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Original question:

LM3404: Texas Instrument data sheet explanation

LM3404: LM3404

Federico Polidori
Prodigy 50 points
Part Number: LM3404

Hi everybody from Italy

As you can see in the linked messages -I'm tryng to drive a Cree COB led with the TI LM3404.

At the beginning I received help from Mr. Irvin and - thanks to this - I was able to realize an excel spreadsheet and afterwards a pr ototype which you can see down here

The schematics were realized according to the one in the LM3404 datasheet page 23.

The components were chosen basing on the following calculations as in the pages 24, 25 and 26 of the same datasheet:

Input voltage: 24 Vin [Volt] 1,34E-10 cost
LED forward voltage: 17,5 V0 [Volt] 0,4 inductor ripple curront increase
LED current:  0,65 IL [Amp] 0,1 ΔiF
Switching frequency: 3,00E+05 Fsw [Hz] 2,25 rD
RON =V0/(1.34 E-10*Fsw) 440298,5075 RON  [Amp] 442000 Ron comm
fSW =V0/1.34E-10*RON 298845,1408 Fsw comm  [Hz]
tON = 1.34E-10*(RON/VIN) 2,46783E-06 ton  [sec]    
ΔiL = 0.4 *If 0,26 ΔiL  [Amp]
LMIN = ((VIN - VO)*tON)/ΔiL  6,35942E-05
LMIN [H]
6,20E-05 H L  comm
ΔiL= ((VIN - VO)*tON)/Ltip  0,266685215 [Amp] 6,10E-05 H L  comm
ΔiL= ((VIN - VO)*tON)/LMax  2,71E-01 [Amp] 6,30E-05 H L  comm
ΔiL= ((VIN - VO)*tON)/LMin  2,62E-01 [Amp]
IL(peak)=IL+0,5*ΔiLMax 7,86E-01 [Amp]
ΔiL(led short)= ((VIN - 0,2)*tON)/LMin  9,47E-01 [Amp]
IL(peak )=IL+0,5*ΔiLMax 7,86E-01
Rsns=(0,2*L)/((If*L)+(V0*tsns)-(((Vin-V0)/2))*ton) 0,345146642 ohm 0,3 ohm  
Zc=(ΔiF*rD)/(ΔiL-ΔiF) 1,349849775 ohm
C0=1/(2*PI*Zc*fSW) 3,95E-07 F 0,33 uF
D=tON/(tON+toff-min) 0,891611971
if=0,2/(Rsns-((Vf+0,2)*tsns)/(C0+(ΔiL typ/2))) 0,666685343
Id=if*D 0,594424633
Pd=0,3*Id 0,17832739

The other integrated circuits that you can see realize a PWM dimming controller and other functions.

The Item is perfect and now a new lamp it is hanging from my lab ceiling.

The only problem that sticks to my mind is that - despite the expected current was 650 mA - the real measured current is 480 mA as you can see in the multimeter down here.

So I realized another prototype by changing the starting data as you can see in the next spreadsheet.

