LM3489: Short Circuit Current Limit

Donovan Porter

Intellectual 1920 points

Part Number: LM3489

Hi,

I am using the LM3489 buck in a TI-Design that converts a barrel jack laptop charger (19.5 V) to USB Type-C PD. Please see schematic below.

I noticed when connected to certain laptops, there would be a small current spike around 4 or 5 A. I tuned the current limit on the LM3489 to get it to work (Radj = 26.1kohms now). Originally Radj was 10.4 kohms. Now when I short the VoutDCDC to GND I get the waveform below. CC is VoutDCDC.

With the 10.4 kohm I had the waveform shown below.

Even though I have raised the current significantly, it seems that the buck should still be limiting like it was before, just at a higher current. However I see the pulsing from 5 A to about 15 A. Any Ideas what is going on?

Regards,

Donovan

over 7 years ago

0 Samuel Jaffe over 7 years ago

TI__Mastermind 25590 points

Donovan,

Figure 22 in the datasheet describes the foldback behavior at current limit.

-Sam

0 Donovan Porter over 7 years ago in reply to Samuel Jaffe

TI__Intellectual 1920 points

Hi Samuel,

Thanks for the quick response. Why is there such a big difference between the two scope shots? Is the buck not current limiting in the second scope capture?

Regards,
Donovan

0 Samuel Jaffe over 7 years ago in reply to Donovan Porter

TI__Mastermind 25590 points

Donovan,

The first has a higher current limit, so it will take a bit longer for the part to realize how high the current has gone. That seems to be what's going on. It would be interesting to get a 50k potentiometer and see how it looks as you turn it from 10k to 20k. Is this a smooth transition or a jump. A smooth transition would support my explanation. A jump might require more thought.

-Sam

0 Donovan Porter over 7 years ago in reply to Samuel Jaffe

TI__Intellectual 1920 points

It is a jump. I'm just quickly shorting the output voltage to GND.

Regards,
Donovan

0 Samuel Jaffe over 7 years ago in reply to Donovan Porter

TI__Mastermind 25590 points

I meant shorting it while changing Radj from 10.4k to 26.1k. To see how it transitions from one waveform to the other.

0 Donovan Porter over 7 years ago in reply to Samuel Jaffe

TI__Intellectual 1920 points

Ok I see what you mean.

Thanks,
Donovan

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LM3489: Short Circuit Current Limit