LM2678: ON/OFF pin voltage of LM2678

Uchida Yasuhide

Prodigy 180 points

Part Number: LM2678
Other Parts Discussed in Thread: LM3880,

Hi,

I use the LM3880 of power sequencer and LM2678 of DCDC.

Please see attached my schematic.

I have a problem.

The ON/OFF voltage of LM2678 becomes approximately 4V.

My expectation is 3.3V.

4V exceeds recommended max voltage (VCC+0.3V=3.6V) of FLAG pin.

Why is this happening ?

Could you tell me the solution ?

over 2 years ago

0 Frank De Stasi over 2 years ago

TI__Guru 57555 points

Hello

The LM2678 incorporates a pull-up current source on the ON/OFF pin and a Zener clamp.

The internal current source is pulling up the voltage on the flag pin of the LM3880.

You will need some kind of clamp circuit or voltage divider. One method is shown attached here.

0 Uchida Yasuhide over 2 years ago in reply to Frank De Stasi

Prodigy 180 points

Hi,

Thank you for your reply.

I tried diode clamp method, but I think difficulty that I can meet completely 3.6V spec of Flag pin.

So I tried to add pull-down resistor 200kΩ.The voltage of FLAG1 pin becomes 3.2V.

I concern becoming 3.6V over by the tolerance of internal current source.

Is the pull-down resistor 200kΩ OK ?

If you have a recommended pull-down resistor value, I want to know it.

0 Arief Hernadi over 2 years ago in reply to Uchida Yasuhide

TI__Mastermind 23815 points

Hi,

Is it possible for the customer to put a zener instead of 200kohm resistor?

You may be able to use 3.3V zener such as this one

https://assets.nexperia.com/documents/data-sheet/TDZXJ_SER.pdf

Thanks

-Arief

0 Uchida Yasuhide over 2 years ago in reply to Arief Hernadi

Prodigy 180 points

Hi,

Thank you for your reply.

I want to use a pull-down resistor.

Why do you put a zener instead of 200kohm resistor?

Do internal current source have large tolerance?

0 Arief Hernadi over 2 years ago in reply to Uchida Yasuhide

TI__Mastermind 23815 points

Hi Uchida-san,

The zener will clamp the voltage of the pin and hence limit the voltage on the flag pin.
At the same time, i noticed that the abs max voltage on flag pin is 6V, therefore 3.6V should not damage the part.

I could not find the current source value of ON/OFF pin, therefore i could not calculate the voltage on the flag pin with 200kohm.

Thanks

-Arief

0 Uchida Yasuhide over 2 years ago in reply to Arief Hernadi

Prodigy 180 points

Hi Arief-san,

Thank you for your reply.

I found current source value on the datasheet(Page6 6.9 Ion/off and Page 11 7.3.6 ON/OFF)

The current source is typ20uA, max45uA.

0 Arief Hernadi over 2 years ago in reply to Uchida Yasuhide

TI__Mastermind 23815 points

Hi Uchida-san,

Thank you for the current source information. I calculated the voltage on the pin and that would also be beyond 3.6V with the 200kohm resistor (expected voltage is 2/3 x 3.3V + 45uA x 66kohm = 5.2V).

You may want to use 75kohm instead of 200kohm in this case to keep the voltage below 3.6V

Thanks

-Arief

0 Uchida Yasuhide over 2 years ago in reply to Arief Hernadi

Prodigy 180 points

Thank you for your reply.

I have a idea in this problem.

I want to add Nch-MOSFET. Please attached figure.

In this case, when the flag1 is low, the FET is ON and the ON/OFF pin is low.

When the flag1 is open, the FET is OFF and the ON/OFF pin is open(DCDC is ON).

Is this solution OK?

Is there problem about this solution?

0 Arief Hernadi over 2 years ago in reply to Uchida Yasuhide

TI__Mastermind 23815 points

Hi Uchida-san,

Yes that should work. Good idea!

Thanks

-Arief

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LM2678: ON/OFF pin voltage of LM2678