequency to improve this.

Best Regards,

Feng