UCC2897: If Vin current load 30mA is within IC normal operating spec while UVP on the LINEUV pin

Hello Tiffany,

I think you emailed me this question also.

Anyway, 20mA or 30mA is a very high current for a controller that is in a standby state.

Figure 3 in the datasheet shows that the quiescent supply current should be about 2mA.

In most applications VIN is only used to startup the device and once the device is operational a bias voltage of greater than 12.7V on the VDD pin is used to power the IC.

Do they actually measure this current on the VIN pin of the ic or is this the input current to the power supply durine a line under voltage ?

Regards

John

Hi John,

Please refer to the attached file.

I_Vin_S issue 20180412.doc

Hi Johnson,
I can see that the VDD power is derived from VIN by means of a discrete linear regulator designed around ZD103 and Q108.
With this arrangement the voltage on the source of Q108 is going to be Vs = 12- VGS_TH = 9V min and the minimum voltage on VDD = 9-Vd = 8.3v
This means that the internal regulator on the VIN input is still on and the UCC2897A is drawing current from the VIN pin.
VDD needs to be biased greater than 12.7V.
I think you should replace Q108 with an npn bipolar transistor and remove D113.
This should ensure that the VDD pin is powering the UCC2897A
Also I don't see any decoupling caps on VIN.
Is this correct ?

Regards
John

Hi John,

Yes, there is no decoupling cap on Vin.

The current I_Vin_S on NG IC is more then Good IC. We think the large I_Vin_s to make the voltage drop on Vin_s. So We want to Know 30mA peak current is meet your IC design?

Thanks.

Johnson

Hi Johnson,

You need to have a decoupling capacitor on Vin.
Most likely you are seeing a variation in peak currents on Vin because of the uncertainty caused by a lack of decoupling.
A peak current rating is not specified  for the UCC2897A 
You should calculate the average current and multiply by the value of Vin.
This should be just a few 100mW and not cause overheating of the UCC2897A
Your maximum power dissipation will occur when the device comes out of UVP condition.
This is really the current you should be measuring if you think there is an issue.

Why do you worry about the device current when in an UVP condition ?

Regards

John

Hi John,

The current I_Vin_S =30mA on NG IC make our DCDC module fail the UVP test.
And, the I_Vin_S =20mA on good IC make our DCDC module pass the UVP test.
We think in UVP condition, if the I_Vin_S is more than 20 mA is NG IC.
Can you help to check it?

Thanks.

Johnson