If it has a power down mode, how to enter the power down mode by software?
There's a low power mode available in hardware - as long as the RSTB pin is held low, the device is in a low-power mode.
As far as I know, there's no way to enter a low-power mode from software.
But I need a software method as I don't have extra IO pin to control the RSTB pin.
Are you sure you can use the chip correctly without any control over the RSTB pin? To what is the pin connected in your design?
I am asking, because for correct operation, you need to hold the RSTB pin low for at least 10µs after power-on. Please refer to the datasheet:
After software has been loaded, the BOOT command causes the Si4684 to enter the OPERATIONAL state, which is the highest-power state.
The Si4684 can be returned to the RESET state by asserting (holding low) the RSTB pin.
Figure 12 shows required reset, startup, and shutdown timings for the Si4684. RSTB must be held low (asserted) during any power supply transitions and remain asserted for 10 µs after all power supplies are stable as specified in Figure 12. Failure to assert RSTB as indicated here may cause the device to malfunction and may result in permanent device damage.
The chip's RSTB is connected to system reset. I just want to power down the chip without resetting the whole system,
The chip's RSTB is connected to system reset.
Ok, that works.
I just want to power down the chip without resetting the whole system,
As far as I know, this is not possible (there does not seem to be a "power down" command.