TPS55288: Output Voltage Setting

Thanks Zack for your quick response.

While using internal feedback, do we need to connect a pull up resistor of VCC??

Hi Sahil,

When using internal feedback, you can leave FB pin floating.

Noted. Thanks.

Actually, we are facing issues using both internal and external feedback resistors. We are not able to generate desired output voltage.

We have made design in such a way that TPS552882 as well as TPS55288 works okay.

Please see attached sanpshot of schematic.

To make it work for TPS55288, I have made following changes:

- Removed R595 & R594

- Populated R582 & R583 to do I2C connections.

We are able to write the registers successfully but not able to get output.

We set OE bit to 1 and FB bit to 1 to set in external feedback mode with Vref to be max (1.2V). In this case, we are getting unstable 740mV.

Kindly help to review the schematic adn provide your feedback.

Thanks!

Hi Sahil,

It looks lots of components value are not selected well. For example:

1. For 400kHz fsw, the recommended inductance is 4.7uH like EVM.

2. For TPS55288, the pull-up resistors for SCL, SDA must exist.

3. CDC cannot must short to GND. Please leave it floating.

4. Compensation network doesn't look fine, should be reason that Vout is unstable.

5. What's the desired Vin range, Vout range, maximum load current of your application? We need that to calculate the correct compensation network.

6. Last but not least, you need to follow the EVM recommended layout. Please see the layout guideline app note. We also saw lots of regulation issue related to layout. 

Hi Zack,

Thanks for the detailed feedback.

Please see my response for all your feedbacks:

1. For 400kHz fsw, the recommended inductance is 4.7uH like EVM.

[Sahil]: We will validate that by changing inductor.

2. For TPS55288, the pull-up resistors for SCL, SDA must exist.

[Sahil]: Pull ups already exist on the host side which is not shown in this snapshot. Apologies for that.

3. CDC cannot must short to GND. Please leave it floating.

[Sahil]: We have already removed that 0R resistor. So it is floating.

4. Compensation network doesn't look fine, should be reason that Vout is unstable.

[Sahil]: Vin range: 4V-12V, Vout range: 1.2V-5V with max current of 3A. Kindly provide compensation netwrok values for the same.

5. What's the desired Vin range, Vout range, maximum load current of your application? We need that to calculate the correct compensation network.

[Sahil]: Please refer my response in Sr#4.

6. Last but not least, you need to follow the EVM recommended layout. Please see the layout guideline app note. We also saw lots of regulation issue related to layout. 

[Sahil]: We have followed the layout guidelines. We are able to get the TPS552882 IC to work properly there. However, we are facing issues with TPS55288 only.

Let me know if you need any moere information.

Thanks!

Hi Zack,

Can we use the compensation network values provided in EVK??

Hi Sahil,

You can try with R590=30kohm, C680=10nF.

HI Zack,

Thanks for the values.

Would this work for output voltage from 1.2V -5V??

Any other comp change required?

Hi Zack,

Awaiting your repsonse.

Hi Sahil,

Could you get correct output voltage now?

You can also adjust the ILIM pin resistance to 66kohm to set average current limit to 5A to protect the inductor and IC.

Hi Zack,

I changed the compensation network to the values you suggested and changed inductor to 4.7uH.

I set the Vref value 1200mV and selected external feedback using I2C registers.

feedback resistor values are Rfbup = 100k and Rfbbt = 56k.

I am getting output voltage waveform as shown below:

What could be the possible causes for this?

Hi Sahil,

Could you also share the layout file? Maybe the layout will cause regulation issue.

Hi Zack,

Please see the attached snapshot of layout.

Hi Sahil,

It looks like there is some places could be optimized but I'm not sure if this cause the regulation issue.

Below is my suggestions:

1. Add several gnd vias close to VCC capacitor ground terminal.

2. Add several GND and Vout vias close to the capacitor C6635.

Could you check SW1, SW2, VCC pin waveform? 

Hi Zack,

We have ordered EVK for this chipset.

We want to use this IC in another application which has configurable output voltage. See the specs below

Vin: 12V

Vout: 1.2V - 5V @ 3A max current.

Will this IC suffice our requirements and generate stable configurable output over I2C interface?

If yes, please provide the Webench design for the same.

Thanks! 

Hi Sahil,

TPS55288 fits for this application. 

One question: your application seems only need a buck converter, why does you choose a buck-boost converter?

You can use WEBENCH to generate the schematic. Please remember to share the schematic with us for review.

Hi Zack,

Thanks for the confirmation.

We used Buck Boost because we didn't found any DCDC Buck switcher which is I2C configurable.

Also, can we use the TPS55288 EVK schematic as per our requirements?

Currently we are planning to use the attached design for our application.5V 3.5A.pdf

Hi Zack,

Awaiting your response. 

Hi Sahil,

The attached file is a buck converter TPS565208 schematic, maybe you share the wrong scheamtic?

Hi Zack,

Apologies for the wrong schematic.

Please see the attached file.

I2C DCDC.pdf

Hi Sahil,

Please see my comments:

1. Add a 100nF ceramic capacitor and put it close to VIN pin.

2. Add a 1uF/0402 package ceramic capacitor and put it close to Vout and GND pin.

3. If you don't need output current limit protection, you can short ISP, ISN pins to VOUT pin as the schematic. If you need this output current limit function, place a current sense resistor between ISP and ISN pin, also add a 100nF filter capacitor close to ISP, ISN pin.

4. Change Rilim resistance to 66kohm.

Hi Zack,

Well noted. Thanks for your detailed feedback.

This feedbacks are for the schematic pdf (I2C DCDC). Am I correct?

Also, please confirm that this schematic design would work for output voltage from 1.2V to 5V @ 3A max output current.

Hi Zack,

What if we use the EVK schematic? Would that work for our application?

Hi Sahil,

1. Yes, the comment is based on the file 'I2C DCDC.PDF'.

2. Yes, the EVK schematic also work for your application.

Hi Zack,

Thanks for the confirmation.

We will have the EVK for this switcher by tomorrow. We will validate the same and let you know if we face any issues.

Thanks!