LM2991 - using as a switch

Hirotaka Matsumoto

Other Parts Discussed in Thread: LM2991

Hi all

Would you mind if we ask LM2991?

On the datasheet Typical Application P1, in case of not implemented R2（R1=240Ω、Cin=Co=470uF）,
this regulator doesn't work as dropper function.
So, only this regulator uses as switching function on/off like as High side switch.
And then, will it be Input = Output? or not under warranty?
Or, if you have some arecommended negative voltage switch, could you let us know it?

Kind regards,

Hirotaka Matsumoto

over 9 years ago

0 Hirotaka Matsumoto over 9 years ago

Guru 27585 points

Hi team,

We guess that your team is so busy.
However, could you let us know it?

Kind regards,

Hirotaka Matsumoto

0 Donald Jones over 9 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 22610 points

My apolgies. Friday 03-July was a US holiday.

Yes, you can do this.

With no R1 in place the control loop will go to the rail and saturate. This means that the output stage will be operating in drop-out mode and the ground pin current will be quite high. There is no specificaction for ground pin current in this condition, my best guess would be around 10mA to 15mA.
Vout = Vin - Vdo

0 Hirotaka Matsumoto over 9 years ago in reply to Donald Jones

Guru 27585 points

Donald san,

Thank you for your very prompt reply!

We have one question from our customer.
Is this method of use(as the high side switch) under manufacturer's warranty or not?
And, in this case, the device isn't broken, is it?
Especially, as you mentioned "there is no specificaction for ground pin current in this condition",
my customer is worried whther this way is the warrantied way of use or not.

We appreciate your help.

Kind regards,

Hirtotaka Matsumoto

0 Donald Jones over 9 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 22610 points

As long as none of the electrical limitations are violated (i.e. Vin), all is good.

While there is no specification for ground pin current while operating in drop-out mode, there is a typical performance curve for it.

0 Donald Jones over 9 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 22610 points

If it makes your customer feel less worried, they can use a value for R1 that sets Vout to equal Vin. The same effect, the internal control loop goes to the rail and the pass transistor saturates.

0 Hirotaka Matsumoto over 9 years ago in reply to Donald Jones

Guru 27585 points

Ronald san,

Thank you for your reply!

If you have "typical performance curve for it(ground current)", could you give us it?

Kind regards,

Hirotaka Matsumoto

0 Donald Jones over 9 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 22610 points

From the LM2991 datasheet.

This is typical PNP LDO behavior for the ground pin current to increase, sometimes dramatically, while operating in dropout mode.

0 Hirotaka Matsumoto over 9 years ago in reply to Donald Jones

Guru 27585 points

Donald san

Thank you for your reply.

And, our customer use R and C constant as followings;
-Cin=Co=470uF（25V, ESR=0.062Ω@20℃）
-Vin=-17.1V～-19.08V
-R1=240Ω
-R2=no connect

As our recognition, it is no problem.
If you have some advice, could you let us know?

Kind regards,

Hirotaka Matsumoto

0 Donald Jones over 9 years ago in reply to Hirotaka Matsumoto

TI__Mastermind 22610 points

I see no problem.

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LM2991 - using as a switch