Switch Mode LED driver is literally just using a comparator -is this ok?

[grizedale]

Hello,

Please can you state if the following LED driver is a robust design?....



Buck LED driver
Vin = 13V
Iout = 2.55A total (obviously the leds are paralleled, and so each led passes 2.55/6 = 425mA.
The LEDs are rated to 1 Amp


Please note that the control scheme involves no LED driver IC, and not even an oscillator, the comparator literally just switches the FET on and off.

This is a flashing LED light, so it flashes ON/OFF (ON = 190ms, OFF = 300ms)

I have pointed out that there will be large surging inrush due to the simple comparator control, which, at start-up, literally just keeps the switching PFET on until the current sense resistor voltage gets above 0.6V.

I believe that that in high temperature operation (ambient = 40degC), the output 100uF electrolytic capacitor's ESR will fall very low, and the delta(Voltage) on the comparator inverting pin will be very small and the comparator will jitter noisily.

Do you agree with me that this design is severly suspect and in need of re-doing.?

Regarding the inductor saturation which will occur at start-up, or at return from Flash-off, the designer says it doesnt matter....because most of the resulting inrush just goes into the electrolytic capacitor and not into the leds.

The inductor is a DR125-330-R:-
**broken link removed**


Incidentally, here is the LTspice simulation of it.

Version 4
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The quote is not the proper tag to use for code because it doesn't limit the length , use code tags instead [alexan_e]

 

[mtwieg]

Bang bang controllers like that aren't too surprising in very simple/cost sensitive designs (like blinky LED drivers). So long as all the components are rated for the appropriate ripple currents, it should be fine. But there should be some hysteresis on the comparator, in order to define the switching frequency and prevent chatter (though maybe there is internal hysteresis).

 

[grizedale]

OK thanks mtwieg, by the way the LED s aren't matched , but are still parallelled with very little dropping resistance.

The LEDs are XPEWHT:-
**broken link removed**

This is ahigh volume product, and if LED drivers can be done ok with just a simple comparator, and with paralleled LEDs, then why is anybody buying LED driver IC'S?

 

[mtwieg]

OK thanks mtwieg, by the way the LED s aren't matched , but are still parallelled with very little dropping resistance.

The LEDs are XPEWHT:-
**broken link removed**

Yeah there should probably be some more series resistors in the LED array to improve balancing.

This is ahigh volume product, and if LED drivers can be done ok with just a simple comparator, and with paralleled LEDs, then why is anybody buying LED driver IC'S?

Because not everyone cares about having features like small size, line rejection, brightness control, high reliability, or ultra high efficiency. Sometimes you just want blinky lights with the minimum effort and cost. Also for some reason a lot of people seem to really like bang bang control, even when it's not entirely appropriate.