LM53600-Q1: oscillation on switch pin

Stefano Vischi1

Part Number: LM53600-Q1

Hi.
We got a question about LM53600-Q1.
We are using PN LM53600NQDSX, with fixed output at 3.3V.
Input voltage is between 11V to 13.5V, but it should operate good at 4V and 16V too.
Current consumption will be lower than 50mA.
We have simulate it with webench before to realize design and we have follow suggested schematic and layout.
We have connected SYNC/MODE to GND according to datasheet note in order to reduce EMI in case of light load.

Now we are debugging it and we are noticing that on switch pins there is undesired oscillation.
This appear over 5V. At 4V we are seeing correct waveform as reported on webench.
Moreover, we have seen that VOUT is a bit more than expected 3.33V.
If we connect SYNC/MODE to high level, VOUT is 3.3V and oscillation desapears.

Do you have any suggestion or explanation?

Thanks in advance
Best regards
Stefano

over 5 years ago

0 Arief Hernadi over 5 years ago

TI__Mastermind 23845 points

Hi Stefano,

This is a normal operation of the part when the MODE is tied to GND because the part is operating in PFM (Pulse Frequency Modulation Mode). When you this pin high, it will operate in FPWM (Forced PWM mode) this is when the part operates at you desired frequency.

On PFM, the frequency will be folding back in order to lower the input quiescent current.

As you increase the load, your waveform should change and similar to when you tie the MODE pin high.

Thanks

-Arief

0 Stefano Vischi1 over 5 years ago in reply to Arief Hernadi

Prodigy 40 points

Hello,
Thank you for your explanation.
The doubt concerns 3.33 V VOUT, while we expect 3.3 V.
30mV is not a problem for our application, but we would like to understand if it is a behavior expected for your point of view in case of oscillation for lower laod.

Thanks in advance
Best regards
Stefano

0 Arief Hernadi over 5 years ago in reply to Stefano Vischi1

TI__Mastermind 23845 points

Hi Stefano,

Yes, a slight bump in the output voltage is expected in this mode at light current.

You can check figure 27 on this datasheet for more information https://www.ti.com/lit/ds/symlink/lm53600-q1.pdf?ts=1594916648047&ref_url=https%253A%252F%252Fwww.google.com%252F#page=26

0 Stefano Vischi1 over 5 years ago in reply to Arief Hernadi

Prodigy 40 points

Hi.

Thanks for your precious suggestions.

Regards

Stefano

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LM53600-Q1: oscillation on switch pin