Input voltage: 24 Vin [Volt] 1,34E-10
LED forward voltage: 17,5 V0 [Volt] 0,4
LED current:  0,8 IL [Amp] 0,1
Switching frequency: 3,00E+05 Fsw [Hz] 2,25
RON =V0/(1.34 E-10*Fsw) 440298,5075 RON  [Amp] 442000 Ron comm
fSW =V0/1.34E-10*RON 298845,1408 Fsw comm  [Hz]
tON = 1.34E-10*(RON/VIN) 2,46783E-06 ton  [sec]    
ΔiL = 0.4 *If 0,32 ΔiL  [Amp]
LMIN = ((VIN - VO)*tON)/ΔiL  5,16703E-05
LMIN [H]
5,12E-05 H L  comm
ΔiL= ((VIN - VO)*tON)/Ltip  0,322939128 [Amp] 5,12E-05 H L  comm
ΔiL= ((VIN - VO)*tON)/LMax  3,23E-01 [Amp] 5,12E-05 H L  comm
ΔiL= ((VIN - VO)*tON)/LMin  3,23E-01 [Amp]
IL(peak)=IL+0,5*ΔiLMax 9,61E-01 [Amp]
ΔiL(led short)= ((VIN - 0,2)*tON)/LMin  1,15E+00 [Amp]
IL(peak )=IL+0,5*ΔiLMax 9,61E-01
Rsns=(0,2*L)/((If*L)+(V0*tsns)-(((Vin-V0)/2))*ton) 0,279882681 ohm 0,274 ohm
Zc=(ΔiF*rD)/(ΔiL-ΔiF) 1,009244104 ohm
C0=1/(2*PI*Zc*fSW) 5,28E-07 F 0,47 uF
D=tON/(tON+toff-min) 0,891611971
if=0,2/(Rsns-((Vf+0,2)*tsns)/(C0+(ΔiL typ/2))) 0,729943435
Id=if*D 0,650826305
Pd=0,3*Id 0,195247891
Cin 3,85599E-06

I increased the led current from 650 mA to 800 mA hoping to obtain 700/750 mA  in the prototype.

The result was miserable:

As you switch on the circuit on - the current starts growing from 400 mA to 500 mA in five minutes; then The  LM4036 becomes hot and the Led starts blinking with a connected current fluctuation.

The LM quickly dies.

I'm wondering if I should settle for this results and give up the idea to get at least 700 mA or someone of you can "LED me to this LED (COB LED)"

I recall that the mentioned COB LED is a CREE  XLamp CXA1507-0000-000F0YE435F .

Best regards from Italy.

Federico Polidori

  • 0
    TI__Mastermind 28655 points
    Hello Federico,

    It is difficult to get a switching current driver to work correct on a bread board. There is no ground plane so noise can cause issues. Bypass capacitors and the current sense need to be low inductance routes. It might make more sense to use a modified EVM for this.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 50 points
    Hello Irwin
    Thank you for your attenction and your tips.
    All right - i see your point..
    At the end of the data sheet there's a PCB sample. I will trasform it in a Gerber and with the service I will obtain some PCB in two or tree weeks.
    But I am wondering: " are my calculations correct?"
    Is the fsw I chose appropriate?
    Is the target tolerance for the LED ripple current right at 0 ,1A - as in the first sample - or is it better at 0,05 A - as in the second one?
    I'm not aware about the tipical rD value for the product I chose.
    Is the ratio between the inductor ripple current and the LED ripple current 40% - as in the first sample - or 30% - as in the second sample?
    Thank you again!
    Greetings from Italy,
    Federico
  • 0 in reply to Federico Polidori
    TI__Mastermind 28655 points
    Hello Federico,

    Your calculations are close. I would move the Vled up to 19.9*V (from the graph in the datasheet of the LED). RD is calculated by looking at the voltage at 700 mA and 600 mA (for 650 mA operation) from the curve and the delta V. I get about 4 ohms ((20.1V-19.7V)/0.1A). I get Iled of about 678 mA. Fsw is about 336 KHz. This is somewhat different because of the Vled I'm using.

    As for ripple current, it's more of a what is desired. I get about 55 mA of peak to peak ripple with the first set of calculations you have. It's almost easier to measure it after it's built then calculating.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 50 points
    Dear Irwin,
    first of all I would like to thank you for such a wide help. I'm preparing the final release of my prototype which includes your suggestions for the driver, a 5 V power supply and an ATmega microprocessor aimed to the PWM generation and to accomplish the functions of memorizing the scenarios.
    By the way - in the meantime - I arranged to put the prototype in a copper box ground-connected. the behavior - as you said - completely changed.
    The current - inside the box - rises from 480 mA to 610mA which is very close to the expected value. I'm sure the next one will be perfect. If you want, I'll send you some informations and photos about the complete item just finished.
    Thank you again and best regards from Italy!
    Federico