Hello,
I am using the TPS74301 LDO to generate a rail that is supposed to simultaneously track a reference voltage during startup. The datasheet says " By properly choosing values of resistor divider used to feed the track pin, one can implement Simultaneous and ratiometric startup". From my understanding of the device, It makes the output voltage track the track pin voltage if the voltage at the track pin is less than 0.8V, and after that the output voltage tracks the Set point.
If we use a voltage divider to generate the track voltage, doesn`t it become ratiometric sequencing by default ?The voltage at the track input (output of the voltage divider) will be a ratio of the input to the voltage divider. Kindly clarify. If that is the case, how does changing the value of the resistor divider enable ratiometric or simultaneous sequencing ?
Thanks in Advance!
Ps: the part i am refering to is the TPS74301, LDO with programmable sequencing .
For the type of sequecing question,
The type of sequencing depends upon the value of R1 and R2. Using voltage divider for generating the track input does not mean that it is ratiometric sequencing.
As shown in the timing diagram in FIGURE 29 of the datasheet of TPS74301, in simultaneous sequencing the slope for both I/O & core is the same but the slopes are different in ratiometric.
In ratiometric, the voltages reach at corresponding regulation at the same time hence the dv/dt required are different. (Because the voltages are different).
In your application, your assumption is correct that if you use simultaneous sequencing then all the three would have same slope i.e. they ramp simultaneously.
All above questions are answered in FIGURE 29 in the datasheet of TPS74301.
Please refer to that.
Thanks.
Thanks for the reply. But what was not clear from the datasheet is how the sequencing method could be changed from ratiometric to simultaneous by just changing the voltage divider values. I am under the impression that using a voltage divider to divide down a voltage to be tracked makes the sequencing inherently ratiometric. Could you please clarify ?
I am using two PTH modules with their Auto track inputs tied together to generate two rails in my application. In addition to this, i am also using this LDO to generate another rail , which must ramp simultaneously with the rails generated by the PTH modules. Hence, i am dividing down the Auto track voltage by means of a voltage divider recommended by the datasheet for simultaneous sequencing and applying that to the track input of the LDO. I am thinking this would cause all three of them to ramp simultaneously. Is my assumption correct ? Is there anyone who has done something similar ? Any help would be greatly appreciated.
What would be even better is a way to verify this ( I am trying to make a board to bench test this in the meantime ) . It would be great if TI could give me some kind of Spice Macromodel/Behavioural model of the regulators in question , so that i could simulate it at my end.
PS: The complete part numbers of the regulators in question is: PTH04000WAZ, PTH05050WAZ and of course, the LDO TPS74301.
Thanks !
Sornam
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