LM2733: LM2733Y need support

Hi Tanvir,

It looks like the device didn't switch at all.

Can you share the layout of your board? How many failure boards did you see? Could you replicate it easily? I ask this question because I want to check if all the IC and external components are soldered correctly. 

Hi Zack,

thanks for your support.

find attached layout PCB.

first i start with step by step voltage increasing but IC dint get boost with required current and voltage.

second i tried with direct 12V suddenly current goes above 500mA and after that IC not work.

i have tried same 2 times.

thanks,

tanvir

Hi Tanvir,

I didn't find LM2733. Why the left-bottom area of this picture is blank.

Hi zack,

U1 IC is LM2733YMF.

left bottom area is another schematic which i cant share with you.

rest of all if you find some query let me know.

thanks,

tanvir

Hi Tanvir,

In the layout, there is a inductor L3 in series with rectifying diode D1. This is not accepted for a boost converter. It seems you didn't follow WEBENCH schematic. Could you share the schematic? 

Hi zack,

 it as per WE BENCH tool refer trailing attached we bench design

find attached schematic.

diode i used ES1J fast switching diode. L3 is ferrite bead as per your suggestion for EMI. but still i have kept black with link for just functional testing of IC.

one thing we bench design as per LM2733XMF switching freq1.6mHz but i am using IC LM2733YMF which switching freq 600kHz so is there any change in parameters?

0844.SCHEMATIC1 _ PAGE2.pdf

Hi Tanvir,

Please follow the WEBENCH recommended schematic. Remove L3, C4, C6,C11. They will affect the normal working of LM2733. Add a 82pF Cf capacitor in parallel with upper feedback resistor. And the layout is not good. It's better to follow datasheet recommended layout.

LM2733Y switching frequency is 600kHz, the inductor ripple will be larger, and output voltage ripple will be larger.

Hi zack

L3, C4,C11 not connected yet just checking functional behavior of IC  C6 is 82pF already parallel to Rupper.

please note: we want to test this board within next 2 to 3 days for EMC testing hence request to support to fast.

thanks,

tanvir

Hi Tanvir,

The ferrite bead is advised to be placed after the output capacitor. 

Could you check SHDN, FB, SW pin voltage? It's strange the converter didn't switch.

Hi zack,

i have shorted SHDN pin for bia current.

just checked waveform at switch pin it seem ok 600kHz and voltage 35V

without load output voltage is 0v and IC get heated and damage.

Thanks,

Tanvir

Hi Tanvir,

Could you share Vin, SW, iL, VOUT waveform?

Hi zack,

thanks for reply.

i have checked SW, Vout Vin waveform. it seems like same

vin 12V, Vout also 12V and switching freq less than 150Khz it means IC doesn't switching at all.

checked with fresh IC .

Thanks,

Tanvir

Hi Tanvir,

Have you check FB pin voltage?

In your previous layout picture, it looks like FB pin is shorted to GND.

Hi zack,

FB pin voltage is 600mV.

i am not getting SHDN pin voltage equal to vin in case of both condition by shorting and 51K resistor.

FB pin not shorted to ground i cross checked the layout.

Thanks,

Tanvir

Hi Tanvir,

It doesn't make sense that FB pin voltage is 600mV. According to your provided schematic, the upper feedback resistor R4 is 383kohm and lower feedback resistor R2 is 13.3kohm. So if Vout is measured to be 12V, the FB pin voltage should be 12V*13.3k/(13.3k+383k)=0.403V. Please double confirm each pin voltage is correctly measured.

Are you sure R1 is soldered correctly? Please also check SHDN pin voltage otherwise IC is always disabled.

Hi Zack,

after many testing and troubleshooting IC working fine.

i closed vout to only 36V by FB network 13K and 370K . without load voltage is 35V

but when ic connect load current get boosted above 600mA and IC get damage.

any comments..?

Thanks,

Tanvir

Hi Zack,

awaiting for your reply..

Thanks,

Tanvir

Hi Tanvir,

It's because of the bad layout if you find the IC get damaged at high load current. When load current is high, the switching spikes at SW pin would be very high, which could destroy the device.

Please follow the recommended layout.

Hi zack

can you let me know what is wrong in the PCB layout.

second by functional how can protect spike at sw pin?

Thanks,

Tanvir

Hi Tanvir,

Below is the recommended layout for LM2733. As you can see, the power trace are really short and wide to reduce the PCB parasitic resistance and inductance. You can find the detailed explanation on page 18 of datasheet.

Hi zack,

i have tested board once again as per recommend layout.

same issue observed IC get damaged after supply switch ON to 12V.

any suggetion..?

Thanks,

Tanvir

Hi Tanvir,

Sorry to hear that.

Can you share the layout please? 

Hi Zack

please find attached  layout PCB.

Thanks,

Tanvir

Hi Zack,

awaiting for reply.

second thing can i add external MOSFET across sw pin of IC so that i can icrease MOSFET drain voltage. and there will be no spike issue .

Thaks,

Tanvir

Hi Tanvir,

Do you have any startup waveform of capturing VIn, SW, Vout at same time? Because you are applying a 12V voltage on Vin pin and this device maximum input voltage is 14V. Is there a overshoot at input voltage during startup? Please check it first.

For the layout, I would say it is better than previous one but still not best. Please send it to me for review next time before sending for malnufacture.

The Cout is not placed across the diode the IC GND pin. But it maybe not the root cause of IC failure. So I need you help to provide some waveform. Such as Vin startup, steady state waveform.

Hi Zack,

i cant show all waeform at a tme.

Sw pin shows spike of 600mV 

Thanks,

Tanvir

Hi Tanvir,

Please share the waveform of Vin, SW, Vout otherwise I can't analyze...

Hi zack,

i am trying to get waveform at starting but IC get suddenly damage.

Thanks,

Tanvir

Hi Tanvir

Jusr let you know taht Zack is out of office. Please allow us reply next week. Thanks for your kind understanding.

Hi Tanvir,

You can use a external power supply to connect to LM2733 circuit input capacitor and set the power supply voltage at around 5V. So that there is enough voltage room for Vin. Then check if the output voltage is correct. Probe Vin, SW, VOUT pin waveform and share it with me.