Does the TPS544b20 support setting VOUT via PMBus?

Hi Dan, TPS544B20 supports adjusting VOUT by PMBus. The nominal set-point is set up using a resistor divider from DIFFO (output of the remote sense amplifier) to FB (terminal of the error amplifier) to AGND. The nominal reference (Vref) is equal to 0.6V. 

Using the VREF_TRIM command, the nominal Vref can be adjusted by -20% to +10%. This can also be combined with the Margin function as described in the OPERATION command. For example, if nominally, VOUT = 1.2V, i.e. VOUT = 2x Vref = 2*0.6V . Changing Vref to 0.66V by changing VREF_TRIM to +60mV, makes VOUT=2*0.66V=1.32V. 

TI does have a similar device which uses the standard VOUT_COMMAND command to set the output voltage. This would be TPS544B25, and TPS544C25. Changing VOUT_COMMAND from 1V to 1.2V, makes VOUT=1.2V. Note however, that there are only 8 initial boot-up voltages configurable by resistor programming for this part - other set-points would require programming via pmbus before inital power-up. The Vout can be changed always after power-up by the VOUT_COMMAND. 

Hi Matt,

Customer is curious if I can use the TPS544b25 to program output voltage from 0.8V to 5.5V. 

Couple questions:

- I am not too concerned with initial voltage as we would have device powered down until SW turns on.  What are performance differences between TPS544B25 and TPS544B20?  Seems B20 uses D-CAP/D-CAP2 and B25 uses voltage mode.  Any words on how these modes compare?

 - I do not want to use an external circuit to program these.  Can you ask last contact why the TPS544B25 wouldn’t just do this?

Thanks,

Dan

Hi Dan, 

TPS544C25 can be programmed via PMBus from 0.8V to 5.5V. BUT... It has three supported VOUT ranges, which correspond to the resistor divider ratio placed from DIFFO to AGND. The output voltage is adjusted by changeing the reference voltage, but the difference is that TPS544C25 supports Vref from 0.5V to 1.5V. 

There are three supported divider ratios (these correspond to the VOUT_SCALE_LOOP) parameter. 

• VOUT_SCALE_LOOP=1, No divider (no fb to Gnd resistor), VOUT=1x VREF... so VOUT = 0.5V to 1.5V
• VOUT_SCALE_LOOP=0.5, 2:1 divider , VOUT = 2x Vref....  so VOUT = 1V to 3V
• VOUT_SCALE_LOOP=0.25, 4:1 divider, VOUT=4x Vref... so VOUT= 2V to 6V. 

Adjustment within any one range can be made by changing PMBus commands only (VOUT_COMMAND). Changing between ranges requires changing the feedback (fb to GND) resistor. 

As you mentioned, TPS544C25 uses voltage mode control. This is a "standard" linear, clocked fixed frequency control scheme. The DCAP/2 control scheme is a TI proprietary form of constant on-time control. The trade-off is that TPS544C25 will be slower to respond to a load transient (more output capacitance required for the same output deviation), but it will operate at a fixed frequency. The TPS544C20 uses pulse frequency modulation, so its operating frequency, while appx constant for a DC load, will change dramatically in response to a load transient step/release.