13.3 Power Supply Sequencing
Four classes of supply voltages exist on the Si5391/Si5391P:
1. VDD = 1.8 V (Core digital supply)
2. VDDA = 3.3 V (Analog supply)
3. VDDOx = 1.8/2.5/3.3 V ± 5% (Clock output supply)
4. VDDS = 1.8/3.3V ± 5% (Digital I/O supply)
There is no requirement for power supply sequencing unless the output clocks are required to be phase aligned with each other. In this
case, the VDDO of each clock which needs to be aligned must be powered up before VDD and VDDA. VDDS has no effect on output
clock alignment.
If output-to-output alignment is required for applications where it is not possible to properly sequence the power supplies, then the
output clocks can be aligned by asserting the SOFT_RST 0x001C[0] or Hard Reset 0x001E[1] register bits or driving the RSTB pin.
Note that using a hard reset will reload the register with the contents of the NVM and any unsaved changes will be lost.
One may observe that when powering up the VDD = 1.8 V rail first, that the VDDA = 3.3 V rail will initially follow the 1.8 V rail. Likewise,
if the VDDA rail is powered down first then it will not drop far below VDD until VDD itself is powered down. This is due to the pad I/O
circuits which have large MOSFET switches to select the local supply from either the VDD or VDDA rails. These devices are relatively
large and yield a parasitic diode between VDD and VDDA. Please allow for both VDD and VDDA to power-up and power-down before
measuring their respective voltages.
13.4 Grounding Vias
The pad on the bottom of the device functions as both the sole electrical ground and primary heat transfer path. Hence it is important
to minimize the inductance and maximize the heat transfer from this pad to the internal ground plane of the PCB. Use no fewer than 25
vias from the center pad to a ground plane under the device. In general, more vias will perform better. Having the ground plane near the
top layer will also help to minimize the via inductance from the device to ground and maximize the heat transfer away from the device.
Si5391 Reference Manual • Power Management
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
61
Rev. 0.5 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • January 11, 2022
61
