Q: TPS54320 SS/TR and Power Good Relationship by 1012266

A customer is having an issue with the tracking function of the TPS54320. Specifically, he has run through the equations in the datasheet for the resistor divider R1 and R2 on page 16 for simultaneous sequencing. The calculated results give him the correct sequencing, however the resulting voltage on the SS/TR pin is less than 1.2V. Consequentially this voltage level keeps the PWRGD signal from being released (page 14 of the datasheet). This seems like a catch-22. Are there any workarounds on those equations to generate a voltage above 1.2V on the SS/TR pin that will maintain simultaneous sequencing and allow PWRGD to be pulled high?

Circuit specifics:
3.3V to 2.5V tracking:
R1 = 39.4k
R2 = 18.54k
Resulting voltage on SS/TR = 1.05V


A: Re: TPS54320 SS/TR and Power Good Relationship by 1012266

The voltage that is being tracked needs to be 1.75x the voltage that you are bringing up due to the 1.4Vmin on SS/TR for PWRGD to be released. For example, 1.2V can track 3.3V just fine because it is 2.75x, but 3.3V is only 1.32x 2.5V.

A modification to the circuit shown in the datasheet is to add a third resistor from the SS/TR pin to the PWRGD of the 3.3V device.  PWRGD will be low for the entire ramp up of the 2.5V.  When the 3.3V is up, PWRGD will go high impedance and the resulting voltage divider onto SS/TR will allow the SS/TR voltage to go above 1.4V.  The parallel lower resistors would equal the R2 in the datasheet equations and the 1 resistor to ground would give you a voltage above 1.4V on SS when the PGOOD resistor is disconnected.

They can also use sequential tracking or an external supervisor/sequencer.