data/text/DLP4/900213

TPS549B22: Reference Voltage Programming

Part Number: TPS549B22 Hi, I need to program the TPS549B22's reference voltage to 0.75000V using the PMBUS (USB Interface Adapter) and the Fusion Digital Power Designer software. I cannot find the reference voltage selection on the software. Thanks

Hi Andrei, For the TPS549B22, the VOUT_COMMAND command should be used to program the reference voltage. This has a max of 1.2V, like when configuring with VSEL, so an external resistor divider is required for higher output voltages. Let me know if you have any further questions. Anthony

Hi Anthony, I am using the TPS549B22EVM development board. I swapped out the sensing resistors on the board with a 10kOhm and 15kOhm resistor and set the reference voltage (VOUT_COMMAND) to 0.75000V. According to the datasheet, this should output 1.25V. Unfortunately, the output voltage is identical to the reference voltage (VOUT_COMMAND). Here is my understanding of the RSP/RSN Remote Sensing Functionality: The TPS549B22 uses TI's D-CAP3 control mode, which is a variation of Constant On-Time (COT) control. The TPS549B22 will adjust its operating frequency to attempt to regulate the voltage between the Remote Sense Positive pin (RSP) and the Remote Sense Negative pin (RSN) to be equal to the internal reference voltage (which is set through an external voltage divider connected to the VSEL pin of the TPS549B22). See Figure 30. The resistor ratio is R1 / R2 = (Vout/Vref - 1) where R1 is the resistor from VOSNS to RSP and R2 is the resistor from RSP to RSN. Using Table 2 (Page 20) of the TPS549B22 datasheet, I set the reference voltage (VOUT_COMMAND) to 0.75000V and physically swapped out the resistors to output 1.25V. With a reference voltage of 0.75000V, R1 = 10kOhm and R2 = 15kOhm. Thank you for your help.

Andrei Marinescu The TPS549B22EVM-847 Users Guide contains details of using the FUSION GUI with the TPS549B22 and the TP549B22EVM-847 evaluation board. <secret URL>/lit/ug/sluubn2/sluubn2.pdf#page=22 Figure 24 on page 23 shows the "All Config" tab of the Configuration page. First, select "Configuration on the lower left of the screen" The Reference voltage is programmed through PMBus via the VOUT_COMMAND. Since the TPS549B22 does not support the command VOUT_SCALE_LOOP, the user will need to program the desired reference voltage into the VOUT_COMMAND value. Note: The VSEL pin-strapping will override the VOUT_COMMAND value stored in NVM by the STORE_DEFAULT_ALL command unless one of the 4 0.975V options are selected. I would recommend using either the "OPEN" (pin pulled to BP) or SHORT (Pin shorted to ground) options, depending on the fault response option desired, Hiccup or Latch-Off. See Datasheet page 20 <secret URL>/lit/ds/symlink/tps549b22.pdf#page=20 for more details.

Andrei Marinescu If you attempted to change the resistor divider to VOSNS to RSP = 10k Ohms and RSP to RSN = 15k Ohms, and the output voltage is directly tracking the reference voltage of 0.75V instead of 1.25V, either the RSP to RSN connection is open, or the VOSNS to RSP resistor is shorted. The schematic is on page 6 <secret URL>/lit/ug/sluubn2/sluubn2.pdf#page=6 Change R10 to 15kOhms and R8 to 10kOhms. I know it's a little complicated in there with the extra components to allow for D-CAP mode with RCC injection, so double check the resistor placement.

Andrei Marinescu Here is the location of R15 and R8 on the PCB:

Hi Peter, I replaced R8 with a 10kOhm resistor and R15 with a 15kOhm resistor. I also desoldered R23 and left the circuit OPEN as shown in Table 2. I set VOUT_COMMAND to 0.75000V, but the development board is still outputting the same voltage as VOUT_COMMAND. The only thing I am unsure about is the resistance across R15. When I check the resistance of R15 it reads between 6-12kOhm even though it is a 15kOhm resistor. Is this my problem? Thanks for your help.

Andrei Marinescu Measuring the resistance of R8 or R15 in circuit is going to be problematic because there is enough capacitance in Cout to look like a short circuit to many Ohm-meters, so you're likely to measure R8 and R18 in parallel when you measurement them in-circuit. If the output voltage you are measuring is equal to VOUT_COMMAND, then it is likely that one of the resistors is not corrected correctly. Turn the output on, measure VOUT with your volt meter, then measure the voltage across R15 and the voltage across R8. If the divider is set-up correctly the voltage drop across R15 should be equal to the VOUT_COMMAND value and the voltage drop across R8 should be equal to 2/3 of that (R8 / R15 = 10/15 = 2/3). If the voltage drop across R8 is near 0V, then check to see if the pads for C31, C32, or C36 are shorted out and let me know what the results are.

When I try to measure the voltage across these resistors, the output goes to 0V. Once the power supply is 0V, it does not return to the 0.75000V until I power the board off and on again.