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Power factor corrected, critical mode Flyback LED drivers.

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Flyback

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Hello,
Please could you answer the following questions about the LT3799 PFC’d flyback LED driver?

LT3799 datasheet:
http://cds.linear.com/docs/en/datasheet/3799fa.pdf

Why is the current sense reference voltage just 100mV?
Other chips, such as LM3450, and all others, have at least half a volt for current sense reference voltage due to noise issues, why is the sense voltage so low for the LT3799?
Can the LT3799 be used with an opto-isolator as in the attached pdf schematic? (In this schematic, I have connected the internal error amplifer thats inside the LT3799, as a voltage follower. LTspice simulation of LT3799 based LED driver is attached, with the optocoupler feedback.

Why does the LT3799 have to use an internal multiplier block and why should it have to sense the input voltage? After all, the NCL30000 PFC flyback controller does not have either of these..
NCL30000 datasheet:-
http://www.onsemi.com/pub_link/Collateral/NCL30000-D.PDF

(the NCL30000 simply uses constant on time, which is all that is needed to give Power factor correction, when a Critical conduction mode flyback is used with virtually no primary bus capacitance.)

LTspice simulation of LT3799 based LED driver is attached also, with optocoupler feedback
 

Attachments

  • PFC Flyback _BCM _LT3799 _opto.pdf
    21.2 KB · Views: 227
  • PFC Flyback _BCM _LT3799 _opto-isolated.asc
    9.1 KB · Views: 205
Why is the current sense reference voltage just 100mV?
Low power loss. Higher efficiency.
Why does the LT3799 have to use an internal multiplier block and why should it have to sense the input voltage?
That way has been used many times.
There are many ways. Some have patents.

Most PWM ICs use constant frequency. Constant on time (I like) is not understood by many.
 
Thanks, do you think the opto-isolated version of LT3799 based PFC flyback is ok? (pictured and simulated in post#1)
 
The error amp and 2.5V is a TL431.
Why voltage regulate when you are driving LEDs? You want to current regulate.

I think you should be able to see LED current from the primary side. Not 0.01% but good enough for LEDs.
 
Why voltage regulate when you are driving LEDs? You want to current regulate.
Yes I agree but the simulation is just to see what is the peak primary current when its supplying 40V, 625mA.

Also We may put a secondary side led driver on and want to run it off a constant input voltage.
I was actually just wondering if the given opto isolation method looks acceptable, because strangely the datasheet doesn't offer an opto-isolated solution, unlike the LM3450 which is the same kind of chip and suggests using it with opto-isolation.
 
I did not think about the opto-isolator much but it seems OK.
I was thinking about getting power from the 40 volts to run the error amp and opto.
You could tap into the string of LEDs at the 1/4 point (10 volts) and steel some power for the opto.
LED current =625mA and error amp current is 1 to 3 or maybe 6mA. The bottom LEDs might be short 0.5% to 1%.
 
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