TPS7A47: TPS74A7

Hi Joey,

I've got a few questions about the failing tests on the device.

The first question is where are you sensing the VIN? This device contains UVLO circuitry. If there is significant load current, which in your case is occurring on all failing tests, then the voltage seen at the device pin may be substantially smaller than what is being fed in. If this voltage is not measured carefully, then it may be pushed into shutdown which would explain all of the following test failures, and why they would subsequently pass once a higher VIN of 8V is applied.

Have you ran other devices?

Is this seen on all devices tested or just this one?

Joey Oweis said:

1 5 5 overall_accuracy VOUT VIN=3V IO=1A                                                                                              1.365000 V    0.002806   1.435000 V   Fail

The device is not failing a spec here, the device is not regulating at all. This seems to be either current limit kicking in, Thermal shutdown, UVLO, or a damaged device.

You cannot get the device to operate at VIN = 3V with a 1A load applied to it? It stays disabled throughout the test?

Joey Oweis said:

1 5 6 overall_accuracy VIN=3V IO=1A                                                                                                        -2.500000 %  99.799601   2.500000 % Fail 

This having a 100% differential seems like the device is not even operating correctly, not just failing an accuracy test. Is this being compared to the proper value? If this is the set output voltage % than the deviation of 0.2004% would be expected and make sense. If it is utilizing the near 0V from the above 1 5 5 condition then the device was not operable so it's accuracy would not be in line.

Joey Oweis said:

1 10 1 current_limit ICL                                                                                                                                  1.000000 A   0.388500    2.000000 A

This device utilizes Brickwall CL tests. This means that regardless of what is occurring, 1.0A at least should flow in a output current limit condition. This lower value may be a result of the part not entering current limit, or more likely the device is not powered on enough to provide operation. Current will stay high, but VOUT should begin to tank. Is this a short circuit CL test?

Regards,

John

Hi John,

My response..

where are you sensing the VIN ?   At the part on the pin and measuring Vin = 3V Ven=3V

Have you ran other devices?   Yes, four devices

Is this seen on all devices tested or just this one?   Yes all devices

You cannot get the device to operate at VIN = 3V with a 1A load applied to it? It stays disabled throughout the test?  Yes, it is not regulating at VIN 3V 1A but is at 35V 1A.  At VIN 3V the highest current which can be regulated is ~150mA

Is this a short circuit CL test?  Yes, forcing the output to Vo=.9*Vo(nom.  .9*1.4v=1.26v

Thanks,

Joey

A couple of additional questions - 

Can the load of 1A be applied continuously or does it need be pulsed ?

How critical is "connect thermal pad to a large-area ground plane"  on page 5 of spec ?  Does this absolutely need to be done for functionality. The failures are occurring at room temperature.

Hi Joey,

Joey Oweis said:

How critical is "connect thermal pad to a large-area ground plane"  on page 5 of spec ?  Does this absolutely need to be done for functionality. The failures are occurring at room temperature.

This is an important spec as doing this will have a significant effect upon thermal performance of the device. Additionally, by floating the thermal pad, which is connected to GND using conductive die attach, you are floating a part of GND which can shift voltages in response to EMI from external sources. This could be causing errors and improper operation, however I don't believe that this is fundamentally the issue with your testing as the 35V = VIN would perform worse if it was solely a thermal issue.

Joey Oweis said:

Can the load of 1A be applied continuously or does it need be pulsed ?

As long as you are not thermally limited, the device can provide a 1A load continuously.

Joey Oweis said:

Yes, it is not regulating at VIN 3V 1A but is at 35V 1A.  At VIN 3V the highest current which can be regulated is ~150mA

This is a concern as if you are sensing at the device pins and you see 3V, the device should be able to regulate a 1A load at VIN=3V. I'm going to need to do some bench testing to see if limits are not correct for this device based upon the testing you are seeing.

Regards,

John

Thanks. I look forward to your results. 

Hi Joey,

Firstly, could I ask for a schematic of your setup, additionally could I ask for information on the testing sequence?

I've spoken with the test engineer responsible and a few other members of the team for this device. This device is known to be sensitive to inductance in particular. Are the output capacitors close to the device? is there large inductance on the input or output of the device? Inductance can cause this device to become unstable, and with a minimum input voltage, the instability at high current could push the LDO into UVLO shutting down the device entirely resulting in the low output voltage that you see.

Regards,

John

Hi John,

Here is the schematic and test code. The capacitors are close to the device under the socket. As far as inductance, this is something I will have to look into.

/cfs-file/__key/communityserver-components-multipleuploadfilemanager/829aa80a_2D00_ef8b_2D00_4e5a_2D00_9f71_2D00_a00256cbd0db-540584-complete/TPS7A4700.pds

hw.relay->open(pin_group.All_Relays);
UF.Timer.Delay(10 mS);

hw.vi->set_dc(pin_group.OUT_, 0.0, 5.0, -1000e-3, 2000e-3);

UF.Timer.Delay(10 mS);

hw.vi->measure_voltage(pin.OUT_S, VOUT_1A, 10e-3);

hw.vi->set_dc(pin_group.OUT_, 5.0, 0e-3);
UF.Timer.Delay(10 mS);

VOUT_NOMACC_1A = ((1.4 - VOUT_1A)/1.4)*100;

smartDatalog(VOUT_1A, POWER_UNIT, 5, pin.OUT_S,DATALOG_AVG);
smartDatalog(VOUT_NOMACC_1A, POWER_UNIT, 6, pin.OUT_S,DATALOG_AVG);

Thanks,

Joey

Hi Joey,

Thanks so much for this.

I've got one question, If you bring just VIN up to 8V or just EN up to 8V does the device operate? Under which condition?

I showed this to our test team, and here were some of the questions that were presented.

What are the arguments in the set_dc command? Are these voltages, time, current or something else? The -1000e-3 specifically was of interest.

Are these fresh devices? Or are they being reused from another application?

Regards,

John

I've got one question, If you bring just VIN up to 8V or just EN up to 8V does the device operate? Under which condition?   Only VIN

What are the arguments in the set_dc command?  set_dc(voltage, voltage_range, current, current_range)

Are these fresh devices? Or are they being reused from another application? 

These are fresh devices.  Are there any parts available which can purchased through TI ? The website indicates none are available.

Hi John,

I appreciate your help.  At this point, I would like order new parts. Can you send me the link of the page above showing stock?  The one I am directed to indicates the part is out of stock.

Thanks,

Joey

I see this when I click on the link - out of stock. Can you confirm for me parts whether or not parts are available ?

Hi John,

I managed to get new parts and am having better success. The only test I am failing is overall accuracy 35V/1A at 125C, room and cold pass. I am able to pass at ~750mA (125C).  Can the current be derated under these conditions?

Thanks,

Joey

It is only happening at 35V/1A passes 3V/1A.