TPS55288: FB connection resistance when using FB_INT

Nomura Kousaku

Other Parts Discussed in Thread: TPS55288, USB2ANY

Tool/software:

Does the FB/INT pin have an internal resistor, so if I want to use FB_INT, do I need to connect a resistor to this pin?
In the evaluation board documentation, 1 MΩ is connected.

”The JP2 jumper is for the external feedback or the internal feedback selection. By default, this jumper is set to
the FB_INT position.”

Do I need to connect 1MΩ to GND when using FB_INT?
Why?

7 months ago

0 Brigitte 7 months ago

TI__Guru 65340 points

Hello Nomura,

For what do you want to use FB/INT? Do you want to use it for setting the voltage with an external feedback divider or do you want to use it as a digital output?

If you want to use it as logic output, have a look to chapter 6.5 in the datasheet: https://www.ti.com/document-viewer/TPS55288/datasheet#GUID-943C6051-5641-4455-86C9-103F8B665E08/TITLE-SLVSF01A_TPS55288_STARFISH_UPDATE_ELECTCHAR

Under LOGIC INTERFACE you can find that the leakage is below 100nA at 5V (internal resistance above 50 MOhm) and when the pin pulls to low, a current of 4mA sinking causes maximum a voltage of 0.1V (internal resistance to GND below 25 ohm).

Best regards,
Brigitte

0 Nomura Kousaku 7 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte

(1) I want to use internal feedback.
　In that case, is it okay for this pin to be OPEN because there is a resistor inside?

(2) If I want to consider both internal and external feedback, is it okay to keep the Rfb_UP and Rfb_BT implemented?

0 Nomura Kousaku 7 months ago in reply to Nomura Kousaku

Prodigy 40 points

Dear Brigitte

SORRY
When using internal feedback on the evaluation board, why is there a 1 MΩ between Vcc and FB?
Is this resistor necessary for internal feedback?

0 Nomura Kousaku 7 months ago in reply to Nomura Kousaku

Prodigy 40 points

Dear Brigitte

Is the R19 of the circuit on this evaluation board 102 kΩ?
Or is it 1MΩ?

Best Regurd
Nomura

0 Brigitte 7 months ago in reply to Nomura Kousaku

TI__Guru 65340 points

Hello Nomura,

The resistor R19 is for pulling up the signal that is coming out of the pin when it is used as output. The value according users guide schematic and BoM is 102kohm.

FB is an output when you are programming the device for internal feedback, so you can leave it floating, but then you cannot see a difference between the pin pulled low internally or having it not pulling low.

FB/INT is a open-drain circuit when the internal feedback is used.

Best regards,
Brigitte

0 Nomura Kousaku 7 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte

The resistor R19 is for pulling up the signal that is coming out of the pin when it is used as output. The value according users guide schematic and BoM is 102kohm.
⇒Is there a formula for making R19 a value of 102 kΩ?　 Is 100 kΩ NG?　 Is 1MΩ NG?
　When should the FB pin be used as an output?

FB is an output when you are programming the device for internal feedback, so you can leave it floating, but then you cannot see a difference between the pin pulled low internally or having it not pulling low.
⇒Does this mean that there is no problem with the FB pin in the open state?
　What is the problem if I can't tell the difference between a pin that is pulled down low and a pin that is not pulled down internally?

FB/INT is a open-drain circuit when the internal feedback is used.
⇒Does it mean that the normal operating state when using internal feedback is OPEN drain?
　Is it okay to cause a drain short circuit if an abnormality occurs inside the IC?
　Does this mean that it is a signal that can be used when an abnormality inside the IC is detected?
　If this anomaly detection is not used, can the FB pin be OPEN?

How do I choose whether to use internal or external feedback?
Right now I'm verifying it with internal feedback. (Use this DCDC as the source power supply for the USB-PD.) ）

Best Regurd
Nomura

0 Brigitte 7 months ago in reply to Nomura Kousaku

TI__Guru 65340 points

Hello Nomura,

FB shall never be used as an output, but this pin has 2 functions, please have a look to the datasheet.

