TPS546C23: Cannot Set right voltage through PMBUS

Rahul Balakrishna

Part Number: TPS546C23
Other Parts Discussed in Thread: TPS546D24A

I am trying to set a particular Vout. For instance, when I wanted to set 1.65 V, I get 2.4V and when I wanted 0.35 V, I get 0.5 V. I have set all values to default including TRIM and SCALE LOOP, below is the screenshot of the datasheet. Please let me know if I am missing something. FYI I am using PMBUS.

Thanks in advance for your help !!!

over 3 years ago

0 Gerold Dhanabalan - Online Design Tools over 3 years ago

TI__Genius 14455 points

Hi Rahul,

Here are some notes on this topic. Can you please check if you have an external feedback divider and if it is set appropriately?

Also, can you share the schematic with us to take a look?

Regards,

Gerold

0 Peter James Miller over 3 years ago

TI__Mastermind 48240 points

Rahul Balakrishna

It sounds like you have external resistors programming the output voltage that do not match VOUT_SCALE_LOOP, so the VOUT_COMMAND to reference voltage ratio does not match the Output Voltage to FB ratio.

As Gerold Dhanabalan - Online Design Tools mentioned, the TPS546C23 only supports a few VOUT_SCALE_LOOP values, 1.0, 0.5, 0.25 Your current resistor divider ratio 0.6875 (1.65V / 2.4V) is not matched the VOUT_SCALE_LOOP, and there is no VOUT_SCALE_LOOP option that matches that divider ratio.

There are 3 potential solutions:

1) Adjust the feedback divider resistor ratio to match the VOUT_SCALE_LOOP that provides the target programmable range.

This will change the initial boot-voltage prior to PMBus programming to 0.6, 1.2, or 2.4V depending on the scale selected

The VOUT_SCALE_LOOP selected should be based on the desired programmable output voltage range:

Upto 1.65V, select VOUT_SCALE_LOOP = 1.0 and no resistor from FB to AGND

Upto 3.3V, select VOUT_SCALE_LOOP = 0.5 and a resistor from FB to AGND equal to the resistor from DIFFO to FB

Upto 5.5V, select VOUT_SCALE_LOOP = 0.25 and a resistor from FB to AGND equal to 3x the resistor from DIFFO to FB.

2) Use the existing resistor divider, leave VOUT_SCALE_LOOP = 1.0 and allow the device programming the TPS546C23 to adjust the VOUT_COMMAND programmed value for the external feedback divider.

3) Change the design to use the TPS546D24A, which replaces the external divider with an internal divider and provide Pin programming options for boot voltages from 0.5V to 6.0V and matches VOUT_SCALE_LOOP to the internal divider ratio.