參數(shù)資料
型號(hào)： MC145192EVK
廠商： Motorola, Inc.
英文描述： 8-Pin High Frequency 4-Amp Sink Synchronous MOSFET Driver 8-SON -40 to 125
中文描述： 評(píng)估板手冊(cè)
文件頁(yè)數(shù)： 16/24頁(yè)
文件大?。?/td> 348K
代理商： MC145192EVK
MC145192
16
MOTOROLA
DESIGN CONSIDERATIONS
CRYSTAL OSCILLATOR CONSIDERATIONS
The following options may be considered to provide a ref-
erence frequency to Motorola’s CMOS frequency synthe-
sizers.
USE OF A HYBRID CRYSTAL OSCILLATOR
Commercially available temperature–compensated crystal
oscillators (TCXOs) or crystal–controlled data clock oscilla-
tors provide very stable reference frequencies. An oscillator
capable of CMOS logic levels at the output may be direct or
dc coupled to REFin. If the oscillator does not have CMOS
logic levels on the outputs, capacitive or ac coupling to REFin
may be used. See Figure 8.
For additional information about TCXOs and data clock
oscillators, please consult the latest version of the eem Elec-
tronic Engineers Master Catalog,the Gold Book,or similar
publications.
DESIGN AN OFF–CHIP REFERENCE
The user may design an off–chip crystal oscillator using
discrete transistors or ICs specifically developed for crystal
oscillator applications, such as the MC12061 MECL device.
The reference signal from the MECL device is ac coupled to
REFin. (See Figure 8.) For large amplitude signals (standard
CMOS logic levels), dc coupling may be used.
USE OF THE ON–CHIP OSCILLATOR CIRCUITRY
The on–chip amplifier (a digital inverter) along with an ap-
propriate crystal may be used to provide a reference source
frequency. A fundamental mode crystal, parallel resonant at
the desired operating frequency, should be connected as
shown in Figure 20.
The crystal should be specified for a loading capacitance,
CL, which does not exceed approximately 20 pF when used
at the highest operating frequency of 10 MHz. Assuming
R1 = 0
, the shunt load capacitance, CL, presented across
the crystal can be estimated to be:
CinCout
Cin+ Cout
where
Cin = 5 pF (see Figure 21)
Cout = 6 pF (see Figure 21)
Ca = 1 pF (see Figure 21)
C1 and C2 = external capacitors (see Figure 20)
Cstray = the total equivalent external circuit stray capaci-
tance appearing across the crystal terminals
The oscillator can be “trimmed” on–frequency by making
either a portion or all of C1 variable. The crystal and asso-
ciated components must be located as close as possible to
the REFin and REFout pins to minimize distortion, stray
capacitance, stray inductance, and startup stabilization time.
Circuit stray capacitance can also be handled by adding the
appropriate stray value to the values for Cin and Cout. For
this approach, the term Cstray becomes zero in the above ex-
pression for CL.
Power is dissipated in the effective series resistance of the
crystal, Re, in Figure 22. The maximum drive level specified
CL =
+ Ca + Cstray +
C1
C2
C1+C2
by the crystal manufacturer represents the maximum stress
that the crystal can withstand without damage or excessive
shift in operating frequency. R1 in Figure 20 limits the drive
level. The use of R1 is not necessary in most cases.
To verify that the maximum dc supply voltage does not
cause the crystal to be overdriven; monitor the output
frequency (fR) at Output A as a function of supply voltage.
(REFout is not used because loading impacts the oscillator.)
The frequency should increase very slightly as the dc supply
voltage is increased. An overdriven crystal decreases in fre-
quency or becomes unstable with an increase in supply volt-
age. The operating supply voltage must be reduced or R1
must be increased in value if the overdriven condition exists.
Note that the oscillator start–up time is proportional to the
value of R1.
Through the process of supplying crystals for use with
CMOS inverters, many crystal manufacturers have devel-
oped expertise in CMOS oscillator design with crystals. Dis-
cussions with such manufacturers can prove very helpful.
See Table 4.
R1*
C2
C1
FREQUENCY
SYNTHESIZER
REFout
REFin
Rf
* May be needed in certain cases. See text.
Figure 20. Pierce Crystal Oscillator Circuit
Cin
Cout
Ca
REFin
REFout
Cstray
Figure 21. Parasitic Capacitances of the Amplifier
and Cstray
NOTE: Values are supplied by crystal manufacturer
(parallel resonant crystal).
2
1
2
1
2
1
RS
LS
CS
Re
Xe
CO
Figure 22. Equivalent Crystal Networks
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