DRA722_17 Datasheet, PDF (343/408 Pages) Texas Instruments – Infotainment Applications Processor

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DRA722, DRA724, DRA725, DRA726
SPRS956B – MARCH 2016 – REVISED JANUARY 2017
• Max PCB Impedance (Z) vs Frequency (F) budget between Device’s power inputs and PMIC’s output
power filter node including ground return is determined by applying the Frequency Domain Target
Impedance Method to determine the PCB’s maximum frequency of interest (Fpcb). Ideally a properly
designed and decoupled PDN will exhibit smoothly increasing Z vs. F curve. There are 2 general
regions of interest as can be seen in Figure 8-14.
– 1^st area is from DC (0Hz) up to Fpmic (typ a few 100 kHz) where a PMIC’s transient response
characteristic (i.e. Switching Freq, Compensation Loop BW) dominate. A PDN’s Z is typically very
low due to power filtering & bulk capacitor values when PDN has very low trace resistance (i.e.
good Reff performance). The goal is to maintain a smoothly increasing Z that is less than Zt1 over
this low frequency range. This will ensure that a max transient current event will not cause a
voltage drop more than the PMIC’s current step response can support (typ 3%).
– 2^nd area is from Fpmic up to Fpcb (typ 20-100MHz) where a PCB’s inherent characteristics (i.e.
parasitic capacitance, planar spreading inductances) dominate. A PDN’s Z will naturally increase
with frequency. At frequencies between Fpmic up to Fpcb, the goal is to maintain a smoothly
increasing Z to be less than Zt2. This will ensue that the high frequency content of a max transient
current event will not cause a voltage drop to be more than 5% of the min supply voltage.
Figure 8-14. PDN’s Target impedance
1.Voltage Rail Drop includes regulation accuracy, voltage distribution drops, and all dynamic events
such as transient noise, AC ripple, voltage dips etc.
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Applications, Implementation, and Layout 343
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