Hello,
I use a system of 4 LMZ31506 in my prototype board.
All modules are powered by a +12V rail.
The first device is configured in Slow start mode and its output is set to +5V.
All other devices are tracked on the +5V output of the first module.
The output voltages are set to +3,3V, +2,5V and +1,8V.
There is a resistor bridge between +5V and the SS/TR pin of each tracked device.
Each bridge has been calculated as indicated in the LMZ31506 data sheet (page 23).
Everything works well, except one curious thing about the PWRGD outputs:
In the data sheet (chapter "Power good"), there is a condition on the SS/TR level, that must be above 1,4V for enabling the PWRGD output.
Based on the resistor bridge recommendations, I have calculated that when the output voltage is set below 2,1V, the SS/TR voltage is above 1,4V, and so PWRGD signal is available.
If the output voltage is set above 2,1V, the SS/TR voltage is below 1,4V and so PWRGD should not be enabled.
The SS/TR levels are:
1,69V for the 1,8V module.
1,2V for the 2,5V module.
0,9V for the 3,3V module.
So, in theory, only the PWRGD output of the 1,8V module should work (other levels are under 1,4V).
In fact, the PWRGD output of the 3,3V module is effectively stucked at 0V, but the PWRGD output of the 2,5V module works, with the SS/TR level at 1,2V.
First, I wanted to use all the PWRGD outputs to sequence other power modules.
The question is: Is the PWRGD output of the 2,5V reliable ?
Daniel Kubiak
