WEBENCH® Tools/LM5088: lm5088_2 poor heat dissipation

hi，
increase load (0A-1A)，the output drop down 0.6V(12.3V down to 11.6V), please tell me how to solove???

hi,
VIN=60V, Iload=1.1A, output down off， Iload=1A VOUT=11.76v；Iload=0A VOUT=12.23v ; please help me . my design:VIN=18~72v, VOUT=12v, Iout=3A;

User,

1. No, that is not normal. It may work okay but this sounds to me like we have an issue to fix.

2. This is also not normal.

Please share the schematic and layout. I'll take a look and get back to you.

-Sam

hi sam,

please check my schematic and layout.

sch_pcb.rar

hi sam,

1.  1 pcs IC  broken down already, please help me .

2.  I give you my design parameter： Css=22nf（soft start）,Rt=14.6K( set frequency 400Khz）, Rramp=196K, Cres_dith=10uF, CVcc=1uF, Ccomp=1.5nF, Rcomp=92K, Chf=22pf, Rcs=20mohm, Cboot=27nf,  M1=CSD19534Q5A, D2=MBRD10200CT, L2=33uH（power  inductance）, Cout=330uf+3*10uf, Cin=22uf+3*4.7uf,  D1=B220A-13-F(output voltage 12V  supply to VCC)

hi Sam，
thank you for your reply.
I changed bottom paste the snubber circuit RC（R=5.1ohm,c=1000pf ）and RS（sense resistance） to top layer, and together with schottky D2---SW waveform is better than before.

1. but above 3A load @18V，SW not normal again.
2. and also above 30V input , the ambient temperature is 25 degrees, the PCB , IC, schottky are so hot, the temperature reached 73℃, 70℃,71℃, and efficency is 88%( 1.8A@30V ), and 60V @1.1A over temperature protection. and I checked the SW normal.

please help me how to improve the temperature. my application ambient temperature is 55 degree. now 25 degrees also unnomal.

hi Sam，
by the way, vout=12v supply voltage can be applied to the VCC pin, so Rramp should be changed to 314K, according to Ios=VOUT*25uA, Rramp=(Vout-Vramp)/(Ios-25), here Vramp is 1V, -----is this right?
best regards

User,

So now R3, C16, and R11 are on the top layer but did you connect the snubber to GND or to R11? It should be connected to GND, not R11.

1. Not normal how? Does it look the same way it looked before or is it not normal in a different way?

2. That efficiency seems okay. The heat is probably due to layout. The IC thermal pad should be connected to a large GND plane. Your layout has it connected to a small SGND plane. This should be as large as possible.

Other layout guidelines:

• Connect SGND and PGND closer to the IC. See the EVM and datasheet for guidelines.
• Do not have a long thin trace go PGND (GND on your Altium file and GND in the datasheet).
• And more. This layout will probably need to be redone. See EVM and datasheet for guidelines.

Ios = VOUT * 5uA, not *25uA but yes, you calculated correctly.

-Sam

hi sam,
R3, C16 connect the snubber to R11 now----just copy the EVM datasheet, I think you are right,this should be connected to GND.
ok ， I will have a try to connect to GND.

User,

I'm checking with the person who made the EVM to see if there was a reason for this. I'll let you know what they say.

Keep me posted on your tests.

-Sam

hi sam,
So now R3, C16, and R11 are on the top layer , connect the snubber to GND ---the sw waveform the same as the before.
1. LM5088 datasheet page 14 : Feature Description : For duty cycles greater than 50%, peak current mode control circuits are subject to sub-harmonic oscillation----is this the reason for SW waveform unnormal by my test

hi sam,
1. according the datasheet of LM5088 , if the sw waveform is normally characterized by my test, so how to adding a fixed slop voltage ramp , the recommend parameter is what？Cramp？ Rramp？ Ccomp? Chf? Rcomp?
2. the vout VPP up to 300mv ---so high, I just want to change the SW working on DCM to CCM ,so maybe can reduce theVPP.

dear Sam,

I have layouted again, please help me check out,   use the EVM and datasheet  to layout.SECOND_TEST0825_3.zip

User,

Thanks for sending the PCB layout. The snubber resistor canbe reduced in size (0805 is sufficient). Generally, the larger component in the RC snubber is connected to GND so that the smaller component (typically the cap) is tied to SW. This reduces the radiated EMI effect. The smaller the SW node copper area, the better.

Also, can you fill out the LM5088 quickstart file - this is available by download in the LM5088 product folder.

Regards,
Tim

HI Tim,
The latest layout test not better than before, but SW waveform look like better than before, sub-oscillation is better. BUT now I just test 1minutes , the inductance so hot（above 100 temperature）, the schottky (MBR10200CT) is broken down（not work） ,please help me, this is the second layout.

HI Tim,
I just copy the EVM(AN-1913), why work worse the before, and the schottky (MBR10200CT) is broken down（not work）, the snubber resistor canbe always copy the EVM, please help as soon as possible

Usre,

Please send the completed quickstart file for review. Have you calculated the expected component power dissipations?

Regards,
Tim

Hi Tim,

1.  I'm confused why the first layout load 2A（ VIN=60V）without any component damaged  and the SW waveform is not regular square waves,  but the second layout( copy EVM layout) schottky damaged, the SW waveform looks like ok.

2.  the  schottky (MBR10200CT)： VF=0.9V,  IOUT=2A， duty=12/60=0.2, IF=Iav=(1-duty)*Iout=1.6A, P=VF*IF=0.9*1.6=1.44w.

3. quickstart file： L=25.6uH ---choose 33uH,   RT=14.61k--choose 14.6K, Cramp=564pf--choose 1.5nf（is good for VPP),  Rramp=194K--choose 200k, Cout=196uf---choose 330uf+10uf+10+10uf+0.1uf（is better for VPP）,  Cin=4.1uf ----choose 4.7uf*5, Rcomp=182.4K--choose 62.2K(better for VPP), Ccomp=218pf----choose 1.5nf, CHF=47pf---choose 22pf. CVCC=0.6uf---choose 1uf, Cres=11.667uf-----choose 10uf( the EVM here is 22nf, but I use this 22nf not work when any load startup, so I change this value to 10uF).

4. the  RC absorbing circuit  5.1ohm ( footprint 2512), you recommend 0805,  VIN=60V, does the 0805 footprint  power  consumption meet ? and  here how  the RC power consumption ?

best regards

M5088_Quick_Start__MEfortim.xls