LM3880: Issue with driving EN pin with open collector driver

Ed Brady

Part Number: LM3880

I am trying to drive an LM3880MF-1AA with an MC33161DMR2G voltage monitor. The MC33161DMR2G output is an open collector driver pulled up to 3.3V via a 10K resistor. When the the two parts are connected, the output will not release and go high. If I disconnect the two parts I see that the output of the MC33161 will release and transition high correctly. I have 6 boards built with this circuit; 2 work fine and 4 fail as described.

Does the enable for the LM3880 have any special requirements that I am missing. Kind of stumped as to what the problem is.

I have attached the schematic and pictures of the MC33161 output when connected to the LM3880 and not connected. The first cursor in the scope plots is where the PS is enabled, the second cursor sits where the MC33161 should release allowing the enable pin on the LM3880 to go high.

This shows the output of the MC33161 when not connected to the LM3880

This show the output of the MC33161 when its connected to the LM3880

over 4 years ago

+1 yihe over 4 years ago

TI__Guru 66230 points

Hello

The EN is a comparator with 7uA pull up current source. if you have a external control source, there is no need to have a capacitor on the EN since the capacitor is charged when LM3880 is power up. You can remove the capacitor.

Regards

Yihe

0 Ed Brady over 4 years ago in reply to yihe

Prodigy 30 points

Thanks for the reply. The circuit is designed to allow either power up/down sequencing where R122 is installed and C207 is not or power up sequencing only where R122 is not installed and C207 is installed. The configuration is customer specific.

0 yihe over 4 years ago in reply to Ed Brady

TI__Guru 66230 points

Thank you for the clarification. The schematic is kind of blurry.

I tested on the EVM, 10k pull up to the VCC works well for the EN on the EVM.

have you checked the soldering to make sure there is no short?

Regards

Yihe

0 Ed Brady over 4 years ago in reply to yihe

Prodigy 30 points

Yes I have verified there is no shorts. But if I take out R122 the MC33161DMR2G transitions high as expected. Not sure what is going on. There definitely appears to be some interaction between the two parts that prevent it from working.

0 yihe over 4 years ago in reply to Ed Brady

TI__Guru 66230 points

Have you measured the impedance of the EN pin and compare with the working board to see whether they are the same?

Regards

Yihe

0 Ed Brady over 4 years ago in reply to Ed Brady

Prodigy 30 points

No I have not measured the impedance of the EN pin on working vs non working board. I will try to look at this but will not be able to get to it for a few days

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LM3880: Issue with driving EN pin with open collector driver