LM7480-Q1: LM7480 Output voltage is lower than expected.

Kadir E

Part Number: LM7480-Q1
Other Parts Discussed in Thread: LM7480

Tool/software:

Hello,

I have made a design following the design instructions in LM7480 datasheet. It is meant for a 12V system and the aim is to pass ISO 7637-2 (Pulses 1,2,3) and ISO 16750-2 (Unsuppressed Load Dump).

When I test the prototype, I am seeing 6.8V on the output and it does not turn on until 13V. The output stays at 6.8V all the way to 30V on the input side. (From the design procedure, I would expect the output to be clamped at around 25V). Is there anything that immediately jumps up from the schematic? If not how can I diagnose the problem?

Thank you

7 months ago

0 Shiven Dhir 7 months ago

TI__Guru 62110 points

Hi Kadir,

Please probe the following signals.

VIN,VS,HGATE,VOUT.

VS,VCAP,HGATE,VOUT.

Regards,

Shiven Dhir

0 Kadir E 7 months ago in reply to Shiven Dhir

Prodigy 10 points

Hello Shiven,

I dont have access to 4 channel oscilloscope so I go 4 captures wrt VIN. Hope this is useful. Last capture has 5V/div for CH2 otherwise the setup is same across all captures.

Thank you,

Kadir E

CH1:VIN/CH2:VS

CH1:VIN/CH2:HGATE

CH1:VIN/CH2:VOUT

CH1:VIN/CH2:VCAP

0 Shiven Dhir 7 months ago in reply to Kadir E

TI__Guru 62110 points

Hi Kadir,

What is your maximum input voltage including transients.

I see that you have chosen common source topolgy which signals that you need it for some kind of clamping.

Regards,

Shiven Dhir

0 Kadir E 7 months ago in reply to Shiven Dhir

Prodigy 10 points

Hello Shiven,

9-16V with 12V nominal, transient Vmax is 100V. Yes would like Vout to clamp at 25V max.

Regards,

Kadir E

0 Shiven Dhir 7 months ago in reply to Kadir E

TI__Guru 62110 points

Hi Kadir,

Thanks for information.

I see that you have used a 62V zener which has maximum clamping voltage of 65.6V which is >65V

I would recommend to use a 56V zener instead which has max clamping of 58.8V

Next, the zener is rated for 1W and hence the series resistor R5 can be decreased.

According to power dissipation equation, you can use ~2.5k resistor and still be <1W dissipation.

Please test with a 2.5k resistor on R5 and it will work.

Regards,

Shiven Dhir

0 Kadir E 6 months ago in reply to Shiven Dhir

Prodigy 10 points

Hello Shiven,

Would that fix the output voltage issue? Currently its clamped at 6.8V wheres Id expect it to be at around 25V.

Best,

Kadir E

0 Shiven Dhir 6 months ago in reply to Kadir E

TI__Guru 62110 points

Hi Kadir,

Yes, it will fix that.

You can confirm it without changing zener also.

Regards,

Shiven Dhir

0 Kadir E 6 months ago in reply to Shiven Dhir

Prodigy 10 points

Hello Shiven,

I have tried 2.5k in place of R5 and the output voltage went up to 8.4V at 12V Vin. I decreased it to 1.8k and then 1.3k and the output voltage went up to 10V and 11.6V respectively. Could i use 1k 2W resistor and get a 3W zener? (1SMB5943BT3G). Would that cause other problems?

Best,

Kadir