Could you give some details here? How does slope compensation cause problems at startup? If anything, current mode control should be much more immune to input surge current than voltage mode control.At high duty cycles , current mode controllers need lots of slope compensation, which means the FET source current sense resistor gets downsized, (to make way for the added slope compaensatory ramp)……………..
……….-this in turn leads to overcurrent problems at Start-up, Power ON/OFF/ON and durint LED flashing ON/OFF sequences.
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What is required in this case is that the current through the LED string should be regulated at 350mA. One string may drop 50V at that current, while another drops 47V. When that one has warmed up, it may only drop 44V. If the voltage across the LED string is set to a constant 48V, results will be quite unpredictable.(eg vin = 10V, Vout = ~48V , Iout = 350mA)
Why on this earth do all semiconductor companys provide “CURRENT MODE” solutions for the above??
It is BLATANTLY obvious that VOLTAGE MODE is what is required.
Are you suggesting that the best way to supply a constant current is to use a voltage mode converter? :???:Sunnyskyguy:
Sorry i dont understand you.
-There is no reason a voltage mode converter cant regulate a LED current accurately to any choosen value.
I don't see why this would be the case, as long as you have control of the slope of the slope compensation ramp. And I realize that there are controllers with fixed internal slope compensation, but even those can have additional external slope compensation added.mtwieg:
I think the first step is to appreciate that a current mode converter with much slope compensation has a degraded (smaller ohmic value) fet source current sense resistor.
Why would this be that case? You should be able to define the peak current limit independently of the Rsense value. If not, then that's a problem with the control IC, not with current mode control.-once you realise that then you realise that the FET source current limit is not as low as it could be, and overcurrents can occur.
Yet voltage mode control has no inherent inrush limiting either, so I still don't see the point.With voltage mode, you can make your fet source current limit much lower, because you dont have the slope ramp to also think about
Why would this be that case? You should be able to define the peak current limit independently of the Rsense value. If not, then that's a problem with the control IC, not with current mode control.
Or just increase the slope compensation ramp.... ultimately all you need is a certain ratio between the current sense ramp and the slope compensation ramp. Decrease one, increase the other, it doesn't really matter....the fact is, pick any LED driver you like off the market, and add slope compensation to it, and you end up having to downsize your current sense resistor.
if final target is to control the output current, why give up current mode control? it is well known that voltage mode control have difficulty in controlling boost and buck-boost converter as the RHP zero.
You still haven't actually demonstrated why this is the case though. I can find many current mode LED drivers with internal switches, sense resistors, and slope compensation which are specified for duty cycles up to and over 90%. They fit your example application fine (though not for quite that much power throughput). And there are some with external FETs which would allow for the high output power, such as the LT3755. What's wrong with such a part?My initial observation here is a genuine one.....the semiconductor companys have got it wrong......The majority of DCDC LED driver IC's should be voltage mode.
But i guess there just supplying what the customers want, and we all know that the latest "fashion" is for current mode IC's....even though for LED driver application, they are often inferior to voltage mode.
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