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MC88LV915T bảng dữ liệu(PDF) 8 Page - Motorola, Inc |
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MC88LV915T bảng dữ liệu(HTML) 8 Page - Motorola, Inc |
8 / 12 page 8 MOTOROLA Figure 3. Recommended Loop Filter and Analog Isolation Scheme for the MC88LV915T 47Ω BOARD VCC 0.1µF (LOOP FILTER CAP) 330Ω 1MΩ 0.1µF HIGH FREQ BYPASS 10µF LOW FREQ BYPASS 47Ω BOARD GND 8 9 10 ANALOG VCC RC1 ANALOG GND ANALOG LOOP FILTER/VCO SECTION OF THE MC88LV915T 28-PIN PLCC PACKAGE (NOT DRAWN TO SCALE) A SEPARATE ANALOG POWER SUPPLY IS NOT NECESSARY AND SHOULDNOT BE USED. FOLLOWING THESE PRESCRIBED GUIDELINES IS ALL THAT IS NECESSARY TO USE THE MC88LV915T IN A NORMAL DIGITAL ENVIRONMENT. Notes Concerning Loop Filter and Board Layout Issues 1. Figure 3 shows a loop filter and analog isolation scheme which will be effective in most applications. The following guidelines should be followed to ensure stable and jitter–free operation: 1a.All loop filter and analog isolation components should be tied as close to the package as possible. Stray current passing through the parasitics of long traces can cause undesirable voltage transients at the RC1 pin. 1b.The 47 Ω resistors, the 10µF low frequency bypass capacitor, and the 0.1 µF high frequency bypass capacitor form a wide bandwidth filter that will minimize the 88LV915T’s sensitivity to voltage transients from the system digital VCC supply and ground planes. This filter will typically ensure that a 100mV step deviation on the digital VCC supply will cause no more than a 100pS phase deviation on the 88LV915T outputs. A 250mV step deviation on VCC using the recommended filter values should cause no more than a 250pS phase deviation; if a 25 µF bypass capacitor is used (instead of 10µF) a 250mV VCC step should cause no more than a 100pS phase deviation. If good bypass techniques are used on a board design near components which may cause digital VCC and ground noise, the above described VCC step deviations should not occur at the 88LV915T’s digital VCC supply. The purpose of the bypass filtering scheme shown in Figure 3 is to give the 88LV915T additional protection from the power supply and ground plane transients that can occur in a high frequency, high speed digital system. 1c.There are no special requirements set forth for the loop filter resistors (1M Ω and 330Ω). The loop filter capacitor (0.1 µF) can be a ceramic chip capacitior, the same as a standard bypass capacitor. 1d.The 1M reference resistor injects current into the internal charge pump of the PLL, causing a fixed offset between the outputs and the SYNC input. This also prevents excessive jitter caused by inherent PLL dead–band. If the VCO (2X_Q output) is running above 40MHz, the 1M Ω resistor provides the correct amount of current injection into the charge pump (2–3 µA). For the TFN55, 70 or 100, if the VCO is running below 40MHz, a 1.5M Ω reference resistor should be used (instead of 1M Ω). 2. In addition to the bypass capacitors used in the analog filter of Figure 3, there should be a 0.1 µF bypass capacitor between each of the other (digital) four VCC pins and the board ground plane. This will reduce output switching noise caused by the 88LV915T outputs, in addition to reducing potential for noise in the ‘analog’ section of the chip. These bypass capacitors should also be tied as close to the 88LV915T package as possible. |
Số phần tương tự - MC88LV915T_01 |
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Mô tả tương tự - MC88LV915T_01 |
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