Constant off time LED Driver??

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gilbertomaldito

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constant off time

can anyone explain the function of R4 , Q3 and C11 for constant turn off time circuit of this LED Driver LM3445 from NS.

**broken link removed** - page 13 figure 11. The functions of these 3 devices were not discussed in the data sheet.

the circuit has a similar circuit mentioned here: http://www.microsemi.com/micnotes/1304.pdf .. but still I can't understand.
 

turn on time led

ON page 17 of lm3445 they speek of it....

q3 must be to isolate r4 from c11....

is not that webbench on NS website a simulator?
 

timeled

eem2am,

Can you check page 22. Why is that they use different Fsw in number 3 and number 12 calculations?
I believe the only switching frequency is the frequency applied to the Gate of the Power MOS (to turn on and off). Is there any Fsw in an LED driver? Am I missing something?
 

constant off-time

it is hysteretic control....no oscillator....no fixed frequency, just switches when the comparators trip.

its a simple way....but is sometimes suspected of making emc worse.
 

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