DRA790 Datasheet, PDF (367/436 Pages) Texas Instruments – Infotainment Applications Processor

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DRA790, DRA791
DRA793, DRA797
SPRS968A – AUGUST 2016 – REVISED FEBRUARY 2017
• Decoupling capacitors must be carefully placed in order to minimize loop inductance impact on supply
voltage transients. A real capacitor has characteristics not only of capacitance but also inductance and
resistance.
Figure 7-6 shows the parasitic model of a real capacitor. A real capacitor must be treated as an RLC
circuit with effective series resistance (ESR) and effective series inductance (ESL).
C
ESL
ESR
SPRS906_PCB_FREQ_01
Figure 7-6. Characteristics of a Real Capacitor With ESL and ESR
The magnitude of the impedance of this series model is given as:
2
Z=
ESR
2
+ω^æç EωSELSL
è
-
1
ωC
ö
÷
ø
where : w = 2π¦
SPRS906_PCB_FREQ_02
Figure 7-7. Series Model Impedance Equation
Figure 7-8 shows the resonant frequency response of a typical capacitor with a self-resonant frequency of
55 MHz. The impedance of the capacitor is a combination of its series resistance and reactive capacitance
and inductance as shown in the equation above.
1.0e+01
S-Parameter Magnitude
Job: GCM155R71E153KA55_15NF;
1.0e+00
1.0e–01
1.0e–02
X_C=1/ωC
X_L=ωL
1.0e–03
Resonant frequency
(55 MHz) (minimum)
1.0e–04
1.00e–002
1.00e+000
1.00e+002
1.00e+004
Frequency (MHz)
1.00e+006
1.00e+008
SPRS906_PCB_FREQ_03
Figure 7-8. Typical Impedance Profile of a Capacitor
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Applications, Implementation, and Layout 367
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