AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 13

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

Voltage Regulators
Line Terminations
MOTOROLA
noise margin. It is also the preferred logic family for interfacing with
microcontrollers, as they are also manufactured using CMOS technology. One
important point regarding CMOS devices is that unused input pins should be
tied either to the power rail or ground. In MCU circuits, noisy environments may
cause these unterminated inputs to behave erratically, and cause the MCU to
execute runaway code.
For the typical regulator circuit, adequate decoupling capacitors should be
placed as close as possible to the output of the regulator because the distance
between the output of the regulator and the load will create an inductive effect
in the trace and causes internal oscillation of the regulator. Typically, a 0.1 µ F
decoupling capacitor is added to both input and output of the regulator to
prevent possible internal oscillation and to filter high frequency noise. In
addition, a relatively large bypass capacitor should be added (10 µ F per
ampere) to reduce output ripple.
capacitors for the regulator. The capacitors should be placed as close as
possible to the regulator device.
When a circuit is operating at high speeds, the impedance matching between
the source and destination is very important. Because mismatching will cause
signal reflection and ringing. The excess RF energy will radiate or couple to
other parts of the circuit, causing EMI problems. Termination of signals help to
reduce these undesirable effects.
Termination not only reduce signal reflection and ringing by matching the
impedance between source and destination, but can also to slow down the fast
rising and falling edges of the signals.
There are several termination methods, each has its advantages and
disadvantages.
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
INPUT
Figure 11. Regulator Bypass and Decoupling
Table 3
PART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES
100 to
470 µ F
lists a summary of the termination methods.
+
0.1 µ F
Figure 11
78XX
0.1 µ F
shows the bypass and decoupling
+
100 to
470 µ F
OUTPUT
AN2321/D
13