The programming of the device defines if the pin is used as FB or as INT. Please check out the datasheet.

Best regards,
Brigitte

0 Nomura Kousaku 7 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte

①Where can I find more information about FB in the datasheet?
　I can see that there are two functions.

②Where does it say that Vcc and FB are connected with 102 kΩ when used with INT?

③The FB/INT pin is the indicator for output short circuit protection, overcurrent status, and overvoltage status (Default).
　⇒How do I use the indicator?

④At the moment we are considering using internal feedback,
　How do I choose whether to use internal or external feedback?
　Right now I'm verifying it with internal feedback. (Use this DCDC as the source power supply for the USB-PD.) ）

Best Regurd
Nomura

0 Brigitte 7 months ago in reply to Nomura Kousaku

TI__Guru 65340 points

Hello Nomura,

1. Please find this information here: https://www.ti.com/document-viewer/TPS55288/datasheet#GUID-50405974-7D31-4FF8-99ED-69ABD074E872/TITLE-SLVSF01SLVSF015743, and here: https://www.ti.com/document-viewer/TPS55288/datasheet#GUID-85F110DA-2FBC-495B-BBBD-179687E2A4BD/TITLE-SLVSF01DEVICE_DEVICE_VOUT_FS and here: https://www.ti.com/document-viewer/TPS55288/datasheet#GUID-5DE3E67F-003B-4BAB-9E1F-99EFD3A89C68/TITLE-SLVSF01DEVICE_DEVICE_STATUS

2. The feedback pin is an open-drain output, so it is only able to pull to GND. To make sure that the voltage on this pin goes high when it is not actively driving to GND, you need a resistor to any voltage which is below the abs max rating of this pin. This is the reason why there is a resistor to VCC on the EVM.

3. If you want to use it, you need to have some logic monitoring when this pin goes low.

4. The default setting of the device is with internal feedback, see here: https://www.ti.com/document-viewer/TPS55288/datasheet#GUID-85F110DA-2FBC-495B-BBBD-179687E2A4BD/TITLE-SLVSF01DEVICE_DEVICE_VOUT_FS so you do not need to do anything if you are using internal feedback. If you want to use external feedback, you need to program the register in above link to this function.
For programming the device on the EVM, you need this GUI: TPS55288-EVM-GUI GUI for evaluation module (EVM) | TI.com as well as the USB2ANY interface box. For more details have a look at the "Getting started with TPS55288" in this link: TPS55288EVM-053 Evaluation board | TI.com

Best regards,
Brigitte

0 Nomura Kousaku 7 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte

１．I confirm.

２．In such an abnormal time, will FB drive and become LOW?
　　How is the resistance of 102 kΩ determined? Is 100 kΩ or 1 MΩ NG?

３．If it is not to be used as an indicator in the event of an abnormality, can the FB pin be OPEN (Vcc and resistor are not connected)?

４．This is how i can set it up to use either internal or external feedback.
　　What are the advantages and disadvantages of external feedback and internal feedback?
　　I would like to know in the case of what output specifications to choose an external FB. And vice versa.

Best Regards,
Nomura

0 Brigitte 7 months ago in reply to Nomura Kousaku

TI__Guru 65340 points

Hello Nomura,

2. Yes, the pin will then be driven low. The value of the external resistor is defined by the voltage you apply to the pin and the maximum current the pin is able to drive. The current is given in the electrical characteristics part of the datasheet.

3. yes, the pin can be floating as long as you keep the programming on internal feedback.

4. The values you are able to set for the internal feedback are given in the datasheet. If a different output voltage value is needed, the external feedback is needed.

Best regards,
Brigitte

0 Nomura Kousaku 7 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte

2.I understood.
　What kind of fail causes FB to become LOW?
　For example, an output short?Is there anything else?

3.I understood.

4.I would like you to tell me a little more about this.

