TPS81256 not preforming

Hi Diego,

do you see the ~2.8V with every of the three mentioned resistors? Measured via scope or DMM?

I guess you are operating just the EVM, having the EN-Pin jumper-ed to VIN and applying a resistive load?

Do you have a chance to provide a scope-plot of startup as well as steady state operation enclosing VIN, VOUT and EN?

Kind regards,

Florian

Hello Florian,

Attached is a link to the start-up scope plot. As for the steady state on the output, it's stable for the output with a ripple voltage of 64mV to 72mV (at 50 - 150 Ohm).

For the setup, I have an Agilent U3606A power supply connected directly to the input of the TPS81256 and set for 3.3V. The output is just the resistor.

Please let me know if there is any other information you need.

Diego Aguilera

http://db.tt/6iAGSCDI

Hello again Florian,

I also have a TPS61240 that I was originally trying to use to convert the 3.3V to 5V. The device I am trying to power has a max load of 300mA. Since it near the limit of this IC, I decided to switch to the TPS81256. I am now thinking there must be something else wrong if neither one of these IC's are working as advertised. The TPS61240 is on a custom PCB I designed from the TI schematic.

I could understand if I made a mistake on the PCB for the TPS61240, hence the bad output. But for the TPS81256 Eval board I've bought, I don't understand why I wouldn't get a 5V output.

Hello Diego,

the waveforms look strange. This is definitely not the behavior we can expect or what I see when i power up this module.

Did you just power it up after unpack - These devices are very ESD sensitive! Do you have just this one EVM, or could you try a different one? If so, I'd suggest that i send you a new one.

Did you get in the same trouble when testing an TPS61240 EVM?

Best regards,

Florian

Yes, I unpacked it and powered it up. I don't remember doing anything out of the ordinary that would cause it to fail. I usually set a current limit, but I don't see that harming anything. We only bought one evaluation kit.

The TPS61240 outputs ~3.29V with no load. With 150Ohm load (33mA) the voltage falls to 1.1V. Where the TPS81256 has an almost constant ~2.8V output. Again, looking at the layout of the TPS61240, there are a few traces that could be made wider. Although I don't believe a bad layout can drop the output voltage so low.

Diego

Florian,

Does the TPS61240's power pad need to be connected to ground? It's not shown on the datasheet. It is connected on the evaluation schematics. I replaced the IC and it was working. I was testing a few things (different loads and enable/disable) and managed to break it again. I have one left and I want to know what's wrong before I destroy the last IC.

Florian,

The TPS61240 is working fine after I grounded the plane. We are convinced the TPS81256 is faulty. What are the steps to getting it replaced?

Thanks again for the support. The new IC's worked.