TPS2421-1: Device Failure - No Fault - Conducting 0.9V

Hi Mark,

The fault asserts when there is an overload event beyond fault timer period or if the device hits thermal shutdown.

Can you please provide test waveforms of Vin, Vout, input current and FLT output.

Best Regards,
Rakesh

Hi Rakesh,

1st off the device is not indicating a fault. 

Vin goes from 0V to 3.6V

There is a load that maximally reaches 1A

Vout rises to 0.9V

The device no longer functions.

This happened to 4 circuits thus far out of 80 circuits in use at another location. At the moment the device failed I was not present. 

I cannot provide scope traces until I can replicate the problem so I am looking for a way to create the problem. So with your knowledge of the chip, can you suggest whtt would cause it to fail and only pass 0.9VDC instead of 3.6VDC

Hi Mark,

During startup, this device operates in constant power output mode (5W typical) to limit the inrush currents. The section 8.4.2 in the datasheet provides information on the maximum load the device is able to do successful startup. The higher output capacitance further restricts the maximum startup load.
Can you verify the function on failed boards by removing C3 470uF.
What is the application use case and end equipment here?

Best Regards,
Rakesh

Hi Rakesh,
Downstream from the 2421 there is a current sensing circuit that switches in 1 of 3 different sensing resistors (with relays for range changing) with the lowest resistor being 0.04 Ohms. Beyond the current sensing circuit is a connector for plugging into a device . This is part of a testing application so the load is a device under test that is plugged into connector before power is applied. This is all being done on a manufacturing floor at a tier 1 electronic manufacturer.

It is possible the device under test has a short between power and return due to a manufacturing problem with the device under test. Even if the 2421 fails to startup should the fault output not be asserted?

The 0.9V output I'm finding occurs even when a device under test is not plugged in and all the relays are open (so there is no load ).

It's important to note that the failed device worked fine for hundreds (if not thousands) of cycles prior to failing and that there are 77 other identical circuits that continue to function properly (for now).

I will remove the 470uF cap and see. 

I can say we have seen this 2421 device failure a couple times in the past in this same application and swapping out the 2421 resolved the issue. This is highly inconvenient as the manufacturing plant is not local to us.

Best Regards,
Mark

Hi Mark,

Thanks for detailing the device use case.
The device provides fault output for an unsuccessful startup. Can you correlate - whether 2421 device failure and device under test failure happened at the same time? I am just checking whether the failure of "device under test" lead to the failure of 2421 device
Please check the operation of failed 2421 device by removing 470uF cap. If needed, we can set up a call to discuss next steps.

Best Regards,
Rakesh