Best Regards,
Nomura

0 Brigitte 7 months ago in reply to Nomura Kousaku

TI__Guru 65340 points

Hello Nomura,

Please have a look at this part of the datasheet for the different voltage levels that can be set with internal feedback: https://www.ti.com/document-viewer/TPS55288/datasheet#GUID-7EE3FC7F-B31E-4BA6-A607-6746D64480B9/TITLE-SLVSF01T5657623-35

and here https://www.ti.com/document-viewer/TPS55288/datasheet#GUID-600E26FA-0F13-43D7-96BC-8DA8C8DCF6BB/TITLE-SLVSF01T5657623-105 review especially the REF and the VOUT_FS registers.

Best regards,
Brigitte

0 Nomura Kousaku 7 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte

2.What kind of fail causes FB to become LOW?
　For example, an output short?Is there anything else?

Best Regards,
Nomura

0 Brigitte 7 months ago in reply to Nomura Kousaku

TI__Guru 65340 points

Hello Nomura,

This information is given here: https://www.ti.com/document-viewer/TPS55288/datasheet#GUID-85F110DA-2FBC-495B-BBBD-179687E2A4BD/TITLE-SLVSF01DEVICE_DEVICE_VOUT_FS

Best regards,
Brigitte

0 Nomura Kousaku 7 months ago in reply to Brigitte

Prodigy 40 points

Dear Brigitte

2.I understood.

I would like to ask you again, about voltage setting using internal feedback
I don't understand how the voltage and the register correspond to the following.
I would like to know how to determine the register for the output voltage.

5V：0x02D00
9V：0x19A00
15V：0x2C600
20V：0x3C000

Best Regards,
Nomura

0 Bryce Xun 7 months ago in reply to Nomura Kousaku

TI__Mastermind 27830 points

Hi Nomura,

When internal feedback is selected, the output voltage is set by Vref/ratio, where Vref is set at REF register, and feedback ratio is set at VOUT_FS register, default internal feedback ratio is 0.0564.

BRs,

Bryce

0 Nomura Kousaku 7 months ago in reply to Bryce Xun

Prodigy 40 points

Dear Brigitte

For example: If the output voltage is 5V, the REF is 00D2h?
The Vref of 00D2h is 282mV?

　　　　　　5V=Vref/ratio=0.282/0.0564

Where do you find that 282 mV is 00D2h?

Best Regards,
Nomura

0 Bryce Xun 7 months ago in reply to Nomura Kousaku

TI__Mastermind 27830 points

Hi Nomura,

You can find the descriptions at the 7.6.1 REF register to set the Vref value.

BRs,
Bryce

0 Nomura Kousaku 7 months ago in reply to Bryce Xun

Prodigy 40 points

Dear Bryce

45mV+1.129mV*(210)=282mV
210=00D2(hexadecimal)

I understood.

What kind of settings are required to support USB-PD PPS output with this DCDCIC?

Best Regards,
Nomura

0 Bryce Xun 7 months ago in reply to Nomura Kousaku

TI__Mastermind 27830 points

Hi Nomura,

You will need to pair with PD controller and send I2C requests to this DC/DC, so the output voltage can adjust accordingly to realize voltage PPS.

And to realize the current PPS, you will need to adjust the ISP-ISN voltage at IOUT_LIMIT register to adjust the output current limit value.

BRs,

Bryce

0 Nomura Kousaku 7 months ago in reply to Bryce Xun

Prodigy 40 points

Dear Bryce

I will first check the PD controller (TPS65992) documentation.

We recognize that the above IOUT_LIMIT is used even for fixed output voltages.
Since we are now putting a 10 mΩ resistor in the circuit, the default is 50 mV / 10 mΩ = 5 A.

If i want to set the current limit to 3A, i need to write 30mV (111100b) to the register, right?
Is it correct to understand that each current value can be set for the output voltage (5V/9V/15V/20V) of each PD?

Best Regards,
Nomura

0 Bryce Xun 7 months ago in reply to Nomura Kousaku

TI__Mastermind 27830 points

Hi Nomura,

Yes, you are right, 30mV/10mΩ=3A current limit.

Yes, each current value can be set for the output voltage (5V/9V/15V/20V).

BRs,
Bryce

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TPS55288: FB connection resistance when using FB_INT