10.3 Power Supply Sequencing
Four classes of supply voltages exist on the Si5386:
1. VDD = 1.8 V (Core digital supply)
2. VDDA = 3.3 V (Analog supply)
3. VDDO = 1.8/2.5/3.3 V (Output Clock supplies)
There is no general requirement for power supply sequencing on this device 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.
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 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 register changes will be lost.
When powering up the VDD = 1.8V rail first, it can be observed 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. Allow for both VDD and VDDA to power-up and power-down
before measuring their respective voltages.
10.4 Grounding Vias
The "Epad" on the bottom of the device functions as both the sole electrical ground and as the 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.
Si5386 Rev. E Reference Manual • Power Management
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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Rev. 1.2 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • December 9, 2021
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