LM74900-Q1: Always on voltage drop

Colin Chen

Part Number: LM74900-Q1
Other Parts Discussed in Thread: LM74900

Dear Experts,

Here is our circuit schematic diagram. We found during testing that the voltage of VBAT_12VSB drops to around 5V when connected to a load, with a voltage drop of nearly 7V. The backend is connected to four CAN controllers, and the total load is less than 10mA when loaded. Currently, this phenomenon only occurs during the initial power on, and the voltage drop disappears when powered off and then on again.

It is currently unclear what the cause is, is it due to circuit design reasons? Thank you

1 month ago

0 Shiven Dhir 1 month ago

TI__Guru 56155 points

Hi Colin,

Please show waveforms with following nodes probed.

VIN, DGATE, HGATE, OUT

VS, DGATE, HGATE, OUT

Regards,

Shiven Dhir

0 Colin Chen 1 month ago in reply to Shiven Dhir

Intellectual 985 points

Hi Shiven,

The following are the voltage waveforms from DGATE to GND and VBAT_12VSB to ground when powered on;The phenomenon only occurs during the power on stage, and the FLT pin remains at a high level without any faults occurring; If no load is added, the voltage is normal, but when a load is added and the power is turned on, the voltage will drop for a period of time.Thank you

0 Shiven Dhir 30 days ago in reply to Colin Chen

TI__Guru 56155 points

Hi Colin,

Please label the signals.

Also, please provide the requested signals with a 4-channel scope if available.

Regards,

Shiven Dhir

0 Colin Chen 23 days ago in reply to Shiven Dhir

Intellectual 985 points

Hi Shiven,

The waveform is as follows,please refer to,thank you

0 Shiven Dhir 22 days ago in reply to Colin Chen

TI__Guru 56155 points

Hi Colins,

Looks like VIN is not able to support the startup. VS is following VIN and hence you see the drop.

Is there any current limit on the supply?

Can you try with some other type of supply?

Regards,

Shiven Dhir

0 Colin Chen 19 days ago in reply to Shiven Dhir

Intellectual 985 points

Hi Shiven

It doesn't seem to be a power issue. We are using DC stabilized power supplies, and even after testing with a different power supply, there will still be waveform dips. The following is the measured waveform. If LM74900-Q1 EN is grounded and LM74900 is turned off, the voltage will be stable. We think that starting LM74900 may affect the voltage output at the midpoint VBAT_12VSB.Thank you

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LM74900-Q1: Always on voltage drop