LM3880: EN pin concern , I have voltage on EN when I power up the Vcc and nothing work

Hello Weidong

Thanks , that LM3880 AB , I have ordered LM3880 AC and AF do test other batches I will post update

on the TI doc there is no available values of EN voltage ( open pin ) but it sound logical that EN is an input so the Voltage shall be 0 V

Regards

David

Hi David,

Did you order LM3880-EVM to try? or you can compare your own schematics with EVM(http://www.ti.com/lit/pdf/snva172) to see any obvious difference.

The flag is a open drain, do you have a pull-up ?

Regards

Yihe

Hello , Yihe

Thanks for your post

regarding the demo board no I have copy and paste the concept

right now I have no load on flag pin I will try this afternoon to set a load on flag

The factory doc is very light regarding DO / DONT so that quite painful to play the apprentice sorcerer

I test with load and post results

Regards

David

Hello

I have other tests

I have change the LM3880 ( same batch from Farnell ) for another I have the same result

a) without load connected EN pin is at 4.50 V and nothing on the flag pin when I put EN 5V on EN pin

b) with LED ( 25 ma ) connected on each flag ( + 100K ballast ) EN pin is at 4,5 V and nothing on the flag pin when i put 5 V on EN

I try now another batch from Farnell , as Mouser parcel is still not delivered

that understandable to lost so much time with a 6 pin chip so simple

Regards

David

Hello

in the aim to progress I have some questions

1) Does LM3880 is very sensitive chip ( ESD or other problem )
2) Does LM3880 can be destroyed by power it up (Vcc to 5V ) without flag connected
3) Does some component as LED are compatible just to load the flag or shall I use other ( to generate a 20 mA load as max is 50 mA )

Regards

David

Hello

I have make test with another PCB , ballast very close to the flag pins , reflow soldered

LM3880 AC from Mouser new batch , previous one's was AB from Farnell

Vcc 5V

I have still 4.7 v on EN ( open pin )

That incredible

Regards

David

Hello, Yine

Thanks

I have copy and paste the TI schematic
I have soldered available LM3880 AB just to test as I am lost
I have still 4.7 on EN ( open pin ) I have 4.7 V on all the LM3880 I can use
and 2.70 V on Flag by using 10K and not 100 k resistor

I have no control of EN

that clearly not understandable

Regards

David

David,

There is 4.7V on the open EN pin because there is a 7uA current source from VCC connected to the EN pin internal to the device.

So if the EN pin is open it will follow the VCC pin. 

And this is basically equivalent to having the EN pin tied to VCC at power-up which the datasheet does not recommend, see below.

The EN pin should be exercised to active (High) after the device is powered up.

For the last recommended case above, External Enable Signal, I would ensure that the EN pin is low before applying VCC otherwise you will be equivalent to the not recommended case again.

Also, if you are seeing 2.7V on the FLAG pins with the EN pin High, then the FLAG pins are active but the voltage from the pull-up is being limited by something external to the sequencer,  I interpreted your post to indicate that the LEDs are tied from the FLAG pin to ground, this would be the likely source of voltage limitation.

If the Enable was low then the FLAG pins should be <0.4V.

The Pull-up resistance is probably too high to light the LEDs if you are using 10K or 100K resistors.

Hello

OK thanks your detailed answer

so I cannot use the same 5 v PSU than the Vcc ?

I link the pcb screen shot I use a manual toggle switch between EN and Vcc

Shall I use another 5 V PSU ? as I have understood that I shall NOT drive EN and Vcc in the same time but that not written I shall use another PSU source ??????

Regards

David