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LM2676 and LM2696 3V3, 5V and 12V problem

srt35
Intellectual 640 points
Other Parts Discussed in Thread: LM2676, LM2696

Hi,

Question 1:

Im working on a project, i need 3v3, 5v and 12v smps circuits. So i choose lm2676 first. I read all datasheet and designed the schemetic for voltage levels what i need. So i bought all specified components with same manufacturer id. I assembly the pcb, I read exactly values with no load, but when i put a load to output for get 1ampere, i read 1v2 instead of 3v3, 10v instead of 5v, 6v-16v instead of 12v. My power supply is stable 17V. Icant figure it out.

For 3v3 SMPS:

IC: 1x LM2676-ADJ, Inductor: 1x P0841, Output Cap: 2x 100uF (10V 1.1A(Irms)), Input Cap: 2x 4.7uF (50V 0.6A(Irms)), Boost Cap: 1x 10nF, Schottky: 1x 40V (3A), FeedBack Res: 1K & 1K74

For 5v SMPS:

IC: 1x LM2676-5v, Inductor: 1x P0841, Output Cap: 3x 100uF (10V 1.1A(Irms)), Input Cap: 3x 4.7uF (50V 0.6A(Irms)), Boost Cap: 1x 10nF, Schottky: 1x 40V (3A), FeedBack Res: No Feedback

For 12V SMPS: ()

IC: 1x LM2676-ADJ, Inductor: 1x PE54039S, Output 
Cap: 2x 47uF (20V 0.9A(Irms)), Input Cap: 2x 4.7uF (50V 0.6A(Irms)), Boost Cap: 1x 10nF, Schottky: 1x 40V (3A), FeedBack Res: 1K & 9K1

Question 2:

I choose LM2696; another smps IC instead of  LM2676. This time i discover Webench and design circuit with it. My parameters are Vin= 13V-24V, 3v3-1.5Amax, 5v-3Amax, 12v-1.5Amax.  i bought all specified components with same manufacturer id again. This time for 3v3;

- 3V3 With no load:             When Vin<15v, Vout=3.3v,

                                               When Vin>15v to 24V, Vout=3.3v going down until Vin=24v and Vout=2.2v

- 3V Wtih load(330ohm and LED):   When Vin<12v, Vout=3.2v,

                                                          When Vin=15v, Vout=2.8v,

                                                          When Vin=20v, Vout=2.1v ,

- 3V3 Wtih load(4R7):                   When Vin<11v its seems Ok,

                                                          When Vin=12v, Vout=1v

                                                          When Vin>12v to 24V, Vout=1v going down until Vin=24v and Vout=0.2v

- 5V With no load:                          When Vin<19.5v, Vout=5v,

                                                          When Vin>19.5v to 24V, Vout=5v going down until Vin=24v and Vout=3.3v

- 5V Wtih load(330ohm and LED):   When Vin=6.5v, Vout=5v,

                                                          When Vin=7.5v, Vout=4.4v,

                                                          When Vin=8.5v, Vout=5v again,

                                                          When Vin>9.2v to 24V, Vout=3.7v going down until Vin=24v and Vout=2.4v

- 5V Wtih load(4R7):                     When Vin<11v its seems Ok,

                                                          When Vin=12v, Vout=1v

                                                          When Vin>12v to 24V, Vout=1v going down until Vin=24v and Vout=0v

- 12V With no load:                       When Vin>10v to 24V, Vout=5.3v

- 12V Wtih load(4R7):                   When Vin>5.3v to 6.6V, Vout=5v,

                                                          When Vin=7.5v, Vout=1v

                                                          When Vin>7.5v to 24V, Vout=1v going down until Vin=24v and Vout=0v

Photos:

Here is LM2676 photos: http://imgur.com/a/3DTFQ

Here is LM2696 photos: http://imgur.com/a/0JPnh

I tried 2 different smps and no success, designs seems Ok, Components seems Ok, PCB seems Ok. Please Help.

  • 0
    TI__Genius 16025 points

    I'm looking into this and will get back to you shortly.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Intellectual 640 points

    Hi Anston,

    Thank you for your reply.

    I try again with LM2676 IC, and get some meaningful results with load, the output voltage not going down, i replace the all output capasitors with 100uF. This design has problem too. But no longer copmlex:

    *3v3 & 5v(for 5V, i use LM2676-5V IC) smps , for example i want to load for 600mA, it gives 200mA. I want to load for 1A, it gives 320mA, seems like divide with 3, when this is happening, output voltage is stable.

    *for 12v smps this problem going like this: i want to load for 2.4A, it gives 2A, i want to load 850mA it gives 650mA. I change inductor with few different options, i didnt think the reason is inductor. The remaining components is capacitors and IC and Diode.

    When i watch the IC output pins, i can see the 260kHz signal and Pwm with duty cycle. Its working like a smps, only not give ampere what should be give.

    I'll keep update here. Waiting for your comments. Thanks.

  • 0 in reply to srt35
    TI__Genius 15810 points

    Hello,

    Quite often when the BOM looks alright and the design doesn't work for no apparent reason it is the layout of the board. The buck regulator has a noisy input side loop. This requires the input capacitors to share the same ground as the anode of the catch diode. In your case, the two grounds are far away and flowing through a single via. In most cases this is the primary reason for bad regulation with load. I have attached a presentation that talks about EMI and it's mitigation techniques using good layout practices. Please go through that. As such on your board, the minimum you need to change is as follows:

    1) Move the input cap ground and diode ground right next to each other. Something like rotating the caps 90degs and the diode 180 degs would help

    2) Use a shielded inductor

    3) Place both the feedback resistors next to each other and very close to the FB pin on the IC. If required, you can route the VOUT sense line to the top of the upper feedback resistor through a via.

    I hope this helps.

    Regards,
    Akshay

    Switching Power Supply Design_ EMI.ppt
  • 0 in reply to Akshay Mehta
    Intellectual 640 points

    For Lm2676 IC:

    I changed my power supply and multimeter,assembly new pcbs today with used all components again. The pcbs is ok, no voltage going down or no current limits. So i changed my power supply and multimeter back and test again, nothing change there is no problem. I cant say anyhing exactly for problem source. I think the IC pad maybe didnt soldered or pcb quality is not enough for smps.

    All smps work perfect. So we have 3v3-3A, 5v-3A and 12v-3A now. I cant test in high ampere mode more than 10 seconds. My resistors is not enough for that watt. When i test for a longer time, i write here the results.

    I cant work on LM2696 IC yet.

    Thank you for your replies.

    for 5V smps:

    IC: 1x LM2676-5V, Inductor: 1x P0841, Output Cap: 3x 100uF (10V 1.1A(Irms)), Input Cap: 3x 22uF (50V 0.6A(Irms)), Boost Cap: 1x 10nF, Schottky: 1x 40V (3A), FeedBack Res: none but connect fb pin to VOUT