Why are these 'same-part-number' optocouplers different?

[treez]

Please explain why these same-part-number optocouplers have totally different If vs CTR graphs?

Avago HCPL4503 optocoupler datasheet (figure 2, page 13)
https://www.google.co.uk/url?sa=t&r...oX_Y6-WrnHmuUvmRQ&sig2=5c3CEfcgdVhvi0auxptxpQ


(Apologies for long web link)

…compared with Fairchild HCPL4503 optocoupler datasheet (figure 3 , page 8)…
https://www.fairchildsemi.com/datasheets/6N/6N136M.pdf

 

[KlausST]

Hi,

You are right, they seem to differ.

Why:
* Two manufacturers, they don't need to be identically in each specification. Other factory, other process...
* This optocoupler is meant to transmit digital data. CTR is more important for linear operation. For digital transmission you won't go to the CTR limit, but you overdrive the output. The datasheets says with 16mA input current it is able to drive one TTL load and a 5kV pullup. This is about 1mA. 16mA * 19% = 3mA. This is plenty of headroom.

Klaus

 

[treez]

This optocoupler is meant to transmit digital data.

thanks but as you know it doesn't have to be for digital operation at all....its just the same as any other optocoupler regarding analog transfer.
Also, the Fairchild graph (referred to in post #1) is unrealistic...CTR never increases as current decreases from nominal in any optocoupler..have Fairchild made a mistake here?

 

[Easy peasy]

the CTR is the same in each data sheet for the "4503" be careful to read same for same....

 

[KlausST]

Hi,

CTR never increases as current decreases from nominal in any optocoupler..

how do you know?

it depends what´s happening inside.
1) there may be a LED protection circuit inside, that generates nonlinear LED current
2) there may be any saturation effects in the photodiode/phototransistor.

I don´t know.

Klaus

 

[Easy peasy]

Once you go over the hump of CTR maximum, as you go back towards this maxima the CTR increases with falling current.

 

[treez]

Thanks, yes, and its the first opto that ive ever seen that does that...it appears to be the wrong way for the feedback loop's requirements....as the opto-diode will see a reducing current, but the opto-transistor current would be the same, at least at first, because CTR increases as opto-diode current decreases.

 

[KlausST]

Hi,

above the max CTR value
with increasing input current: CTR is decreasing, but output current is still increasing.

--> I_In increases from 10mA to 20mA (increase of 100%), while CTR is decreasing from 110% to 90%, --> output still increases about 65%
(Fairchild datasheet)

Klaus

 

[Easy peasy]

Fig 3 on the fairchild data sheet, above 3mA (led) if you reduce the current the CTR goes up, of course the xtor current drops as you reduce led current but the CTR increases as you go back towards 3mA, this is not a reverse operation of the opto... Ic still drops as I led drops...