TPS549B22: default Vout setting

Allen Li2 

a) Does it means for my designthe Vout  will go to Vsel=0.75V firstly during bootup?

[PJM] No.  It means that the VOUT_COMMAND and the RSP to RSN voltage will always boot-up to 0.75V regardless of the last VOUT_COMMAND value stored to NVM.

b) after power on, CPU is possible to adjust the Vout by VOUT _COMMAND.

[PJM] Yes, the RSP to RSN voltage can be changed via VOUT_COMMAND after power-up, but with an external resistor divider from VOUT to RSP, the output voltage will not follow the standard VOUT = VOUT_COMMAND x 2^-9V scaling.  The output voltage will follow a scaling of:

VOUT_COMMAND x 2^-9V x (1-Rtop/Rbot)

Where Rtop is the resistor from VOUT to RSP and Rbot is the resistor from RSP to RSN.

This is also equal to Vout/Vsel x VOUT_COMMAND x 2^-9V

Where Vout is the default output voltage (0.84V) and Vsel is the selected default reference voltage (0.75V) .

For 0.84V output with a 0.75V default reference, that gives VOUT_COMMAND and LSB value of 2.188mV instead of  the standard 1.953mV

 Can the default Vout be changed by VOUT_COMMAND and storing that value to NVM using STORE_DEFAULT_ALL? 

[PJM] the power-up default output voltage and initial VOUT_COMMAND value can not be changed by changing VOUT_COMMAND and using STORE_DEFAULT_ALL when VSEL is connected to a 100k / 16.5k divider.

For an integrated FET converter that can:

1) Program the initial output voltage to 0.84V

2) Adjust the default output voltage by storing an updated VOUT_COMMAND value to NVM

3) Continue to use the standard -9 Exponent ULINEAR16 format for VOUT_COMMAND

You could consider the TPS546D24A, which offers a PIN_DETECT_OVERRIDE command that allows the user to direct the TPS546D24A to use stored NVM values on future power on resets, overriding the Pin Detection.

Allen Li2 

Unfortunately your images did not come through correctly, so I can't comment on the images.

However:

1) Yes, VOUT_MODE is read only and can not be changed, and the LSB from VOUT_COMMAND to RSP - RSN is still fixed at 1.9531mV / LSB, but since VOUT is not directly connected to RSP, the LSB of VOUT is not 1.9531mV, but is 2.188mV since ever 1.9531mV on RSP - RSN translates to 2.188mV at VOUT.

2) Below should be an image of how to route VOUT sense points back to RSP and RSN.  In this example there are 3 VOUT sense points.  There is a local resistive VOUT sense point for VOUT and GND, close to the output of the TPS549B22 converter and a remote resistive sense point.  The TPS549B22 will regulate the output voltage based on the average between these two, weighted by their sense resistance.  Typically designers will select a resistor value 10x higher for the local sense so that the remote-sense can maintain an accurate regulation voltage at the final output voltage.

There is also a capacitive connection to the local output.  This is useful when there is resistance and inductance in the power delivery path between the TPS549B22's local output and the remote sensed output close to the load.  The capacitor allows the converter to compensate for the power-path delay and stabilize the full loop despite the phase lag introduced by this parasitic power-path filter.

Allen Li2 

The schematic looks pretty good.  I see an estimated 10mV of output ripple and 27mV of transient deflection on the output voltage on a 20A load step, so make sure those meet your requirements.