LM5576: Abnormal SW node under no load condition

Ella KIM

Part Number: LM5576

Hi there,

I have two questions while I'm looking for the reason why Vout appears lower value under no load condition in the customer board. It shows normal Vout value if there's a load, though.

Please find the SW node waveform under the no load condition. I believe it's a wrong operation, because SW node never falls into -Vf.

From TINA model simulation, while IL>0A, I can find 1. HSFET on-period and 2. LS re-circulating diode forward-biased period. (I attached TINA model. I ran the sim from 0s to 3ms.snvmbp5.TSC)

Q1. Could you confirm that the customer waveform is abnormal?

Q2. Then, could you share any possible culprit of this abnormal operation?

Thanks!

over 4 years ago

+1 iAlejandro over 4 years ago

TI__Expert 8130 points

Hi Ella,

The waveform is normal. The SW node waveform you provided is expected. This is happening because the device is a non-synchronous buck. Therefore, the device will enter DCM when the inductor current goes to 0A.
Adding a dummy load at the output should help with this behavior and also connecting the PRE pin to SW.

Regards,
Alejandro

0 Ella KIM over 4 years ago in reply to iAlejandro

TI__Expert 4212 points

Hi Alejandro,

Would you help me to understand why the waveform is normal? I believe there should be -Vf period before entering the DCM, because the inductor current is decreasing but still above 0A.

+1 iAlejandro over 4 years ago in reply to Ella KIM

TI__Expert 8130 points

Hi Ella,

I believe the off-time is so fast that it is not seen among the rise/fall times and the ringing. I believe the following section in the datasheet will be very helpful: 7.4.6 Boost Pin.

Regards,
Alejandro

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LM5576: Abnormal SW node under no load condition