IMIFS741 データシートの表示（PDF） - Unspecified

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IMIFS741

Low EMI Spectrum Spread Clock

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FS741

Low EMI Spectrum Spread Clock

BI-DIRECTIONAL BUFFERS

Two pins on the FS741 are connected to Bi-Directional buffers. Using Bi-Directional buffers is a method of

sharing an input circuit and an output circuit with the same pin on the IC assembly, thereby reducing the pin count.

Each Bi-Directional I/O acts as an input during power up and as an output after power has reached a certain

voltage. For the FS741, that voltage is approximately VDD/3. At VDD/3, the FS741 latches the logic state of the

respective line into an internal register for as long as power is applied to the FS741. After VDD/3 has been

reached and the power on reset has occurred, the respective pin is switched from an input gate to an output

driver. This pin remains an output driver for as long as power is applied.

Loop Filters

The FS741 requires an external loop filter to provide the proper operation and bandwidth for a given input

frequency. Since the FS741 operates over a wide range of frequencies, the loop filter will change depending on

the frequency of operation. The following loop filter values are recommended for best performance and

modulation profile at 5.0 volts and 3.3 volts VDD, measured across pin 8 (VDD) and 5 (VSS).

LF

R1

C2

C1

Figure 4.

The tables on pages 7 and 8 contain the loop filter values for the power supply voltages of 5.0 and 3.3 VDC, +/-

10%. The values in both tables 6 and 7 were bench tested for accuracy and optimal performance. The loop filter

values were determined by taking 4 MHz segments of the overall operating range and testing for the optimal

performance at the center frequency of each 4 MHz band. This means that in the first band in the table below, 4 -

8 MHz, the loop filter values shown in the table produce the most optimized performance for 6 MHz. It is possible

to deviate slightly from these values for optimal performance at some other center frequency. Also note that the

values listed in these tables are all commonly manufactured components.

International Microcircuits, Inc.

525 Los Coches St., Milpitas, 95035 408-263-6300, FAX 408-263-6571

http:/www.imicorp.com

4/5/1999

Rev. 2.1

Page 6 of 16

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