Tracking / softstart & FB pin tricks

Question

Here is the circuit under analysis:

A few questions:

1. Z_DRV is a signal from a DAC. From what I can understand, this signal is amplified by U28:B. This in turn causes the output of the switcher IC to track the voltage on the TRK/SS pin up until the feedback voltage reaches its regulation voltage (on pin 5). Why would someone want to have the switcher track? I understand that it can be used for soft starting but I am not familiar with what else this type of setup can be used for.

2. The same DAC signal goes to U28:A. Typically I have seen some circuits where a FET is placed with another resistor on the feedback pin in order to quickly change the output voltage. But here, it is an op-amp with its feedback path along with the feedback path of the switcher. What exactly are they trying to achieve here? A more advanced startup sequence?

3. If the two above questions are commonplace, is there an app note source so that I may get better informed on these techniques and circuits?

Answer 1

The TRK/SS pin is used if the voltage is below 0.6V so maybe you need to use both FB and TRK/SS to fully control the output voltage across all ranges of the output. I'd test it in LT spice to find out.

Output Tracking and Soft-Start Pin. Allows the user to control the rise time of the output volt- age. Putting a voltage below 0.6V on this pin bypasses the internal reference input to the error amplifier, instead it servos the FB pin to the TRACK voltage. Above 0.6V, the tracking function stops and the internal reference resumes control of the error amplifier. There’s an internal 2μA pull-up current from INTVCC on this pin, so putting a capacitor here provides soft-start function.

What exactly are they trying to achieve here? A more advanced startup sequence?

Don't know, but an advanced startup sequence is likely, they aren't trying to control FB1 directly because U28 is AC coupled, so it would likely be only during ramp.

If the two above questions are commonplace, is there an app note source so that I may get better informed on these techniques and circuits?

I'd talk to the linear reps and see if they know anything. App notes might describe this. You could download LT power cad and if the LTC3605 is in there you could autogenerate a spice file and play with the simulation and find out. Keep in mind switchers don't always simulate well so you may need to change the solver and sim parameters. Keep the time short because they also take a lot of processor time to simulate.