TPS22990: Calculation of CT capacity

user201583

Part Number: TPS22990

Dear Sir or Madam,

I am trying to determine the value of CT using Equation 1 of the data sheet (JAJSCB8C) to adjust the rise time of TPS22990.
However, the data sheet (JAJSCB8C) says “An approximate equation ... when VBIAS is set to 5 V”.
Is it possible to use Equation 1 as it is when using at VBIAS = 3.3V?

Redards,

Hasegawa

over 6 years ago

0 Rakesh over 6 years ago

TI__Guru** 105361 points

Hi Hasegawa,

Thanks for reaching out!

We will check and get back to you shortly.

Best Regards,

Rakesh

0 Aleksandras_Kaknevicius over 6 years ago

TI__Genius 15770 points

Hi Hasegawa,

When VBIAS = 5V and VBIAS = 3.3V, the rise time is slightly different. This comparison can be seen in figures 17 and 18.

In your design, what input voltage is being used? In general, VBIAS = 5V and VBIAS = 3.3V will have similar rise times for lower voltages, but it will vary a bit more once VIN = VBIAS.

Thanks,

Alek Kaknevicius

0 user201583 over 6 years ago in reply to Aleksandras_Kaknevicius

Prodigy 140 points

Hi Alek-san,

Thank you for your response.

My design is VIN = VBIAS = 3.3V.

There are several DC / DC converters after the TPS 22990.
The sum of their input capacities is about 100 uF, and I think that this value corresponds to CL.

If the rise time is affected by CL, I would like to know the formula of rise time including CL.

Regards,

Hasegawa

0 Aleksandras_Kaknevicius over 6 years ago in reply to user201583

TI__Genius 15770 points

Hi Hasegawa-san,

We have some data for VIN=VBIAS=3.3V, and using this data I was able to come up with the following equation:

tR(us) = 0.0372 x CT(pF) + 23.41

Load capacitance does not have a large effect on rise time, so this should be a good estimate even for 100uF CL.

Thanks,

Alek Kaknevicius

0 user201583 over 6 years ago in reply to Aleksandras_Kaknevicius

Prodigy 140 points

Hi Alek-san,

Thank you for your reply.

I would like to refer the answers to the design.

Best regards,

Hasegawa

Power management

Power management forum

TPS22990: Calculation of CT capacity