Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Current monitor ic..maximum "vout"?

Status
Not open for further replies.

Flyback

Well-Known Member
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
 
Hi FB,
I think they are saying that Vo max is Vs-0.75, so for a maximum output of 2.5 Volt, then the supply must be 3.3-0.75=2.55.
I think the numbers are due to the system requirements when the LTC 1286 is used. The diagram on Page 1 says;'typical application'. I think if you look at the LTC1286 data sheet, you'll find that the dynamic range of the LTC is related to a maximum input voltage of 2.5 volt.
Thats my take on it.
 
I think it is more of a testing issue. The part is tested for that spec of the electrical characteristics table at a supply voltage (Vs) of greater than 3.3V. I see no reason why you cannot use it below this voltage. The 0.75V spec mentioned later is the saturation voltage of Q2 in FIG 1.

I thought that the input offset voltage of the sense amp would suffer below Vs = 3.3, and this might explain why the part is only tested above 3.3V. However, looking at the spec of Vos vs negative supply voltage, the input offset voltage is still respectable, hence accuracy will not be compromised.

So, I think it is just a testing issue
 
Just had a look at the LTC1286 spec.
The input voltage range for the 1286 is uni-polar from 0 to Vref. When Vin = Vref, then the data value is 7777 (octal) (all 1's)
Fig 4 in the application note (1787) goes over the method for measuring negative currents in the sensing resistor. A bias is applied and this shifts the output range from the ltc1787 and to utilise this feature, there are restrictions on the minimum supply voltage.
theres no mystery in the 3.3 volt Vs; its just a limitation on Vs min when some specific configuration is used.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top