LM7480-Q1: Part misbehave

Slavko Kocjancic

Prodigy 10 points

Part Number: LM7480-Q1

Tool/software:

Hello...

We have problem in our design as part doesn't work as expected. I have already replaced part but have same issue.

The problem is that we don't get output at all.

The attached is part of schematics and image of chip marking.

I have measured all voltages and here is result:

With test condition with X51 set to deliver 0 V to EN/UVLO the measured voltage at pins are:

Item	Voltage	Our opinion
VBAT	12.02 V	Supply voltage
VS, VSNS, SW	12.02 V	As should be
OV	0.40 V	Overvoltage not triggered (Under 1.13 V)
OUT, A	0.00 V	OK as EN/ULVO is low
HGATE, DGATE	0.00 V	OK as EN/ULVO is low
C	0.00 V	Excepted as gate is low
CAP	11.36 V	Probably ok as EN/ULVO is low
EN/ULVO	0.00 V	connected to GND

And other set of measurements with test condition with X51 set to deliver 12 V to EN/UVLO the measured voltage at pins are:

Item	Voltage	Our opinion
VBAT	12.02 V	Supply voltage
VS, VSNS, SW	11.97 V	50 mV drop on R59 indicating 5 uA power draw
OV	0.40 V	Overvoltage not triggered (Under 1.13 V)
OUT, A	0.00 V	Expected as GATE is LOW
HGATE, DGATE	0.00 V	It should be around 20V
C	0.00 V	Excepted as gate is low
CAP	11.36 V	Probably wrong as charge pump should run
EN/ULVO	11.97 V	connected to VS

Really don't know what is wrong there so any help is welcome.

9 months ago

0 Shiven Dhir 9 months ago

TI__Guru 53385 points

Hi Slavko,

Thanks for all the information.

Can you DNP R63 and test again?

Also change R65 to 50-100ohms range resistor.

0 Oliver Filsch 9 months ago

Intellectual 450 points

Hello Slavko,

in addition, please make sure that C33 (Cvs) is >= 10x C36+C38 (Ccap)

See LM7480: Charge pump current draw

At the moment, in your schematic Cvs is even smaller than Ccap. Maybe the charge pump doesnt work properly.

0 Slavko Kocjancic 9 months ago in reply to Shiven Dhir

Prodigy 10 points

I removed R65 so slew rate is completely disabled. No changes detected.

0 Slavko Kocjancic 9 months ago in reply to Oliver Filsch

Prodigy 10 points

We miss that requirement but even with 47uF tantalum doesn't work.

The consumption is also weird. As supply goes thru 10k resistor we observe voltage drop here. With EN/ULVO at 0 V, the drop is under noise flor. According to data sheet the shutdown current is typically 2.87 uA so we should measure around 30 mV drop. When EN/ULVO is high the consumption should rise to around 420 uA, but we can measure only 5 uA.

0 Shiven Dhir 9 months ago in reply to Slavko Kocjancic

TI__Guru 53385 points

Hi Slavko,

Can you share the startup waveforms for analysis.

Please probe the following signals.

VIN,HGATE,VS,VCAP

Please capture with EN high and low when capturing.

Regards,

Shiven Dhir

0 Slavko Kocjancic 9 months ago in reply to Shiven Dhir

Prodigy 10 points

Test conditions are:

R59 Shorted (so fast voltage rise)

R65 Not placed (no inrush limiting)

Scope traces:

1-Yellow    VBAT
2-Pink        VS
3-Blue        HGATE
4-Green     CAP

0 Slavko Kocjancic 9 months ago in reply to Slavko Kocjancic

Prodigy 10 points

... I raised input voltage to 15V

0 Shiven Dhir 9 months ago in reply to Slavko Kocjancic

TI__Guru 53385 points

Hi Slavko,

I hope the EN was high during the tests.

I see that charge pump is not working and there is potential IC damage.

Can you replace the IC and test again?

Regards,

Shiven Dhir

0 Slavko Kocjancic 9 months ago in reply to Shiven Dhir

Prodigy 10 points

EN is connected to VS.

The same thing happens on 3 board, and I already replaced part in my board. All parts are from same source, but seems geniue.

0 Shiven Dhir 8 months ago in reply to Slavko Kocjancic

TI__Guru 53385 points

Hi Slavko,

Did this happen on 3/3 boards, or it happens on 3 boards only, but you have more, and others work fine. I dont see an issue in schematic that is leading to damage of IC.

Did replacing the IC bring-up the board?

Regards,

Shiven Dhir

0 Slavko Kocjancic 8 months ago in reply to Shiven Dhir

Prodigy 10 points

None of 3 assembled boards works. On one board I replaced the chip and doesn't work either.

0 Shiven Dhir 8 months ago in reply to Slavko Kocjancic

TI__Guru 53385 points

Hi Slavko,

Do you have our EVM on which the same chip can be tested?

Do you think there are any issues with your board build? Related to layout or manufacturing?

Regards,

Shiven Dhir

0 Slavko Kocjancic 8 months ago in reply to Shiven Dhir

Prodigy 10 points

I waiting to get EVM board to check this. The board should be fine. The only concern is about thermal pad. In data sheet is marked as do not connect so in our board is floating.

0 Shiven Dhir 8 months ago in reply to Slavko Kocjancic

TI__Guru 53385 points

Hi Slavko,

Thermal pad floating is the correct way.

Regards,

Shiven Dhir

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LM7480-Q1: Part misbehave