# Current monitor max "vout" = ?

Discussion in 'General Electronics Discussion' started by eem2am, Sep 11, 2013.

1. ### eem2am

414
0
Aug 3, 2009
Hello,
This concerns single supply operation of LT1787 as in figure 8, page 12 of the datasheet.
The LT1787 current monitor, on page 3 of its datasheet quotes "vout" as "v(bias) + 1.024V" if "Vs+ > 3.3V".

Why does "Vs+" have to be greater than 3.3V?

Surley LT1787 can produce vout = v(bias) + 1.204V if "VS+" = 2.5v?
(assuming v(bias) = 0V.

Further down the page on page 3, "VOmax" is stated as VS+ -0.75v.

So why are they saying that "vs+" has to be greater than 3.3V for "vout" to be 1.024V?
Surely if "Vs+" is just 2.5v, then "vout" could be 1.024V ?

LT1787 DATASHEET:
http://cds.linear.com/docs/en/datasheet/1787fc.pdf

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,271
2,718
Jan 21, 2010
I can't say why, but they are telling you that.

Perhaps the generation of the internal reference voltage requires a little over 2V of headroom?

Perhaps you could do your own tests to see if it works at a lower voltage.

3. ### eem2am

414
0
Aug 3, 2009
I am wondering if its actually possible for "vout" to be 1.024V if "vs+" is just 2.5V.

I see your point, but even if I order one and it does give vout=1.024v at vs+=2.5v, -if the datasheet doesn't corroborate this, then i'll be worrying if the next batch of LT1787 will behave so well?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,271
2,718
Jan 21, 2010
Exactly.

And that's why you stay within specified limits. You have a far higher degree of certainty that what works will work next time.

5. ### eem2am

414
0
Aug 3, 2009
I am just wondering what is the guaranteed maximum achievable value of "Vout" if "vs+" is just 2.5v?