LM3670: Back Power Protection

Stewart Marlow

Part Number: LM3670

We have used the 3.3V fixed version of the LM3670 in our application and have an issue with the device being damaged due to back-power, i.e. if we have no voltage present on the input due to the batteries being removed and the output connected to our Vcc is powered through alternative means, i.e. if it is powered via a USB port, which is connected to Vcc, the Reg is damaged due to the fact that the FB pin is pushed higher than the 0V on the input. I have tried connecting a Schottky from Vcc to the Reg inputas I have done on linear Regs in the past, which should limit FB to Reg input difference voltage to -0.2V but the Reg is still damaged.

Does anyone know a method of protecting the Reg from back-power when there is no voltage present on the input?

Thanks

over 5 years ago

0 Alfonso Furio over 5 years ago

TI__Expert 3645 points

Hello,

Please have a look into the absolute maximum ratings on page 4 of the D/S, FB can sustain maximum Vin+0.2V ( in your case Vin=0) for this reason the device was damaged. A suitable solution for you can be to use a load switch. I attached here the link to select the part ideal for your application:

www.ti.com/.../overview.html

0 Alfonso Furio over 5 years ago in reply to Alfonso Furio

TI__Expert 3645 points

www.ti.com/.../overview.html

0 Stewart Marlow over 5 years ago in reply to Alfonso Furio

Prodigy 20 points

Hello Alfonso

Thanks for sending the reply, yes I had thought about switching the rails with FETs but I don't have much space in that PCB area, which is why I thought a Schottky from output to input would be best and should address the issue of the FB being only <0.2V above the input, if we have no connection to the battery. Maybe it needs to be a larger Schottky to ensure that the forward voltage is low enough. Do you think it should be adequate? I have attached a schematic to show you where I connect the Schottky which is actually a double BAT54 connected in parallel.

Best Regards

Stewart

0 Alfonso Furio over 5 years ago in reply to Stewart Marlow

TI__Expert 3645 points

Hello,

Unfortunately, I don't think it could be a suitable solution to prevent the failure mentioned above. At the output, you have a USB that is acting as a voltage source, and even if the threshold of the diodes is shallow, you need to be sure to not operate above 0.2V at the FB pin. At the moment I don't have another suggestion for your case.

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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LM3670: Back Power Protection